Fertilizer material

There is great variety among fertilizer materials. In general, fertilizers fall within two major categories: commercial

fertilizer sources and organic sources. While it is difficult to make direct comparisons between these two sources, a few

loose comparisons can be made.

First, commercial sources are typically high analysis fertilizers, while organic sources are low analysis. This means that

commercial fertilizers contain a larger percentage of a given nutrient than organic sources. As a result, commercial

fertilizers are applied in lesser amounts than organic sources, since it take less commercial fertilizers to achieve a given

rate.

Secondly, composition of organic fertilizer is generally much more varied than commercial fertilizers. This lack of

consistency can make it difficult to predict how much organic fertilizers should be applied in order to obtain a desired

rate.

Thirdly, commercial fertilizer production is fossil fuel intensive. As a result, the price of commercial fertilizer can be

relatively expensive.

Commercial fertilizer sources

NITROGEN FERT IL IZERS

Anhydrous ammonium is the starting block for most inorganic nitrogen fertilizers. Anhydrous ammonium is

manufactured by reacting N2 with H2 under extreme heat and pressure in the presence of a catalyst, known as the

Haber-Bosch technology. The Haber-Bosch technology requires large energy input, but allows for the manufacture of

high N analysis fertilizers.

Anhydrous Ammonium

Anhydrous ammonium has the highest nitrogen analysis out of all inorganic fertilizers

It is comprised of 82% nitrogen.

It must be kept under pressure since it evaporates under normal atmospheric pressure.

It is very harmful to human tissue, such as eyes, skin, and lungs. Thus, there are many safety precautions

associated with the handling of NH3.

Ammonium sulfate

Contains 21% nitrogen and 11% sulfur

Sugarcane and pineapple production

Ammonium sulfate is acid forming and lowers soil pH.

Ammonium phosphate

Monoammonium phosphate (MAP)

11-18% nitrogen and 48-55% P2O5

MAP is a water soluble fertilizer

The soil pH temporarily lowers to about 3.5 in areas where MAP initially reacts with soil.

Diammonium phosphate (DAP)

18-21% nitrogen and 46-53% P2O5

DAP is a water soluble fertilizer.

The soil pH temporarily reduces to 8.5 in areas where DAP initially reacts with soil.

DAP may produce free ammonia in high pH soils, which may cause seed injury if placed too close to seed

rows.

Potassium nitrate

13% nitrogen and 44% K2O

Provides soil with readily available nitrate, which generally increases soil pH.

Calcium nitrate

15% nitrogen and 34% CaO

Provides soil with readily available nitrate.

However, calcium nitrate is hygroscopic (absorbs moisture from air) and must be kept under air-tight storage

conditions.

Urea

45-46% nitrogen

Advantages of urea over other nitrogen sources include:

o reduced caking of fertilizer material

o less corrosion on equipment

o decreased costs associated with storage, transportation, and handling

Once applied to the soil, an enzyme known as urease transforms urea to NH4+ and HCO3-.

o This transformation readily occurs under warm, moist conditions.

Urea temporarily increases the pH of the soil it contacts, due to the initial release of NH3. However, the soil pH

may ultimately decrease as the NH4+ nitrifies to NO3-, which is an acid producing reaction.

In soils with high pH, NH4+ may volatilize and escape from the soil in the form of NH3. Volatilization losses are

reduced by incorporating or washing urea into the soil.

Urea can contain biurate, which is phytotoxic to most plants.

o Although most plants tolerate up to 2% biurate levels, pineapple and citrus are sensitive to biuret. The

urea should contain less than 0.25% biuret.

Sulfur-coated urea

22-38% nitrogen and 12-22% sulfur

Sulfur-coated urea is a controlled release fertilizer.

o It contains a coat of sulfur that surrounds a urea granule, which controls its release.

o Urea is only released after the sulfur coat is oxidized by microorganisms.

o The rate at which urea becomes available depends on the thickness of the sulfur coat.

Sulfur coated urea is advantageous in coarse textured soils and/or soils that have a great nitrate leaching

potential.

PHOSPHATE

The major source of inorganic phosphorus fertilizers is rock phosphate. Rock phosphate is a naturally occurring

mineral, which is mined from the earth. Deposits of rock phosphate occur around the world, such as in the United

States, Russia, Morocco, and China.

Rock phosphate (RP)

27-41% P2O5 and 25% Calcium

The minerals that make up RP are various forms of apatite. The reactivity of RP depends on the type of apatite

and its inherent purity. RP is not water soluble and only becomes available to plants under acidic conditions.

RP is most reactive when it is finely ground and incorporated into warm, moist, acidic soils with long growing

seasons. Although the availability of RP is slow, it has a great long term residual effect.

Superphosphate

Single superphosphate (SSP)

16-22% P2O5, 11-12% sulfur, and 20% calcium

SSP is manufactured by reacting RP with sulfuric acid.

SSP does not have a great influence on soil pH.

Triple superphosphate (TSP)

44-52% P2O5, 1-1.5% sulfur, and 13% Ca

TSP is produced by treating RP with phosphoric acid

Like SSP, TSP does not have a great effect on soil pH.

Ammonium phosphate

Monoammonium phosphate (MAP)

11-13% N, 48-62% P2O5, and 0-2% S

MAP is water soluble.

MAP temporarily lowers the soil pH to 3.5 in areas where MAP initially reacts with the soil.

Diammonium phosphate (DAP)

18-21% N, 46-53% P2O5, and 0-2% S

DAP are water soluble.

The soil pH temporarily lowers to 8.5 in areas where DAP initially reacts.

DAP may produce free NH3 in soils with a high pH, which may cause seed injury if placed close to seed rows.

POTASS IUM

Potassium is mined from the earth as soluble potassium salts, or potash, with varying degree of purity. Canada is home

to the world’s largest potash deposit.

Potassium chloride (muiate of potash)

60-63% K2O

KCl is the most commonly used K fertilizer.

KCl readily dissolves in water

Potassium sulfate (sulfate of potash)

50-53% K2O, 17% S K2SO4-

Potassium sulfate is completely water soluble.

In comparison to KCl, potassium sulfate:

o has a lower salt index

o may be used on crops that are sensitive to Cl- (i.e. avocado).

Potassium nitrate

44% K2O and 13% N

Potassium nitrate is also water soluble.

Increases soil pH

Potassium nitrate is also a source of nitrogen.

Potassium-magnesium sulfate

22% K2O, 11% Mg, and 22% S

This inorganic fertilizer does not have a significant effect on soil pH

CALC IUM

Lime

Soil amendment which is commonly used to raise the pH of the soil.

Ground coral in Hawaii contains 38% Mg and 0.6% Mg

Calcium Carbonate

Approximately 38% Ca, depending upon its source

A common liming material, calcium carbonate also supplies calcium to the soil.

Dolomite

22% Ca and 12% Mg, depending upon the dolomite source

In addition to raising the pH, dolomite is a source of calcium and magnesium.

Gypsum

23% Ca and 19% S

Unlike liming materials, gypsum does not increase the soil pH.

In addition to providing calcium and sulfur, gypsum may be used to correct soil physical problems and/or

aluminum toxicities.

Calcium nitrate

15% N and 20% Ca

Calcium nitrate is very soluble in water.

Superphosphates

Single (SSP)

18-21% Ca

SSP supplies both calcium and phosphate.

Triple (TSP)

12-14% Ca

Like SSP, TSP supplies both calcium and phosphate

MAGNESIUM

Dolomite

22% Ca and 12% Mg, depending upon the source

Dolomite is a source of both Ca and Mg, in addition to its liming affect.

Magnesium sulfate (Epsom salt)

9.8% Mg and12% S

Epsom salt is very soluble and does not alter soil pH.

Magnesium oxide

55% Mg

Magnesium oxide increases soil pH.

It is not highly water soluble. For maximum reactivity, it is often mixed into the soil.

SULFUR

Elemental sulfur

In its elemental form, sulfur is a solid

Elemental sulfur is insoluble in water.

When finely-ground elemental sulfur is incorporated into the soil, microorganisms oxidize and convert it to

sulfate.

o The finer the sulfur, the greater its oxidization potential when incorporated into the soil.

Ammonium sulfate

Contains 24% S and 21% N

Ammonium sulfate can have a strong acidifying effect on soil

MICRONUTRIENT

Iron

Iron (ferrous) sulfate

o Contains19% Fe

o May be used as a foliar spray to correct Fe deficiencies

Iron chelate (iron EDTA)

o Contains 5-14% Fe

o May be used as foliar spray or directly applied to the soil

o Though expensive, chelates prevent the formation of insoluble Fe compounds

Zinc

Zinc sulfate

o Contains 35% Zn

o Due to its low soil mobility, zinc sulfate should be mixed into the soil when broadcasted

o Band placement is favorable in finely textures soils that are low in Zn

o Available as a foliar spray

Zinc chelate (EDTA)

o Contains 14% Zn

o May be applied as a foliage spray or directly to the soil

o Zn chelates are very soluble and may be incorporated into liquid fertilizers

Copper

Copper sulfate

o Contains 25% Cu

o May be applied to the soil and/or foliage

o Incorporating Cu into the plant root zone increases the efficiency of Cu

Copper chelate (EDTA)

o Contains 13% Cu

o Very soluble

o May be applied as a foliar spray

Manganese

Mangenese sulfate

o Contains 26-28% Mn

o May be applied as a foliar spray and/or directly to the soil in a band application

Manganese chelate (EDTA)

o Contains 5-12%

o Not recommended as a broadcast

Boron

Sodium borate, or borax

o Contains 11% B

o May be applied to soil as a band or broadcast

o Available as a foliar spray

o Since boron has a small sufficiency range, it should be mixed uniformly into the soil

o Care should be taken to prevent B toxicity.

Sodium tetraborate

o Contains14-15%

o Most widely used B fertilizer

Granusol

A manufactured product that contains 5.4% Fe, 5.2% Zn, 5.6% Mn, 5.4% Mg, 2.6% Cu, and 0.5% B. Since it is

largely insoluble, it should be incorporated into the soil.

Blends (Mixed Fertilizers)

There are many available inorganic fertilizers that contain various combinations of N, P, and K fertilizers. If a particular

formulation of N, P, and K is desired, a blend can conveniently meet the needs of the farmer or gardener, while

reducing the costs associated with buying and applying multiple fertilizers.

For more information on different types of fertilizers, click on the following link:

http://instruct1.cit.cornell.edu/Courses/css412/mod5/ext_m5_pg8.htm

Fertilizer Calculations

When applying fertilizers to your field or garden, you will add fertilizers at a specific rate of application. To accomplish

this goal, it is necessary that you can perform two calculations:

First, you must know how to determine the percentage of nutrients, particularly N, P, and K, that a particular

fertilizer contains.

Secondly, you must know how to calculate the quantity of fertilizer that must be added to a given area in order

to achieve the recommended rate of fertilization for a particular nutrient.

The following provides a detailed explanation of how to perform these calculations, which was prepared and written by

Jay Deputy of Tropical Plants and Soil Science.

Fertilizer Application Calculations

The major nutrients

A complete fertilizer contains all three of the major nutrient elements nitrogen (N), phosphorus (P), and potassium (K).

The total percentage of the nutrients contained in a fertilizer is given as three numbers, which together is known as the

analysis. These numbers are usually in large print on the front of the container or bag. An example would be 10-30-10.

Nitrogen (N)

Nitrogen is reported as total N and may take one of three chemical forms:

NO3- or nitrate-N

NH4 or ammonium-N

Urea-N

Most fertilizers contain a mixture of two or all three of these N forms.

The percent of total-N is represented by the first of the three analysis numbers. For example, a bag with an analysis of

10-30-10 contains 10% N by weight of all nitrogen forms. Therefore, a 50 pound bag of 10-30-10 contains 5 pounds of

total-N, which accounts for 10% of the bag’s 50 pounds weight.

Calculation of %N: 10% of 50 pounds = (.10 x 50 pounds) = 5 pounds of total N

Phosphorus (P)

Phosphorus is never present as pure elemental P. Instead, P is reported in fertilizers as the chemical compound P2O5

or ortho-phosphate. The percent of P2O5 in a complete fertilizer is represented by the second of the three analysis

numbers. For example, a bag with the analysis of 10-30-10 contains 30% P2O5 by weight. Therefore, a 50 pound bag of

10-30-10 contains 15 pounds of P2O5.

Calculation of % P2O5: 30% x 50 pounds = (.30 x 50 pounds) = 15 pounds of P2O5

However, notice that the above calculation determines the amount of P2O5 in the bag of fertilizer, rather than the

amount of total P. To report the quantity of total P, the percent of elemental, or pure, P must be determined.

To calculate elemental P, we must determine the percent by weight of P in P2O5, which is 44%. Thus, 44% of P2O5 is

elemental P. To convert the percent of P2O5 to percent elemental P, multiply the percent P2O5 by 44%.

Therefore, a bag of 10-30-10 contains 15 pounds of P2O5 (see above calculation) and 6.6 pounds elemental P (15

pounds P2O5 x .44 = 6.6 pounds P)

Calculation of % P = % P2O5 x 44% = 15 pounds P2O5 (see above calculation) x .44 = 6.6 pounds of P

Potassium (K)

Potassium is also never present as pure elemental K, but is reported as its oxide form of K2O, commonly called potash.

The percent of K2O in a bag of blended fertilizer is represented by the third of the three numbers of the analysis. For

example, a bag of fertilizer with an analysis of 10-30-10 contains 10% K2O by weight. Therefore, a 50 pound bag of 10-

30-10 contains 5 pounds of K2O 10% of 50 pounds.

Calculation of % K2O = 10% of 50 pounds = (.10,x 50) = 5 pounds of K2O

As with P, in some cases potassium is reported as percent elemental (or pure) K. To calculate elemental K, we must

determine what percentage (by weight) of K2O is elemental K, which we know to be 83%. This means that 83% of K2O

is elemental K. To convert percent K2O to percent elemental K, multiply the percent K2O by 83%.

Therefore, a bag of 10-30-10 contains 5 pounds of K2O (see above calculation) and 4.15 pounds elemental K

Calculation of % K = % K2O x 83% = 5 pounds K2O x .83 = 4.15 pounds elemental K

Calculating fertilizer application rates

The recommended amount of fertilizer to be applied to a crop at any one time has been experimentally determined for

the major nutrients. In most cases, the most essential nutrient under consideration is nitrogen. In the case of turfgrass

nutrition, the recommended amount of fertilizer per application is given in terms of pounds of nitrogen per acre or per

1000 square feet. The normal recommended rate for turf is one pound N per 1000 sq. ft. The frequency of applications

will vary with the species of turfgrass.

In order to calculate the total amount of fertilizer being applied at any one time, several things need to be considered.

These are:

Recommended rate in terms of pounds of N per 1000 square foot.

The analysis of the fertilizer being used. (The quantity of N that the fertilizer contains, which is indicated by the

first number of the analysis.) Keep in mind -- the lower the N %, the more fertilizer that will be required.

The total area being fertilized.

Must be mathematically calculated depending upon the overall shape of the plot

Once these have been determined, the following calculation will give the total amount of fertilizer needed to cover the

designated area.

(Rate of N / 1000 sq. ft) X (Area in sq. ft) / (% N in fertilizer) = Pounds of fertilizer

Remember that when working with percentage figures, convert to a decimal before calculating. Therefore, convert 33%

N to .33 for the calculation

Example 1

Using a fertilizer with analysis 33-5-5 at a rate of one pound N/1000 ft2, how much fertilizer is required to cover a turf

plot that measurers 100 ft x 50 ft.

First calculate the area of the plot,

area = L x W 100 x 50 = 5000 ft2

(Rate of N / 1000 sq. ft) X (Area in sq. ft) / (% N in fertilizer) = Pounds of fertilizer

Example 2

(1 lb /1000 sq ft) X 5000 sq. ft / .33 = 15.15 lb of 33-5-5 fertilizer

This time use a different fertilizer, 20-5-10 at the same rate on the same plot of turf

(1 lb /1000 sq ft) X 5000 sq. ft / .20 = 25 lb of 20-5-10 to cover the same area

Why the difference?

33-5-5 contains more N per pound of fertilizer, and therefore, requires less material to provide one pound of N / 1000

ft2. However, this is not the only criteria that should be used in deciding what analysis to use. The nitrogen formulation

is often a more important consideration.

Organic sources

Proper maintenance of soil organic matter is an important part of nutrient management, as increasingly supported by

the scientific community. Organic matter enhances both chemical and biological soil properties, as well as supplying

sources as macro- and micronutrients. The most stable form of organic matter—humus—plays an all-important role in

improving soil structure, nutrient retention, and water storage. Additionally, it has been shown that additions of animal

and green manures, as well as compost, enriched microbial diversity and populations.

NITROGEN

Animal manure

The amount of nitrogen that manure provides and its subsequent availability to plants is influenced by a several factors:

Nutrient analysis of the animal feed

Storage and handling procedures of the manure

Amount and type of materials added to the manure

Timing and method of application

Properties of the soil

Choice of crop

Nitrogen Analysis

Manures can contain between 0.5 and 6% total nitrogen, though typical values range from 0.5 to 1.5%.

Of the total nitrogen, approximately only 25% to 50% is in the form of ammonium and directly available to

plants./li>

The remaining 50-75% is organic nitrogen and must be mineralized before it is utilized by plants. Thus, the

same conditions for optimal mineralization of organic matter are the same for the optimal mineralization of

organic nitrogen in manure.

Organic Nitrogen

Organic nitrogen is further divided into two categories:

unstable organic nitrogen

stable organic nitrogen

Unstable organic nitrogen

urea or uric acid are the primary forms of unstable organic nitrogen

mineralization into ammonium occurs rapidly

highly vulnerable to volatilization and denitrification losses

it is recommended that manure be incorporated into the soil to prevent nitrogen losses to the atmosphere

Stable organic nitrogen

mineralizes at much slower rates than the unstable fraction

the stable nitrogen that is less resistant to decomposition (approximately 30% to 60% of the total nitrogen)

mineralizes during the first year of application

the stable nitrogen that is more resistant to decomposition mineralizes during the following years with declining

rates of mineralization each year that passes

The following table contains nutrient analysis information for various types of animal manures and composts.

Table 9. Nutrient Composition of Various Types of Animal Manure and Compost (all values are on

a fresh weight basis).

Manure Type Dry Matter Ammonium-N Total Na P2O5 K2O

  % ------------------------- lb/ton ---------------------------

Swine, no bedding 18 6 10 9 8

Swine, with bedding 18 5 6 7 7

Beef, no bedding 52 7 21 14 23

Beef, with bedding 50 8 21 18 26

Dairy, no bedding 18 4 9 4 10

Dairy, with bedding 21 5 9 4 10

Sheep, no bedding 28 5 18 11 26

Sheep, with bedding 28 5 14 9 25

Poultry, no litter 45 26 33 48 34

Poultry, with litter 75 36 56 45 34

Turkey, no litter 22 17 27 20 17

Turkey, with litter 29 13 20 16 13

Horse, with bedding 46 4 14 4 14

Poultry compost 45 1 17 39 23

Dairy compost 45 <1 12 12 26

Mixed compost: Dairy/Swine/Poultry 43 <1 11 11 10

aTotal N = Ammonium-N plus organic N

Sources: Livestock Waste Facilities Handbook, 2nd ed., 1985, Midwest Plan Service; Organic Soil Amendments and

Fertilizers, 1992, Univ. of Calif. #21505.

Legume /green manure

A particular advantage of implementing a legume/green manure rotation into the soil/cropping system is the added

source of organic matter. However, green manures also improve soil structure by reducing bulk density. Green manures

are generally grown for less than a growing season and are plowed under before producing seeds. Examples of

common green manure crops are sunnhemp, annual ryegrass, sudangrass, sudex, and sesbania. Legumes, such as

sunnhemp and sudex, are particularly beneficial since they are nitrogen fixing species and are a good source of

nitrogen.

Management of organic matter also helps to reduce the occurrence of soil erosion, thus improving soil conservation. In

addition to rotations of green manures, cover crops, companion plantings, mulching, and stripcropping with grass

species can help minimize the depletion of soil resources, as well as providing a good source of organic residue on the

soil surface.

Sewage sludge

Sewage sludge consists of the solid products formed during sewage treatment

It is not uniform in mineral composition

Generally, it contains less than 1 to 3% total nitrogen

PHOSPHORUS

Animal manure

Animal can contain 0.1 to 0.4% phosphorus.

Like nitrogen, the amount of phosphorus in animal manure depends upon several factors, including type of

animal feed, handling, and storage of manure.

Out of the total amount of phosphorus in fresh manure, approximately 30 to 70% is organic. Thus,

mineralization must occur before the organic phosphorus becomes available to plants.

Sewage sludge

Sewage sludge contains approximately 2 to 4 % total phosphorus.

Microbial Phosphorus

Certain bacteria in the soil are capable of increasing the availability of phosphate, by increasing its solubility.

The most abundant P-solubilizer is Bacillus spp.

POTASS IUM

Manures

Potassium content may range between 0.2 and 2% in manures.

Sewage sludge

Potassium primarily exists as soluble, inorganic K+.

SULFUR

Animal manure and Sewage sludge: 0.2-1.5%

CALC IUM

Animal and municipal wastes: 2-5% (dry)

MAGNESIUM

Animal and municipal wastes: 0.2-1.5%

For more information about N, P, and K organic fertilizer sources, click on the following link:

http://www.ctahr.hawaii.edu/tpss/research_extension/rxsoil/organic.htm

MICRONUTRIENTS

Animal wastes and municipal wastes

Fe: 0.02% - 0.1% (benefit increased chelation)

Zn: 0.01-0.05%, municipal (up to 0.5%) (benefit chelation)

Cu: Animal small (0.002-0.03%), municipal (0,1%) (natural chelation)

Mn: animal (0.01-0.05%) municipal (0.05%) (chelation)

B: animal (0.001-0.005%) municipal (0.01%) (chelation)

Cl: most low because Cl is highly soluble and mobile

Mo: animal (0.0001-0.0005%) municipal (0.0001%)

Distinctions between manure fertilizers and commercial fertilizers

Nutrient analysis: While commercial fertilizers may have a relatively high analysis of the major macronutrients

(nitrogen, phosphorus and potassium), the nutrient content of manures is much less.

o As a result, a larger quantity of manure must be applied to the soil as compared to the addition of

commercial fertilizer at an equivalent rate. It may take up to 30 tons of manure per acre to achieve the

desired nutrition.

The nutrient content of manure fertilizers is highly variable.

o Factors that affect nutrient content include animal type and diet, handling, storage, and water content.

o Chicken manure generally contains more nitrogen, but also quickly decomposes and subsequently

releases ammonia.

o Since manure is an organic source, the availability of nutrients is also largely influenced by the

biological processes of mineralization and immobilization.

BENEF ITS AND D ISADVANTAGES OF MANURE FERT IL IZERS

Benefits

Provides a source of ammonium

Increases the availability of certain essential elements, including phosphorus and various micronutrients

Increases the mobility of phosphorus and micronutrients in the soil

Increases soil organic matter content

Improves water holding capacity

Increases water infiltration rates

Improves soil structure

Reduces aluminum toxicity

Recycles nutrients

Disadvantages

Contains variable nutrient analysis

Requires high rates of application due to lower analysis (especially N)

Variable quality

Undergoes variable rates of mineralization, therefore difficult to predict nutrient availability

Less flexibility involved in applying specific nutrient combinations

Risk of nitrogen losses volatilization during handling and placement

High costs associated with transportation

Has relatively low nutrient content per unit weight as compared to mineral fertilizers

Potential weed problem through the transfer of weedy seeds which can be minimized through composting

Fertilizer placement

The placement of nutrients is an important issue in nutrient management because placement strongly influences the

subsequent availability of nutrients.

Improper placement can:

reduce yield potential

result in economic loss

Provided that a soil test indicates a particular nutrient deficiency, considerations of nutrient placement involve:

The type of fertilizer being applied

Tillage and crop rotation practices

Choice of crop

Access to necessary equipment

Nutrient mobility in the soil

Soil characteristics

Broadcast

PREPLANT

Prior to planting, fertilizers and/or liming materials are applied uniformly over the soil surface.

After broadcasting, the fertilizer can be incorporated into the soil through tillage

o Incorporation usually reduces losses of nitrogen due to volatilization and denitrification from the soil

surface.

Since phosphorus is an immobile nutrient, broadcasting phosphorus fertilizers is not advantageous. For

greater efficiency, phosphorus should be placed closer to the plant roots in bands.

Broadcasting provides a way to apply needed micronutrients

Surface Band

PREPLANT

Prior to planting, fertilizer is applied to the soil surface in a band.

After application, the fertilizer can be incorporated into the soil.

Under certain conditions, nitrogen availability is increased when applied in a surface band as opposed to

broadcasted.

AT PLANT ING

During planting, fertilizer is applied in a band along the top or side of rows.

This can be effective technique for applying immobile nutrients.

Subsurface Band

PREPLANT

Fertilizer is placed in bands that lie 2 to 8 inches below the soil surface.

This can be an effective option for nutrient placement in reduced tillage systems.

AT PLANT ING

During planting, fertilizer is applied below the soil surface close to the seed row.

Often, the fertilizer is placed 1 to 2 inches below (or below and to the side) of the seed row.

In cool, wet areas, a “starter application” of fertilizer is placed in a subsurface band to boost seedling growth.

Band application and seedling growth

A major advantage of band application is enhanced seedling growth. Stronger seedlings are less prone to suffer from

pests and diseases.

Nitrogen

To prevent seedling injury, high rates of nitrogen should not be placed near seeds.

Phosphorus

Banding phosphorus fertilizers near the seed row can increase phosphorus efficiency by reducing the degree

of P fixation.

Despite an increase in efficiency, phosphorus recovery is typically lower than nitrogen and potassium.

o While plants typically recover less than 20% of the applied phosphorus, 50 to 75% of applied nitrogen

and potassium is generally recoverable.

o The low rate of phosphorus recovery should not necessarily be considered a drawback, since the

build up of P fertility of your soil may be a long term benefit.

Potassium

Banded potassium below or to the side of the seed row typically enhances early seedling growth and reduces

the risk of salt damage.

Banding potassium is usually a more effective method than broadcasting, although this difference becomes

less significant as the rate of applied potassium increases.

NPK and Micronutrient Fertilizers

When applying micronutrients with an NPK fertilizer, the fertilizer should be placed a couple inches away from

seeds to avoid seedling injury.

Salt damage and the salt index

An important consideration when applying fertilizer bands is the fertilizer’s salt index, which is a measure of the potential

salt damage to the plant.

Salt damage

If highly concentrated, dissolved (soluble) salts in the soil solution can have a negative impact on plants. Soluble salts

can originate from minerals in the earth and/or heavily applied fertilizers. Soluble salts accumulate in the soil when there

are high rates of evaporation and insufficient leaching.

Problems associated with salt damage include:

If the concentration of salt in the soil is greater than the salt concentration in plant roots, water will not be

absorbed by the plant. Instead, water will leave the plant and enter the soil.

High concentrations of soluble salts may also result in elemental toxicities of sodium and chlorine.

Salt index

The fertilizer salt index was developed to classify fertilizers according their potential to cause salt injury to plants.

Sodium nitrate is the standard and has an index of 100.

Other fertilizers are assigned a salt index value relative to 100, which describes the fertilizer’s potential to

cause salt injury as compared to the damage caused by an equal amount of sodium nitrate.

A fertilizer with a salt index less than 100 has a lesser potential to cause salt damage in comparison with a

fertilizer with a salt index greater than 100.

Click on the web link below to see a table of common fertilizers and their salt indices. This site also presents a simple

method for calculating the salt index of any fertilizer using the information provided on the fertilizer bag and the salt

index of each component of the fertilizer.

http://www.spectrumanalytic.com/support/library/ff/salt_index_calculation.htm

Topdress

AFTER PLANT ING

When topdressing, fertilizers are applied over the soil and plant surface.

While topdressing of nitrogen is common in turf and pastures, this method is not recommended for phosphorus

and potassium.

Sidedressing

AFTER PLANT ING

When sidedressing, fertilizers are applied in surface or subsurface bands along the side of plant rows.

Care must be taken to avoid damage to the crop, especially the plant’s root system.

Sidedressing provides a valuable opportunity to split the recommended N into smaller applications and apply N

throughout the season.

o Splitting the total nitrogen application into smaller doses throughout the season can be favorable,

especially in coarse soils that have a high nitrate leaching potential.

Sidedressing is not effective as an effective method as preplant banding for immobile nutrients since

sidedressing does not allow time for these nutrients to become available to plants.

Foliar Applications

Foliar fertilizers contain soluble nutrients that are suspended in water.

Foliar fertilizer is directly applied to the above ground plant parts.

With the exception of certain micronutrients, it is difficult for most plants to absorb sufficient nutrients through

their leaves to meet their yield potential.

FOL IAR APPL ICAT ION VERSUS SOIL APPL ICAT ION

When nutrients are obtained from the soil, the nutrients pass first through the root system and then travel

through the xylem before reaching plant cells.

In contrast, nutrients from foliar fertilizers pass through cracks and/or stomata openings in the cuticle of the

leaf and directly enter plant cells.

Foliar fertilizers supply plant cells with nutrients more rapidly than the soil. Thus, foliar fertilizers can provide a

quick way to correct nutrient deficiencies.

o However, due to the risk of foliage burn, the rates of nutrients in foliar fertilizers are much smaller

(less than 1-2%) and several applications may be necessary.

o Foliar P fertilizers have a greater risk of causing damage than N and are applied in lower

concentrations (less than 0.4-0.5%).

o Foliar fertilizers are a common way to apply micronutrients since micronutrients are required in much

smaller quantities than macronutrients.

In high-value horticulture crops, foliar fertilizers may be used in addition to soil nutrients.

Fertigation

Fertigation is the application of fertilizers to the soil through an irrigation system, which applies both water and

nutrients to plants.

It provides an additional way to supply nitrogen, sulfur and potassium.

It allows for a high degree of flexibility in nutrient management because nutrients may be applied continually

throughout the growth of the crop.

o Fertigation makes it possible to synchronize nutrient applications with crop demand. This is an

effective strategy to prevent luxury consumption of nutrients.

o Special features in certain fertigation designs allow for the recovery and recycling of irrigation water,

which may reduce costs and negative environmental impacts.

o Fertigation may also reduce losses of nitrogen due to leaching and denitrification.

o Finally, fertigation may reduce operation costs associated with repeated applications by broadcasting,

banding and sidedressing.

Successful fertigation requires a well-managed and equipped irrigation system for uniform, maximum

efficiency.

Applications of phosphorus and anhydrous ammonia are not as common because these nutrients form

precipitants if the irrigation water contains Ca, Mg, and HCO3- and clog the irrigation system. Click on the

following web link to learn more about fertigation: http://www.ext.colostate.edu/Pubs/crops/00512.html.

Timing

NITROGEN

In warm climates, nitrification occurs readily. As a result, soil ammonium converts quickly to nitrate.

o Losses of nitrate increase due to nitrate leaching during periods of intense rainfall.

o Losses of nitrate due to denitrification occur readily in waterlogged soils.

To prevent nitrate losses, nitrogen can be applied throughout the season in smaller amounts, rather than

applying the total nitrogen at once before the season. This is known as split application.

o Split applications can be applied as a sidedressing or fertigation.

Another way to reduce nitrate losses is to apply fertilizers that contain nitrification and/or urease inhibitors or

are slow release.

o Nitrification and/or urease inhibitors slow the processes of nitrification and urea hydrolysis,

respectively.

o Slow release fertilizers contain a coat of sulfur which must break down before urea is released.

PHOSPHORUS

During a single season, the availability of phosphorus is limited by P-fixation.

To increase the efficiency of phosphorus fertilizers, it is recommended to apply phosphorus before or at

planting.

In soils with high P-fixing capacity, banding is recommended.

Broadcasting is only effective if the P-fixation is low.

POTASS IUM

Like phosphorus, potassium is a relatively immobile nutrient in the soil. As a result, it should be applied before

or at planting.

Potassium can either be broadcasted or banded.

Sidedressing of K is less effective.

Tillage systems

CONVENT IONAL T ILLAGE

The primary purpose of tillage is to loosen the soil.

Conventionally, tillage practices involve the use of equipment to break up and overturn the soil surface, while

simultaneously incorporating surface residues into the plow layer.

In addition to leaving the soil surface relatively free of residues, conventional tillage:

o aerates the soil

o decreases compaction of surface soils

o increases water infiltration in surface soils

o facilitates proper seed emergence

o eliminates and/or controls weeds

Initial plowing may be followed by secondary tillage operations to remove weeds and further loosen the soil.

The most common and oldest tillage practice is moldboard plowing, often using a disk plow.

SOIL T ILTH

Soil tilth is a term used to describe the suitability of a soil toward optimal plant growth.

Tilth refers to the workability of the soil, which describes the ability of plant roots to proliferate and for seeds to

emerge. It is highly influenced by soil structure, texture, and organic content.

A soil with good tilth holds nutrients and water, but is also well drained and aerated.

Under natural vegetative conditions, the majority of soils have rapid infiltration, low compaction, good drainage,

little soil erosion, and the desired bulk density and water holding capacity. These characteristics describe

“good” soil tilth.

However, soil tilth can diminish by long term tillage due to increased subsurface soil compaction, reduced in

soil organic content, and nutrient degradation.

HOW WORKABLE IS YOUR SOIL?

Soil tilth is intimately related to soil aggregation.

Soil aggregation often determines the workability of a soil.

The soils of Hawaii differ greatly in their degree of aggregation.

o Some Hawaii soils do not form stable aggregates. And so, as it rains, the aggregates break up and

water infiltration declines. Poorly aggregated soils are said to swell when wet and shrink when dry.

Subsurface soil compaction is a major concern, especially if these soils are tilled when wet. Over

time, plow pans can develop in areas that are compacted by heavy equipment. Soil compaction can

ultimately decrease the workability of these soils.

o Other soils in Hawaii have more stable soil aggregates. An example of a well aggregated soil is a

highly weathered soil. In this case, the soil aggregates do not break up as readily when wet. Since

these soils are less “sticky,” they are more workable. Thus, the impact of tillage on soil compaction is

less.

ORGANIC MATTER

In addition to soil structure, tillage has a large affect on soil organic matter.

Since tillage enhances soil aeration, the activity of soil organisms increases. As a result, the rate of

decomposition of organic matter speeds up, while the total soil organic matter declines.

The management of organic matter is important for soil tilth because organic matter:

o reduces soil compaction and bulk density

o increases water holding capacity and infiltration

Tillage also directly removes vegetation from the soil surface. This leaves the soil bare and exposed to rain

and wind. As a result, soil erosion increases.

CONSERVAT ION T ILLAGE

Conservation tillage is a way to reduce the negative impacts of conventional tillage.

In conservation tillage practices, farmers may choose to adopt minimal tillage or no-till practices.

o In both minimal and no-till practices, there is minimal disturbance of plant residues.

o In comparison to moldboard plowing, conservation tillage includes practices such as chisel plowing,

ridge tillage, and stubble mulching.

Reduced tillage

Includes any system that maintains 30% of surface residues.

Chisel plowing

A chisel plow disturbs less soil by “stirring” the soil surface. This technique leaves 30% of the soil surface

covered with plant residues, while incorporating the remaining 70% into the soil.

Ridge Tillage

In ridge tillage, areas between rows are left undisturbed.

Thirty percent of the surface residues along the rows remain, while the rest is incorporated into the soil. Crops

are then planting along permanent ridges.

Stubble mulching

Residues are uniformly distributed onto the field, and the soil is minimally tilled.

Much of the incorporated residue remains near soil surface.

No-till

The soil is left undisturbed. Fifty to one hundred percent of the residue from a previous rotation crop remains

on the soil surface.

Benefits

Surface residues:

o enhance aggregation

o soil organic matter

o water infiltration

o drainage

o water holding capacity

o lower soil bulk density

o soil compaction

Minimal tillage enhances the activity and the diversity of soil organisms, which helps to prevent pests and

disease problems.

Reduced tillage increases total organic matter content in surface layer.

o Initially, reduced tillage systems may lead to the immobilization of nutrients. In comparison,

conventional systems initially encourage the mineralization of incorporated residues, although

reducing overall soil organic matter content.

o However, after initiated, mineralization increases to an even greater level than conventional systems.

Since soil amendments are applied to the surface of the soil and not incorporated, they tend to build

up in the surface layer.

o While acidification from organic residues may occur, lime may correct this problem.

Manure Placement

Like fertilizers, manures may be broadcasted or placed in surface or subsurface bands.

o When broadcasting, the manure may either be applied as a solid, liquid, or slurry.

o When applying manure to subsurface soils, slurry or liquid manure may be injected into subsurface

bands.

o Manures may also be placed in surface bands before and after planting.

Deficiency symptoms

To view illustrations of typical nutrient deficiencies, click on the following link: Deficiency Symptoms of Some Common

Crops in Hawaii

NITROGEN

Plants often have stunted growth.

Leaves develop a yellow color, which is a condition known as chlorosis.

Since nitrogen is a mobile nutrient within the plant, nitrogen moves from older growth to new growth

when deficient.

As a result, nitrogen deficiencies first appear in older leaves.

Since deficiency symptoms are sometimes difficult to diagnose, the location of the symptom (new or

old growth) helps us determine which nutrient, if any, is deficient.

When nitrogen is severely deficient, chlorotic leaves may die and fall off the plant.

PHOSPHORUS

Plants often have overall stunting, particularly during the early stages of growth.

Phosphorus is a mobile nutrient; and so, symptoms first appear in older growth.

When deficient, older leaves develop a dark green to blue green color.

In certain corn and grass species, older leaves may develop a purple coloration.

Phosphorus deficiencies can cause poor fruit and seed development as well as delay crop maturity.

POTASS IUM

Plants often experience stunted growth.

Like nitrogen and phosphorus, potassium is a mobile nutrient. Older leaves may develop chlorosis along the

margin, or edge, of leaves.

Certain crops may have weaken stalks, which causes lodging (toppling over).

SULFUR

A common symptom of sulfur deficiency is uniform chlorosis of leaves.

Sulfur deficiency symptoms may resemble nitrogen deficiencies, except the symptoms first appear on

new growth of most crops since sulfur is mostly immobile.

Growth may be stunted, with spindly and thin stems.

CALC IUM

Symptoms first appear in new growth since calcium is immobile within the plant.

Areas of active growth, such as buds, new leaves, and root tips, fail to develop, and eventually turn

brown and die.

Leaf tips are often chlorotic or colorless.

Sticky substances may be excreted from the growing points causing leaf tips to stick together as new leaves

emerge. This may cause tearing of plant tissue.

Young leaves of certain crops may develop a cupped or crinkled appearance.

Buds, blossoms, and fruit may rot and fail to reach maturity.

MAGNESIUM

Deficiency symptoms first show up on older leaves since magnesium is a mobile nutrient. Commonly, plants

develop interveinal chlorosis.

Interveinal chlorosis is a condition in which the plant tissue becomes yellow while the veins remain

green.

Interveinal tissue in some crops may turn reddish, purplish, and bronze.

If severe, the entire leaf may become chlorotic and eventually die.

BORON

Deficiency symptoms first appear in new growth. Leaves may be thickened, curled, and brittle. Stems may also

become cracked.

Other symptoms include rotting and discoloration of fruits and roots.

The plant may also have stunted growth.

COPPER

Plants may have chlorosis, stunted growth, and curling of young leaves.

Leaf tips and leaf edges may begin to die back.

Leaves may develop a dark bluish-green cast.

IRON

Deficiency symptoms include interveinal chlorosis, which first appears on young growth. In severe cases,

entire leaf may turn white and die.

MANGANESE

Like iron, interveinal chlorosis may develop on young leaves, except the chlorosis appears as yellow dots.

In monocot plants, black spots also appear on the base of young leaves.

The plant may have stunted growth.

MOLYBDENUM

Older growth may become chlorotic.

o Molybdenum deficiencies may resemble nitrogen deficiency since molybdenum is involved in the

major nitrogen processes that occur in plants.

The margins of leaves may develop spots of dead leaf tissue.

Z INC

Like iron deficiency, interveinal chlorosis may form on younger leaves. In contrast to iron deficiency, distinctive

bands of chlorosis form between the midrib and the edges of leaves.

In some crops, interveinal chlorosis develops on older leaves, leading to eventual death of the leaf.

Elemental Toxicity

BORON

Leaf margins may develop a red or yellow color, which may lead to necrosis, or death of plant tissue.

Plant may experience stunted growth.

ALUMINUM

The plant may have stunted growth.

Root growth may be severely restricted.

MANGANESE

Older leaves may develop small, dark brown spots.

Leaf edges and tips also become chlorotic, which leads to the death of the leaf.

Like calcium deficiency, young leaves may become cupped and crinkled.

CHLORINE

Plant develops thickened, rolled leaves.

The plant appears to be wilting due to reduced water uptake.

Soil analysis

Soil analysis is a very valuable tool in nutrient management. Most importantly, it enables us to predict and determine

the proper amount of nutrients that should be added to a given soil based upon its fertility needs.

To read more about the value of soil analysis, click on the following link:

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/SCM-9.pdf

If correctly done, soil analysis eliminates much of the uncertainty involved in the application of nutrients, as well as

reducing wasteful applications of valuable resources.

Soil testing involves two major processes:

obtaining a representative soil sample of your soil

laboratory testing to determine the nutrient content of your soil

OBTAIN ING REPRESENTAT IVE SO IL SAMPLES

Soil sampling is the most important step in soil analysis. Since the amount of soil sampled is only very small fraction of

your field, obtaining a representative sample is critically important. If care is not taken during soil sampling, the nutrient

recommendation may not reflect the true needs of your field in its entirety.

Incorrect recommendations could cause:

the loss of

o money

o time

o nutrients

reduce yields

environmental risk

In this section, we suggest ways in which you may eliminate error so that you may obtain reliable nutrient

recommendations.

Suggested Method

A common way to reduce error is to take multiple samples and subsamples from various parts in your field.

Sampling steps

Make a detailed map of your field or garden.

Divide the map into soil sampling areas. Each area should contain a relatively uniform soil.

A uniform soil should not differ in soil type, color, slope, drainage, texture, past management, and

natural vegetation. It is important that soils with variation be sampled separately.

Include no more than 1-5 acres in each test area.

Label each test area clearly so that you may recall which areas you sampled to make later comparisons with

the nutrient recommendations.

For your convenience, you can use the same labeling system to mark the sample bags, which will

allow you to correctly identify the soil samples with the appropriate areas.

It is recommended that you use a waterproof marker when labeling the bags.

Throughout each sampling unit, it is recommended to randomly or systematically collect 10-15 soil cores.

A soil core is a sample that covers a one inch square area and extends to a specific depth.

The depth to which the core is taken varies upon the field. Usually, soil cores are taken to a depth of

4 inches in pasture, turf, or no-till land, but 8 inches in tilled fields or gardens.

If systematically taken, you may choose to develop a zigzag pattern from which to collect cores. A

zigzag pattern helps to cover the entire area.

Even within one sampling area, there may be some variation. If this is the case, only take soil samples that are

representative of the overall area and avoid soils that are uncharacteristic.

You may use a spade or shovel to collect samples. If available, a soil “probe,” is a useful tool, because it is

designed for collecting soil cores.

Tools made out of steel are favored because other metals may contaminate the samples with copper

or zinc.

When using a shovel or spade, dig a hole to the desired soil core depth. Then, remove a 1 inch slice

of soil from the side of the hole. From the 1 inch slice, remove a 1 inch portion to obtain your 1 inch

by 1 inch soil core.

Collect subsample soil cores in either plastic bucket or bag. It is important to use clean tools and materials to

avoid contamination that will result in misrepresentation.

At the end of sampling, the 10-15 cores should be thoroughly mixed together into a composite sample. From

this, a final sample of approximately 1 pint of soil may be taken and placed in an appropriately labeled, thin

plastic bag. You should not use brown paper bags because these bags contain traces of boron.

If you are sampling fields used for tree crops, it is recommended that additional core samples are taken from 8

to 24 inches of soil depth.

Further recommendations include:

soil sample are collected and sent for analyses about two to three months prior to planting

soil samples be taken each season for annual crops and every 2 to 3 years for orchard crops

ANALYZ ING SAMPLES

While personal test kits are available, samples can be sent to an agricultural testing laboratory. Since laboratories are

equipped with professional technicians and the equipment and materials that are necessary to follow the proper

procedures for analyses, you can have confidence in the results that you receive. However, it is recommended that you

send your samples to an accredited soil testing laboratory.

Sample form

Soil samples can be sent to the CTHAR’s Agricultural Diagnostic Service Center for laboratory analysis (ADSC). The

following link is the web address to the ADSC:

http://www.ctahr.hawaii.edu/adsc/

The ADSC requires that a sample form be completed and submitted along with the samples. The sample form contains

information that is helpful in assisting the ADCS make recommendations. Thus, it is important to fill out the sample form

as completely and accurately as possible.

In addition to the ADSC, the following web addresses are linked to other laboratories that perform soil analyses:

Agri-Food Canada http://www.agtest.com

Spectrum Analytic Inc. http://www.spectrumanalytic.com

North Caroline State University http://www.ncagr.com/agronomi/pwshome.htm

A&L Canada Laboratories http://www.alcanada.com/guides_planttissue.html

Types of soil analyses at the ADSC

Routine analyses

Sample Preparation

Soil pH

Soil Salinity

Extractable P

Extractable cations

Special Analyses

Organic C

Total N

Extractable Al

Extractable B

Extractable micronutrients

RECOMMENDATIONS

In approximately two weeks after submission, the ADSC will send you a nutrient recommendation.

A good recommendation includes:

Type of nutrients that your field or garden lacks, or doesn’t lack

Proper timing for nutrient additions

Suggested rates for nutrients application if the calibration information is available for your soil type and crop

choice

o When calibration data is not available, general recommendations for nutrient application rates may be

given. However, there is a chance that the general recommendation will be inaccurate. To

inaccuracies, see calibration section.

It is important to keep in mind that nutrient supply is just one of many factors that can limit plant growth. For instance, it

is possible that other soil properties, such as soil compaction, may be responsible for poor crop performance. Thus,

there are many factors that must be considered when identifying the source of limited growth and implementing the

correct management strategies to resolve these growth problems.

DIAGNOSIS

Soil conditions: Even when nutrients are sufficiently present in the soil, unfavorable soil conditions will affect nutrient

availability. Soil compaction, waterlogging, acidity, and alkalinity are among the many soil conditions that can limit

nutrient availability.

Visual symptoms: While visual symptoms of nutrient deficiencies can be helpful, they do not always provide an

accurate description of the problem.

For instance, a nutrient deficiency does not necessarily provide a visual symptom.

Nor is the presence of a symptom always sufficient. Distinguishing between a particular nutrient deficiency and

mechanical, insect, disease, and pesticide damage can be difficult.

Tissue and soil analyses: Plant and soil analyses are important tools in nutrient management and are the basis for

nutrient recommendations. Together, these analyses make the connection between the nutrient needs of plants and the

nutrient availability in the soil, and indicate the adjustments that are needed. However, tissue and soil analyses, too,

can be compromised by other soil factors that limit crop production.

To read more about how fertilizer recommendations are made, click on the following link:

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm6.pdf

CRIT ICAL LEVELS

ADSC nutrient recommendations are accompanied by a generalized statement “very low,” “low,” “sufficient,” “high,” or

“very high.”  Although these statements do not tell you how much of a particular nutrient should be added (or not added)

to your soil, it may indicate which action you may need to take. Understanding the statements is the first step in

evaluating the fertility of your soil.

The categories “low” or “high” generally indicate that a particular nutrient simply should or should not be

added.

In contrast, “very low” or “very high” nutrient levels usually suggest that the nutrient management program is in

need of change.

To learn more about what the recommendation levels mean and what action you should take, click on the following link:

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm7.pdf

Tissue analysis

Tissue sampling allows us to determine whether plants are receiving proper nutrition.

A tissue analysis is useful because it can indicate the possibility of particular nutrient deficiencies.

o Perhaps your crop requires a larger amount of a particular nutrient or a lesser amount.

Most importantly, tissue analyses can be compared with soil analyses. Such comparisons help evaluate your

present soil fertility program.

T ISSUE SAMPL ING

Tissue sampling is the most crucial step in tissue analysis. Like soil sampling, a representative tissue sample must be

taken, which characterizes the entire plant population.

Accurate soil sampling may be achieved by sampling a multitude of plants.

It is recommended that you:

identify which plant part is recommended for sampling

sample healthy plant parts, and not plants that are damaged or dead

remove any soil and dust from sampled parts

place samples in plastic bag protects it from dirt and contamination

As with soil sampling, an information form must also be submitted along with the sample, which provides the ADSC with

the information needed in making sound nutrient recommendations.

TYPES OF T ISSUE ANALYSES

Routine analyses

P

K

Ca

Mg

Fe

Cu

Zn

Mn

Mo

Al

Na

Total N

Special

Nitrate

Sulfur

Silicon

RECOMMENDED NUTRIENT LEVELS FOR VARIOUS CROPS

Click on the link below to obtain recommended nutrient levels for some common Hawaii crops. The following

information has been complied by the College of Tropical Agriculture and Human Resources.

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm4.pdf

Calibration

Once you receive a nutrient recommendation from the ADSC, how do you know how much nutrients to add to your soil?

Certainly, the terms “sufficient” or deficient” mean nothing without the following information:

Nutrient levels that are considered sufficient for the crops that you intend to grow.

The application rate of nutrients to obtain the sufficient levels in the soil to support crop growth.

We obtain this knowledge through a process called calibration. Luckily, calibration information is available for some

crops grown in a few of Hawaii soils.

Calibration data enables us to:

Acquire the recommended nutrient levels for a specific crop grown in a specific soil type

Compare calibration data with the nutrient levels of your soil as indicated by soil analyses

Apply any nutrients that are not sufficiently present as recommended

Unfortunately, calibration data is not always available for every crop grown in all of Hawaii’s soil. While a general

recommendation may be available, it is not always wise to adhere to a general recommendation. If a nutrient is applied

in too great of a quantity, it may result in the waste of resources, potential environmental harm, and possible nutrient

toxicities.

As a way to prevent possible economic and environmental damage, a calibration experiment can be performed. By

performing a calibration experiment, we answer the question, “How much of a particular nutrient is enough for sufficient

growth?

A calibration experiment allows us to:

determine the sufficiency, deficiency and critical levels for plant growth in a particular soil

interpret soil test analyses and make an appropriate nutrient/fertilizer recommendation

CAL IBRAT ION EXPER IMENT

Experience

The easiest way to perform a calibration is through simple comparison. Through your experience, you may choose to:

gather information about plant and soil nutrient levels from successful plant growth in a particular soil

compare your soil and plant nutrient levels with this information. If your nutrient levels are not sufficient, you

may consider adding additional nutrients.

Field Experiments

Instead of relying upon personal experience, you may take a more scientific approach to determine the appropriate

nutrient levels for a crop of interest. This approach involves an experiment. Therefore, it is advisable to keep detailed

accounts and maintain consistency.

Design of the Experiment

The following calibration experiment is not commonly performed by farmers alone. Instead, farmers can work with

university research personnel and perform ‘on-farm research.’ If you are interested, contact your extension agent.

Nutrient Levels

Choose at least three different nutrient levels, or treatments, that you will apply to your soil.

o You may choose to double (0,100, 200, 400 lb/acre) or triple (0, 50, 150, 450 lb/acre) each increment.

o In a graphical representation of yield versus nutrient additions, the maximum yield is the point at

which the curve begins to plateau.

Replications

In order to minimize the random effects within the experiment, it is important to have a set of at least two

replicates for each nutrient level.

o The variance between the two replicates can be determined statistically.

o Replicated treatments also allow you to determine significant differences between nutrient levels with

greater certainty.

Random Assignment

Within one set of replicates, the plots that receive a specified fertilizer addition should be randomly assigned.

Random assignments reduce the chance that certain plots are treated differently (i.e greater sunlight, more

water, more disturbance).

Collection of soil samples before and after experiment

Soils samples should be taken and analyzed before the experiment is installed.

After harvest, soil samples should be collected and analyzed again from each plot.

From these analyses, you can determine the significant effects of various nutrient applications on both yield

and soil fertility.

Collection of plant tissue

You can also compare how different levels of soil nutrients affect the nutrient levels in plant tissue.

We can use this information to determine sufficiency and deficiency levels of the plant of interest.

Collection of yield data

Yield data can be used to establish deficiency, sufficiency and toxicity ranges of nutrients. You can also

include crop quality information in your report.

Micronutrients

The micronutrients that are managed by growers and we will discuss include:

Iron

Boron

Manganese

Zinc

Molybdenum

Cobalt

There are three additional micronutrients that have been classified as essential, but are generally not managed by

growers. These additional three nutrients, listed below, are rather managed under experimental conditions:

Nickel

Chlorine

Cobalt

Forms and Functions of Micronutrients

IRON

Form: Iron is taken up by plants as either Fe2+ (ferrous cation) or Fe3+ (ferric cation).

Function: Iron is involved in photosynthesis, respiration, chlorophyll formation, and many enzymatic reactions.

BORON

Form: Boron is taken up by plants primarily as H3BO3 (boric acid) and H2BO3- (borate).

Function: Boron plays an important role in the movement and metabolism of sugars in the plant and synthesis

of plant hormones and nucleic acids. It also functions in lignin formation of cell walls.

MANGANESE

Form: The primary form of manganese uptake is Mn2+ (manganous ion).

Function: Manganese is a component of enzymes and is also involved in photosynthesis and root growth.

Additionally, it is involved in nitrogen fixation.

Z INC

Form: The Zn2+ cation is the predominate form taken up by plants.

Function: Zinc is a component of many organic complexes and DNA protein. It is also an important enzyme

for protein synthesis. Also, zinc is involved in growth hormone production and seed development.

MOLYBDENUM

Form: Molybdenum is primarily taken up as MoO42- (molybdate ion).

Function: It is involved in nitrogen fixation (conversion of N2 to NH4+) and nitrification (conversion of NH4

+ to

NO3-).

COPPER

Form: Copper is taken up as Cu2+ (cupric ion).

Function: Copper is also a component of enzymes, some of which are important to lignin formation in cell

walls. It is also involved in photosynthesis, respiration, and processes within the plant involving nitrogen.

Cycling:

IRON

The iron cycle includes both mineral and organic forms.

Mineral Iron

Iron may exist:

in the soil solution

o includes soluble iron and organic matter complexes in the form of chelates

as primary minerals and/or precipitated minerals

cation exchange site on soil particles

Fe containing minerals may dissolve to replenish the soil solution as iron is removed by plants. Little iron is retained by

the cation exchange sites of soil particles as compared to base and acid cations.

Organic Iron

Organic cycling is an important process that ensures iron availability through the processes of mineralization and

immobilization.

Iron Chelation

Iron can also form strong complexes with organic matter known as chelates (a Greek word meaning “claw”).  Chelation

occurs between soluble organic compounds and certain metals in the soil through processes involving microorganisms.

Chelates are very important in micronutrient management because chelation increases the solubility and plant uptake of

many metal micronutrients. We will encounter chelation again when discussing zinc, copper and manganese.

MANGANESE

The manganese cycle is very similar to the iron cycle. The manganese cycle, too, has four fractions:

manganese cations in soil solution

o includes soluble manganese and organic matter complexes known as chelates

exchangeable manganese on soil particles (cation exchange sites)

primary and secondary manganese-containing minerals

soil organic matter

Like iron, little manganese is retained by the cation exchange sites of soil particles. Manganese may undergo

precipitation/dissolution, sorption/desorption on the CEC, mineralization/immobilization, and chelation.

Z INC

Zinc cycling includes:

zinc cations in soil solution zinc

o includes soluble zinc and organic matter complexes known as chelates

zinc retained by soil particles on the cation exchange sites

primary and secondary zinc-containing minerals

soil organic matter

Zinc bearing minerals can dissolve and supply zinc to the soil solution. Once in the soil solution, zinc can be

immobilized, taken up by plants, retained by soil particles, or chelated with soluble organic matter. Organic matter

containing zinc must undergo mineralization before it becomes available for plant uptake.

COPPER

Like Zinc, the copper cycle includes:

Solution copper

o Includes soluble copper and organic matter complexes known as chelates

Exchangeable copper on the cation exchange sites of soil particles

Primary and secondary copper minerals

o Copper may be occluded, or buried, within the structures of various minerals, such as iron and

aluminum oxides

Organic copper

o Copper is more tightly bound to organic matter than the other micronutrients

o Copper deficiencies can occur in organic soils

Copper-containing minerals can dissolve and supply Zn to the soil solution. Like zinc, copper can be immobilized by

microorganisms, taken up by plants, or exchanged on soil particle surfaces. Copper may also form chelates with soluble

organic matter. Organic copper must be mineralized before it is available for plant uptake.

MOLYBDENUM

Unlike the previous metal micronutrients, molybdenum exists as an anion in the soil solution. Nonetheless, the

molybdenum cycle is similar to the others. The molybdenum cycle includes:

Soil solution

Exchangeable molybdenum on the anion exchange sites

Primary and secondary molybdenum minerals

Organic matter

Instead of being held onto the cation exchange capacity, molybdenum is held to soil particles with an anion exchange

capacity (including amorphous materials, iron oxides, acidic kaolin clays). Organic molybdenum undergoes

mineralization and immobilization.

BORON

Boron exists in the soil as:

soil solution boron

exchangeable boron on the anion exchange capacity sites

primary and secondary boron minerals

Boron and organic matter complexes

Boron is the only nonmetal micronutrient described in this section. H3BO3 is most common form of boron in soils that

have a pH between 5 and 9. The exchangeable boron buffers changes in the boron levels of the soil solution. Organic

matter supplies plant available boron. Boron should be carefully managed when applied to the soil since the range

between boron sufficiency and toxicity levels is very narrow.

Factors affecting micronutrient availability

IRON

Soil pH: The availability of iron may be limited in soils with high pH, especially in arid, calcareous soils.

o Excessive liming can induce iron deficiencies.

Soil Moisture and Aeration: Poorly aerated soils with excessive moisture in calcareous soil can promote iron

deficiencies.

o However, flooding of non-calcareous soils can improve iron availability.

Organic Matter: Organic matter improves iron availability due to chelation, which increases iron solubility.

Additions of manure can increase chelation.

Interactions with other nutrients: Excessive amounts of other micronutrients, particularly copper,

manganese, zinc and molybdenum, can decrease iron availability

MANGANESE

Soil pH: Soils with high pH have limited manganese availability since manganese precipitates at high pH.

o Overliming soils can cause Mn deficiencies.

Soil Moisture and Aeration: High soil moisture and poor aeration increases the availability of manganese due

to an increase in solubility.

Organic Matter: Manganese availability increases with the addition of natural organic matter (i.e. compost)

due to favorable chelation which increases the level of exchangeable and solution manganese.

Climate: Wet conditions and warm temperatures increase manganese availability.

Interactions with other nutrients: High amounts of copper, iron, and zinc may induce manganese deficiency.

Z INC

Soil pH: Zinc availability decreases as pH increases.

o Overliming decreases Zn solubility.

Zn adsorption: Though the relative amount of zinc on the cation exchange capacity is low, zinc is attracted

and held tightly to magnesite, dolomite and CaCO3 minerals. As a result, soils containing these minerals can

develop zinc deficiencies.

Organic Matter: Soluble zinc chelates increase zinc availability.

Climate: Cool, wet weather generally has a negative effect on zinc availability.

o Increasing soil temperatures increases zinc availability.

Flooding: Flooding generally decreases zinc availability.

o However, lowering the pH of flooded soils may increase zinc availability.

Interactions with other nutrients: Copper, iron, manganese, and phosphorus can interfere with zinc uptake. 

COPPER

Soil texture: Copper availability is lower in highly leached, coarse textured soils.

Soil pH: Copper availability decreases as pH increases, primarily due to decreased solubility of copper

minerals.

Organic matter: Copper forms very tight bonds with organic matter (more so than any other micronutrient),

which may reduce its availability in organic (peat and muck) soils.

Buried Cu: Copper may be occluded, or “buried,” within the structure of clay minerals and oxides. Occluded

Cu is not available to plants.

Interactions with other nutrients: Copper availability to plants may be reduced when zinc, iron, and/or

phosphorus contents are high in the soil solution.

MOLYBDENUM

Soil pH: Unlike the other micronutrients, the availability of molybdenum increases with increasing pH.

o As a result, liming acidic soils increases molybdenum availability.

Fe/Al oxides: Molybdenum is strongly held onto the surfaces of aluminum and iron oxides, which reduces its

availability.

Interactions with other nutrients: Copper and manganese can reduce the uptake of molybdenum by plants.

Phosphate enhances molybdenum uptake.

Soil moisture: Low levels of soil moisture reduce molybdenum availability.

BORON

Soil pH: Boron availability decreases as pH increases.

o Liming can temporarily induce boron deficiencies, or lessen boron toxicities.

Soil organic matter: Organic matter increases boron availability.

Soil texture: Highly leached, coarse textured soils tend to have low boron availability.

Plant factors: The range between boron sufficiency and boron toxicity is very narrow. Crop sensitivity to boron

varies, and it is important to become familiar with the boron sensitivity of your crop.

Interactions with other nutrients: Crops are less sensitive to boron when there is ample amount of calcium.

This is because calcium acts to reduce boron availability. Boron may become deficient when the Ca:B range is

greater than 1,200:1.

Soil Moisture: Dry environments reduce the availability of boron.

Base Cations

The base cations described in this section include potassium (K), calcium (Ca) and magnesium (Mg).

FORMS AND FUNCT IONS OF BASE CAT IONS

Forms of the Base Cations

K: K+

Ca: Ca2+

Mg: Mg2+

Functions of the Base Cations

Potassium

Involved in many enzymatic reactions

Functions in the synthesis of the energy compounds

Required for translocation of carbohydrates within the plant

Involved regulating gas exchange and water relations during transpiration

Calcium

An important component of the cell wall

Influences the permeability of the cell wall

Involved in cell growth

Participates in the translocation of carbohydrates and nutrients within the plant

Magnesium

A necessary component of chlorophyll, which is the site of photosynthesis

Involved in protein synthesis

Involved in the transfer of energy within the plant

CYCL ING

There are no organic forms of potassium, calcium, and magnesium. Instead, these nutrients exist only in their cationic

form (K+, Ca2+, Mg2+). Notice that potassium only has a single positive charge, while calcium and magnesium have

two positive charges. We refer to potassium as a monovalent cation, and calcium and magnesium as divalent cations.

Potassium:

There are four cycles that are associated with potassium.

First, potassium exists as a component of several soil minerals.

Secondly, potassium can be captured within the structure of expanding clay minerals (i.e. montmorillonite),

which is referred to as potassium fixation.

Third, potassium is held onto the cation exchange capacity.

Finally, the soil solution contains potassium that is readily available for plant uptake.

Mineral K

Most of the potassium found in the soil exists as a mineral, such as feldspar and mica. The transfer of mineral

potassium to other states in the potassium cycle is a very slow process. Essentially, mineral K is not available for plant

uptake during a single growing season.

Captured K

Potassium can also be trapped within the structure of expanding clay minerals. This captured K is referred to as “fixed

K.” “Fixed K” should not be confused with nitrogen “fixation.” When nitrogen fixation occurs, plant-available nitrogen

increases. In contrast, fixed K is not presently available for plant uptake. Although “fixed K” can slowly become available

with time, it is generally unavailable within one crop’s growing season. K-fixation should not always be viewed as a loss,

however, since it conserves potassium for future crop seasons.

Exchangeable K

The exchangeable fraction includes potassium that is retained by the cation exchange capacity of soil particles.

Exchangeable potassium is in equilibrium with the soil solution potassium and may rapidly replenish the soil solution as

potassium is removed.

Solution K

This fraction represents the potassium that can be directly removed from the soil solution by plants.

Fates of potassium in the soil solution

Plant uptake is just one possible fate of potassium in the soil solution. Potassium is a mobile nutrient in the soil and may

be:

lost to leaching

retained by soil particles

precipitated as secondary minerals

Factors affecting K availability:

Amount of K-bearing minerals in the soil: Soils that are inherently high in potassium minerals generally have

greater potassium availability than soils which are inherently low in potassium.

Type of clay minerals in the soil: Since highly weathered clay soils typically have a low cation exchange

capacity, exchangeable potassium may be limited in these soils.

Soil moisture: Potassium moves through the soil largely by diffusion. Diffusion occurs more rapidly at adequate

moisture levels. On the other hand, too much moisture will result in potassium leaching. Since leaching is a

source of potassium loss, it should be minimized.

Soil Temperature: Warm temperatures quicken the release of potassium from K-bearing minerals. And so,

mineral K and “fixed” K become available more quickly at higher temperatures.

Aeration: Adequate oxygen is required by plants to take up potassium.

Soil pH:

o Under acidic conditions, aluminum and manganese toxicities may cause poor root development,

which hinders potassium uptake. When acidic soils are limed, exchangeable K increases due to

increases in the cation exchange capacity.

o If there are excessive amounts of calcium and magnesium, the potassium saturation on the cation

exchange capacity is reduced by increased competition with calcium and magnesium.

Calcium:

There are three components of the calcium cycle. They are:

Calcium precipitation

Exchangeable calcium

Solution calcium

Ca-bearing minerals

Various minerals in the earth provide natural sources of calcium. Among these are the common liming agents, calcite

and dolomite.

Exchangeable Ca

Calcium is the dominant cation on the cation exchange capacity in most soils. It can readily desorb and replenish soil

solution as needed for plant uptake.

Soil solution Ca

Calcium in the soil solution is readily available for plant uptake.

Fates of calcium in the soil solution

Like potassium, plant uptake is only one of the possible fates of calcium in soil solution. Since calcium is a very mobile

nutrient in the soil, it may be:

lost to leaching

retained by soil particles

precipitated as secondary minerals

Factors determining calcium availability:

Total calcium supply: Soils that have a low cation exchange capacity are typically low in calcium.

o Soils that tend to have a low cation exchange capacity are heavily leached, highly weathered soils

and/or coarse textured soils.

Soil pH: Acidic soils tend be low in calcium due to high aluminum saturation.

Type of soil: Moderately weathered soils typically have greater amounts of available calcium as compared to

highly weathered soil.

Calcium saturation: If the cation exchange capacity contains less than 25% calcium, it is recommended that

calcium should be applied to the soil.

Magnesium

The magnesium cycle is very similar to the calcium cycle. Like calcium, magnesium can be contained by:

Magnesium bearing minerals

The cation exchange capacity

Soil solution

There are a variety of primary and secondary minerals that contain magnesium. Magnesium becomes available when

these minerals dissolve, or weather. After release, magnesium is held by the cation exchange capacity of the soil

particles or resides in the soil solution. Magnesium in the soil solution may precipitate into secondary minerals, be taken

up by plants, or leached from the soil.

Factors determining availability

Similarly to calcium, magnesium is limited in soils that are:

Inherently low in magnesium-containing minerals

Acidic

Highly leached

Limed with non-magnesium-containing material

Contain excessive amounts of other cations, such as potassium, calcium and ammonium, which compete with

magnesium and reduces its presence on the cation exchange capacity

With a Ca: Mg ratio greater than 10:1 to 15:1, magnesium will likely be deficient.

Phosphorus

In the tropics, phosphorus is often the most limiting plant nutrient.

This is primarily due to the challenges in the management of phosphorus.

In plants, the concentration of phosphorus ranges from 0.1-0.5%.

Phosphorus Forms and Functions

Forms of Phosphorus available for Plant Uptake

The orthophosphates, H2PO4- and HPO4

2-, are the primary forms of phosphorus taken up by plants.

When the soil pH is less than 7.0, H2PO4- is the predominate form in the soil.

Although less common, certain organic phosphorus forms can also be directly taken up by plants.

Functions of Phosphorus in Plants

Phosphorus is involved in many plant processes, including:

Energy transfer reactions

Development of reproductive structures

Crop maturity

Root growth

Protein synthesis

The Phosphorus Cycle

In contrast to nitrogen, the atmosphere does not provide phosphorus. Instead, orthophosphates originate largely from

primary and secondary minerals and/or from organic sources. However, the phosphorus cycle is by no means less

complex than the nitrogen cycle, and there are many factors that affect the availability of phosphorus in the soil. The

diagram below is an illustration of the phosphorus cycle.

Figure 11. A representation of the phosphorus cycle.

PHOSPHORUS UPTAKE BY PLANT ROOTS

Plant roots absorb phosphorus from the soil solution. In comparison to other macronutrients, the phosphorus

concentration in the soil solution is much lower and ranges from 0.001 mg/L to 1 mg/L (Brady and Weil, 2002). In

general, roots absorb phosphorus in the form of orthophosphate, but can also absorb certain forms of organic

phosphorus. Phosphorus moves to the root surface through diffusion. However, the presence of mycorrhizal fungi,

which develop a symbiotic relationship with plant roots and extend threadlike hyphae into the soil, can enhance the

uptake of phosphorus, as well especially in acidic soils that are low in phosphorus.

For further information on mycorrhizal fungi and its use in Hawaii, click on the link below:

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm14.pdf

PHOSPHORUS SORPT ION AND DESORPT ION

P-sorption occurs when the orthophosphates, H2PO4- and HPO4

2-, bind tightly to soil particles.

Since phosphate is an anion, particles that generate an anion exchange capacity will form strong bonds with

phosphate.

Particles with anion exchange capacity:

Aluminum and iron oxides

Highly weathered kaolin clays (under acidic conditions)

Amorphous materials.

These particles are commonly found in many of the most highly weathered soils and high weathered volcanic soils of

Hawaii. Since P-sorption results in a decrease of plant available phosphorus, P-sorption can become a major issue in

many Hawaii soils.

Additionally, in calcareous soils P-sorption may occur as phosphates sorb to impurities such as aluminum and iron

hydroxides or displace carbonates in calcium carbonate minerals.

Factors that affect P-sorption

Soil Mineral Type: Mineralogy of the soil has a great effect on P-sorption.

o Volcanic soils tend to have the greatest P-sorption of all soils since volcanic soils contain large

amounts of amorphous material.

o Following volcanic soils, highly weathered soils (such as Oxisols and Ultisols) have the next greatest

P-sorption capacities. This is due to the presence of large amounts of aluminum and iron oxides and

highly weathered kaolin clays.

o On the other end of the spectrum, less weathered soils and organic soils have low P-sorption

capacities.

Amount of clay: As the amount of clay increases in the soil, the P-sorption capacity increases as well. This is

because clay particles have a tremendous amount of surface area for which phosphate sorption can take

place.

pH: At low pH, soils have greater amounts of aluminum in the soil solution, which forms very strong bonds with

phosphate. In fact, a soil binds twice the amount of phosphorus under acidic conditions, and these bonds are

five times stronger.

Temperature: Generally, P-sorption increases as temperature increases.

Factors that decrease P-sorption:

Other anions, such as silicates, carbonates, sulfates, arsenate, and molybdate, compete with phosphate

for a position on the anion exchange site. As a result, these anions can cause the displacement, or desorption,

of phosphate from the soil exchange site. Desorption causes phosphate availability in the soil solution to

increase.

Organic matter increases P availability in four ways.

o First, organic matter forms complexes with organic phosphate which increases phosphate uptake by

plants.

o Second, organic anions can also displace sorbed phosphate.

o Third, humus coats aluminum and iron oxides, which reduces P sorption.

o Finally, organic matter is also a source of phosphorus through mineralization reactions.

Flooding the soil reduces P-sorption by increasing the solubility of phosphates that are bound to aluminum

and iron oxides and amorphous minerals.

PHOSPHATE PREC IP ITAT ION AND D ISSOLUT ION

Phosphate precipitation is a process in which phosphorus reacts with another substance to form a solid mineral.

In contrast, dissolution of phosphate minerals occurs when the mineral dissolves and releases phosphorus.

Precipitation and dissolution reactions greatly influence the availability of phosphate in the soil.

Phosphate minerals can dissolve over time to replenish the phosphate in the soil solution. This reaction

increases the availability of phosphorus.

On the other hand, phosphate minerals form by removing phosphate from soil solution. This reaction

decreases the availability of phosphorus.

However, both precipitation and dissolution are very slow processes.

Solubility of Phosphate Minerals

The solubility of phosphate minerals is very dependent upon soil pH.

The soil pH for optimum phosphorus availability is 6.5

At high or neutral pH, phosphate reacts with calcium to form minerals, such as apatite.

Under acidic conditions, phosphorus may react with aluminum and iron to form minerals, such as strengite and

varescite.

MINERAL IZAT ION AND IMMOBIL IZAT ION OF PHOSPHATE

In an average soil, approximately 50% of total phosphorus is organic. Thus, soil organic phosphorus is a very important

aspect of the P cycle.

The various sources of organic phosphorus include

Phytin

Nucleic acids

Phospholipids

Like nitrogen, organic phosphorus is converted to inorganic phosphate through the process of mineralization.

The immobilization of inorganic phosphate, in contrast, is the reverse reaction of mineralization. During immobilization,

microorganisms convert inorganic forms to organic phosphate, which are then incorporated into their living cells.

Mineralization and immobilization of phosphorus occur simultaneously in the soil. Ultimately, the C:P ratio determines

whether there is net mineralization or net immobilization.

When the C:P ratio is less than 200:1, net mineralization prevails. Net mineralization indicates that there is

enough phosphorus in the soil to sustain both plants and microorganisms.

When the C:P ration is between 200:1 and 300:1, immobilization and mineralization rates are fairly equal.

When the C:P ratio is greater than 300:1, net immobilization occurs. During immobilization there is not enough

P to sustain both plants and microorganisms; and so, microorganisms scavenge the soil for P.

Factors affecting mineralization and immobilization

The factors that affect P mineralization and immobilization are the same that affect nitrogen mineralization and

immobilization:

Temperature

Moisture

Aeration

Management of phosphorus—P-fixation

P-fixation is a term that is used to describe both P-sorption and P precipitation. Since both P-sorption and P

precipitation reduce phosphorus availability, a soil with a great P-fixation capacity has less available phosphorus after

fertilization than a soil with a low P-fixation capacity.

In other words, when the same amount of fertilizer is applied to a volcanic soil and a moderately weathered grassland

soil, the volcanic soil has less P available due to its greater P-fixation capacity.

How do we determine how much phosphorus to add?

The answer is that we must account for the P-fixation capacity of the specific soil. For some Hawaii soils, researchers

have determined the P-fixation capacity as various levels of phosphorus is added to the soil. This information allows us

to predict how much phosphorus must be added to the soil to achieve a target phosphorus level. To view the P-sorption

curves for selected soils, click on the link below:

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm9.pdf

PHOSPHORUS LEACHING AND RUNOFF

In Maui County and other tropical regions, highly weathered soils often provide little available phosphorus for plant

growth. To further compound this issue, agricultural systems can experience phosphorus losses as the result of erosion

by wind and runoff water. Erosion by wind can carry particles that contain sorbed-P to water systems, where

phosphorus may later desorb. Sediments containing phosphorus can also contaminate ground and/or surface waters.

Additionally, phosphorus availability is reduced by the removal of plant material (which can serve as a source of organic

phosphorus) during harvest.

Although phosphorus leaching is normally limited in most Hawaii soils due to their high P-fixing characteristics,

phosphorus leaching can occur if the soil reaches maximum phosphorus holding capacity, especially when P fertilizers

are overapplied. Sandy soils are most susceptible to phosphorus leaching. The consequence of phosphorus leaching is

the contamination of ground water reserves.

For further reading on the possible effects of excess phosphorus, click on the link below:

Nitrogen

Of all the essential nutrients, nitrogen is required by plants in the largest quantity and is most frequently the limiting

factor in crop productivity.

In plant tissue, the nitrogen content ranges from 1 and 6%.

Proper management of nitrogen is important because it is often the most limiting nutrient in crop production

and easily lost from the soil system.

Nitrogen Forms and Function

Forms of nitrogen available for plant uptake

Ammonium

Nitrate

Functions of nitrogen in plants

Nitrogen is an essential element of all amino acids. Amino acids are the building blocks of proteins.

Nitrogen is also a component of nucleic acids, which form the DNA of all living things and holds the genetic

code.

Nitrogen is a component of chlorophyll, which is the site of carbohydrate formation (photosynthesis).

Chlorophyll is also the substance that gives plants their green color.

o Photosynthesis occurs at high rates when there is sufficient nitrogen.

o A plant receiving sufficient nitrogen will typically exhibit vigorous plant growth. Leaves will also

develop a dark green color.

The Nitrogen Cycle

Figure 5. The nitrogen cycle

Source: http://www.physicalgeography.net

Gains of Nitrogen to the Soil

Biological and Atmospheric Fixation: Conversion of atmospheric nitrogen to ammonium which is subsequently

available for plant uptake

Direct additions of commercial and organic fertilizers

Transformations in the Soil

Mineralization: Conversion of organic nitrogen to ammonium

Nitrification: Conversion of ammonium to nitrate

Losses of Nitrogen from the Soil

Denitrification: Conversion of nitrate to atmospheric forms of nitrogen

Volatilization: Loss of gaseous ammonia to the atmosphere

Run-off

Leaching

Consumption by plants and other organisms

Nitrogen is a very dynamic element. It not only exists on Earth in many forms, but also undergoes many transformations

in and out of the soil. The sum of these transformations is known as the nitrogen cycle.

Table 5. Various forms of nitrogen

Form of Nitrogen Formula Availability for plant uptake

Nitrogen gas N2 Although 78% of our atmosphere is nitrogen gas, this form of nitrogen must

be transformed to usable forms before it is available for plant uptake.

Ammonia NH3 Ammonia is a gas. Ammonium can escape from the surface of the soil under

certain conditions and is harmful to plants in high quantities. Ammonium is

the basic building block of commercial nitrogen fertilizers.

Ammonium NH4+ Soil particles attract and retain ammonium on cation exchange complexes.

This form may be directly taken up by plants.

Nitrate NO3- Nitrate is the second form of nitrogen which is available for plant uptake. In

most soils, nitrate is highly mobile. However, in the highly weathered soils of

Hawaii, nitrate is stored in soils with ‘anion exchange capacity’ and becomes

less mobile.

Nitrite NO2- Nitrite is an intermediate product in the conversion of ammonium to nitrate

(nitrification). It is usually present in low quantities, but is toxic to plants.

Organic Nitrogen Various compounds Organic nitrogen must be converted to ammonium before it is used by plants.

This conversion occurs with time and is known as mineralization.

Though complex, the nitrogen cycle:

Helps us to understand the complex relationships that exist between the many forms of nitrogen

Provides us with insight pertaining to the availability of ammonium and nitrate, which are the only nitrogen

forms usable by plants

To understand the many ways in which N may be lost from the soil

In this section, we will discuss eight major transformations of nitrogen in the soil: nitrogen fixation, mineralization,

immobilization, nitrification, denitrification, volatilization, and leaching.

NITROGEN F IXAT ION

Although atmospheric nitrogen gas(N2) makes up approximately 78% of the air, it cannot be directly used by plants.

Instead, atmospheric N2 only becomes available to plants through three unique processes. The final product of each of

these processes is ammonium, which is then available for plant uptake.

The three processes which convert atmospheric nitrogen to ammonium

biological nitrogen fixation

chemical nitrogen fixation

atmospheric addition

Biological nitrogen fixation

Certain soil organisms have the special ability to convert atmospheric nitrogen to ammonium. These organisms include

several species of bacteria, actinomycetes, and cyanobacteria.

In the soil, nitrogen fixating organisms can form special relationships with plants, called “symbiotic” associations.

Symbiotic is a term that means “living together.” Although a symbiotic relationship can be antagonistic, the symbiosis

that occurs during biological nitrogen fixation is generally mutual and beneficial.

Legume-Rhizobium Symbiosis

The most abundant symbiotic relationship in nitrogen fixation forms between legumes (i.e. alfalfa, soybeans, etc.) and

the Rhizobia bacteria species.

As the roots of legumes grow, Rhizobium bacteria infect the root hairs where they begin to multiply.

As a response to this colonization, the legume forms nodules, which are structures that form around the

Rhizobia.

Within these nodules, Rhizobia bacteria are able to continue multiplying and converting the N2 from the soil

air to ammonium.

o However, the presence of nodules is not a sufficient indicator that nitrogen is being converted to

ammonium. Active and effective nodules are generally greater than 2 mm, have pink to red interiors,

and concentrate around the tap root.

o On the other hand, non-effective nodules are generally smaller in size with white, green or brown

interiors.

The image below shows cross sections of soybean (Glycine max) nodules. Nodules in the first row are highly effective

at converting N2 from the atmosphere into ammonium. The nodules in the second row are moderately effective at

biological nitrogen fixation, while the bottom row of nodules do not fix any nitrogen at all. Notice the lack of color in the

interior of these nodules, which indicates that they do not have an active ‘nitrogenase system’. A ‘nitrogenase system’ is

the bacterial enzyme that is necessary to convert N2 gas into ammonium through this biological process.

Figure 6. Relationship between color and effectivity of nodulation in soybean.

Source: Legumes Inoculants and Their Use, 1984. University of Hawaii NifTAL Project and FAO.

The next image shows how a well nodulated soybean root system looks when it contains many highly effective rhizobia

in the soil or is applied as an inoculant.

Figure 7. Depiction of effective nodulation of soybean by rhizobia.

Source: J. Burton. Legumes Inoculants and their Use, 1984. University of Hawaii NifTAL Project and FAO

Specificity

Some Rhizobium species are only capable of nodulating a particular legume species and cannot successfully nodulate

other legumes.

For example, the Rhizobium that nodulates alfalfa is a different species from the Rhizobium that nodulates

soybean.

This phenomenon is known as Rhizobium specificity. However, not all Rhizobium are legume-specific. Thus, some

may nodulate a number of different legumes.

The figure below gives examples of some common cross-inoculation groups that may assist you in selecting the proper

rhizobial inoculant for a particular legume host. The proper combination of rhizobia and legume will result in the optimal

nodulation and most nitrogen fixation. From this figure, we see that using soybean rhizobia with a soybean plant forms

an effective symbiosis, while using a soybean rhizobia with a leucaena plant does not. However, cowpea rhizobia is

capable of nodulating both mungbean and peanut.

Figure 8. Specificity of Rhizobia for successful nodulation of certain legumes.

Source: Singleton et al. 1994. BNF Technology for Extension Specialists. NifTAL Project. College of Tropical Agriculutre

and Human Resources.

Biological Nitrogen Fixation Management Program

The occurrence of the symbiotic relationship is heavily dependent upon a variety of soil conditions. If your program

incorporates nitrogen fixation, the following considerations can determine your success.

First and foremost, the Rhizobium must be compatible with the legume. If your crop is Rhizobium specific, you

must use the correct Rhizobium species.

If your inoculum (which contains the Rhizobium bacteria) is applied to seeds, the procedures must be properly

followed.

Nitrogen fixation takes place when total soil nitrogen is insufficient. When sufficiently present, the plant will

instead rely on the nitrogen available from the soil.

Rhizobia are sensitive to any growth factor that limits root development. Such conditions as aluminum and

manganese toxicities will limit inoculation.

Rhizobia are influenced by mineral nutrient imbalances.

o Low levels of calcium, phosphate, molybdenum under acidic conditions will limit nitrogen fixation.

o Under alkaline conditions, phosphate, cobalt, boron, iron, and copper levels become a concern.

Any growth factor (such as light, water temperature stresses or soil compaction) and any management factor

(such as nutrient management, salinity) that detrimentally affects growth of the legume will detrimentally

impact nitrogen fixation.

Table 6. A summary of biological nitrogen fixation measurements by different legumes.

Source: Singleton et al., 1993. The importance of legume-based BNF in world agriculture - an examination of the major

commercial and environmental issues, IFDC Muscle Shoals Alabama.

When does inoculation of legumes with rhizobia increase yield and biological nitrogen fixation?

This question is commonly posed by farmers who are deciding when to apply an inoculant to their crops. Since

inoculation is relatively inexpensive (less than $5.00/acre), growers should error on the side of caution and inoculate

their legume crops unless they have good evidence that inoculation is not needed.

The figure below is a conceptual model which integrates all the factors controlling biological nitrogen fixation.

Additionally, it explains when the inoculation of legumes with rhizobia will result in an increase in plant growth and

biological nitrogen fixation activity. The model is based on the fact that if the plant’s need for nitrogen is greater than the

nitrogen that is supplied by both the existing soil nitrogen and the rhizobia already present in the soil, the inoculation of

supplementary effective rhizobia will result in increased yield and biological nitrogen fixation.

Figure 9. The various Factors that control nitrogen fixation.

Source: Legume response to inoculation in the tropics: Myths and realities, 1992, p135-155. In R. Lal and P. Sanchez

(eds) Myths and Science of Soils of the Tropics. SSSA, Madison.

Amount of Nitrogen Fixed by Legume

When nitrogen is converted to ammonium during biological nitrogen fixation, ammonium becomes available to the

legume and the microorganism that fixes it. Typically, the bacteria can fix anywhere between 20 and 80% of the total

legume N.

Perennial legumes may fix 100 to 200 lb/a/yr

Annual legumes fix 50 to 100 lb/a/yr.

Small amounts of ammonium can also be released by roots of the legume into the rhizosphere, or the surrounding soil.

Availability of Nitrogen in Subsequent Cropping Systems

What about subsequent cropping systems? Can a legume rotation benefit later plantings of a nonlegume crop?

Research shows that yields of nonlegume crops can increase when following a legume rotation. It is believed that the

legume rotation increases the N content of soil, thus making it an effective nutrient management strategy.

However, when the legume is incorporated into the soil, the major benefit of the legume rotation lasts only during the

first year following the legume rotation.

Other symbiotic relationships

Legumes and Rhizobia are not the only species that can establish a mutual symbiotic relationship needed for nitrogen

fixation to oocur.

In wetland rice production, a symbiotic relationship may form between Anabaena azolla (a blue green algae)

and the Azolla fern. As a result, wetland plants can benefit by incorporations of Azolla as a green manure.

Certain tree species (i.e Causarina sp.) can form symbiotic relationships with certain species of Actinomycetes

and the Frankia bacteria. Though this relationship has lesser agricultural importance, it may gain significance

in forestry, or wood production.

Free-Living Nitrogen Fixation

“Free-living” nitrogen fixating organisms are also capable of nitrogen fixation, but are not associated with any plant

species.

Examples of these organisms are azotobacteria, azolospirillum, and clostridium. However, free-living species

do not contribute largely to agricultural production

Chemical nitrogen fixation

Since the 1950s, ammonium-based fertilizers have been manufactured using the Haber-Bosch technique. In this

catalytic process, N2 reacts with hydrogen under 1,200 degrees Celsius and 500 atm.

Since the production of chemical fertilizers requires large inputs of fossil fuel, chemical fertilizers can be relatively

expensive.

The impact of the Haber-Bosch technology on agriculture has been very dramatic. The Haber-Bosch technology

enables high-analysis ammonium fertilizers to be produced quickly. As a result, the reliance on biological N fixation and

manures as N sources has declined.

Atmospheric nitrogen additions

Nitrogen is deposited onto the earth’s surface by:

Rain

o In the form of ammonium, nitrate, nitrite

Finely divided organic N swept along the earth’s surface

Lightning

o Responsible for approximately 10-20 % of soil nitrate (114, Fertilizers)

Industrial wastes

o In Hawaii, as compared to the major industrial regions of the Mainland, industrial wastes do not

significantly contribute to atmospheric N.

Table 7. This table presents estimates of the different sources of atmospherically fixed nitrogen that was deposited onto

the earth in the latter half of the twentieth century. Biological sources account for around 20% of total nitrogen

deposition.

Source: Singleton et al., 1993. The importance of legume-based BNF in world agriculture - an examination of the major

commercial and environmental issues, IFDC Muscle Shoals Alabama.

NITROGEN MINERAL IZAT ION IN SO ILS

When absorbed by plants, ammonium and nitrate are incorporated into plant cells as organic, or living, forms of

nitrogen. When plants die, microorganisms break down, or decompose, dead plant cells. During the decomposition

about plant matter, organic nitrogen is once again converted to inorganic ammonium and released into the soil.

The process that converts organic N to ammonium is called mineralization and plays a significant role in the

management of nitrogen.

To calculate the amount of nitrogen which can potentially be mineralized from your organic fertilizer source, click on the

link below:

http://www.qpais.co.uk/nable/minrate.htm

Conditions affecting N mineralization

The amount of ammonium that is released to the soil through mineralization depends on several factors:

Quantity of Organic Nitrogen: The amount of organic nitrogen originally present in the organic matter

determines the amount of N that can ultimately be mineralized.

Temperature: The optimal range for mineralization to occur is between 77-95 degrees Fahrenheit.

Oxygen: Microorganisms need oxygen and since microorganisms mediate mineralization, sufficient oxygen

must be available in the soil.

Moisture content: Ideally, water should fill 15 – 70 % of pore space for maximum mineralization. This roughly

corresponds to field capacity.

Ratio of carbon to nitrogen (C:N): The C:N ratio is a term used to describe the relative amount of total

carbon in comparison the amount of total nitrogen present in the soil and/or organic matter.

This ratio is very important in determining the rate of mineralization that should occur for a given type

of organic matter.

Since the microorganisms living in the soil need both carbon and nitrogen, net mineralization occurs when C:N ratio is

less than 20:1. This means for every two parts of carbon, there should be 1 part nitrogen for net mineralization. If you

are applying organic amendments to your soil, it is important to become familiar with the C:N ratio to ensure N

availability.

After mineralization of N occurs, ammonium can be:

Taken up by the plants

Consumed by other organisms

Nitrified

Volatilized

IMMOBIL IZAT ION

Immobilization is process that converts inorganic nitrogen to organic nitrogen. It is the reverse reaction of

mineralization.

Immobilization occurs when decomposing organic matter contains low amounts of nitrogen. Thus, immobilization occurs

if the source of organic matter has a high C:N ratio. Microorganisms, who also need nitrogen to live, scavenge the soil

for nitrogen when plant residues contain inadequate amounts of nitrogen. As inorganic ammonium and nitrate are

incorporated into the cells of living microorganisms, the total N levels in the soil are reduced. Immobilization can

ultimately result in nitrogen deficiencies.

When nitrogen is immobilized in the soil, there may be little nitrogen available for crop growth. As a result, plants can

suffer from nitrogen deficiency and develop a yellow coloration. This is the reason why organic materials with a high

C:N ratios, such as grass clippings and grain stover, are usually composted before they are incorporated into the soil or

planting. This allows time for soil microorganisms to decompose the materials and begin to release nitrogen and other

nutrients back into the soil

Mineralization or Immobilization?

The processes of mineralization and immobilization are constantly occurring simultaneously. As organic matter

decomposes, inorganic nitrogen will be released into the soil. As both plants and microorganisms grow, they utilize the

nitrogen in the soil. Once plants and microorganisms die, they decompose and release inorganic nitrogen to the soil

through mineralization.

Even though mineralization and immobilization are both occurring, we can determine which process, mineralization or

immobilization, predominates.

When mineralization occurs at a greater rate, we say that there is net mineralization.

Likewise, when immobilization occurs to a greater extent, there is net immobilization.

What determines whether there is net mineralization or net

immobilization?

The answer is the C:N ratio of the decomposable organic matter.

The C:N ratio is a characteristic of all organic matter, which includes:

Crop residues

Soil organic matter (including humus)

Soil microorganisms (remember microorganism have both carbon and nitrogen)

Rule of thumb

When the C:N ratio of decomposing organic residues is between 20:1 and 30:1, mineralization and

immobilization occur at fairly equal rates.

Net mineralization occurs at C:N ratios less than 20:1.

Net immobilization occurs at C:N ratios greater than above 30:1.

Most well decomposed organic matter in soils have a C:N ration near 10:1

Management of organic residues

If your program involves the addition of organic residues, it is important to know its C:N ratio. This knowledge allows

you to predict whether net mineralization or net immobilization will occur. If the residue has a wide C:N ratio range, it

may be necessary to apply additional amounts of nitrogen to your soil or choose a residue with a narrower range.

NITR IF ICAT ION

In most aerobic soils under optimal soil conditions, ammonium is rapidly converted to nitrate by soil bacteria through a

process known as nitrification.

Nitrification involves two steps:

o First, ammonium is converted to nitrite

o Then, nitrite is converted to nitrate.

As you can see from the outline of steps above, the intermediate product of nitrification is nitrite. If conditions are

unfavorable to undergo the second step of nitrification, nitrite can leach into the ground water and pose as a health risk.

The process of nitrification produces hydrogen ions. When large quantities of ammonium-containing fertilizers are

applied to soil over time, this process can acidify the soil. See figure below for a simplified presentation of the

nitrification process.

Figure 10. Basic process that causes soil acidity by ammonium fertilizers.

Source: Singleton, P. Nutrient Management Concepts: pH & Nutrient Formulation, University of Hawaii Cooperative

Extension Service, Hilo Jul 25 2006.

Factors affecting nitrification

There are many factors that affect nitrification. Since nitrification is mediated by microorganisms, environmental factors

that affect biological life will also influence nitrification. In general, the optimal conditions for most plant growth are also

the optimal conditions for nitrification:

Presence of ammonium in the soil: In order for nitrification to occur, there must be a source of ammonium in

the soil. Sources include mineralized ammonium or additions of ammonium-containing synthetic fertilizers

Presence of microorganisms: Microorganisms that carry out nitrification must be present in the soil.

Soil pH: The optimal pH for nitrification is 8.5, but it may occur over a fairly wide pH range. However, acidity

(less than 5.5) has a detrimental effect on the nitrifying bacteria, thus reducing nitrification.

Soil moisture: Nitrification is optimal at the field capacity of the soil. Nitrification is reduced at moisture levels

greater and below field capacity.

Field capacity is the amount of water that remains in the soil after free drainage in a saturated soil

ceases.

Field capacity is also the optimal soil moisture for most plant growth.

Soil aeration: Nitrification requires oxygen. Any management factor that improves soil aeration, such as

adding organic matter, will help optimize nitrification.

Soil temperature: Nitrifying bacteria are sensitive to temperature. The optimal temperature range for

nitrification is between 77 and 95 degrees Fahrenheit. However, nitrification can occur between 41 and 95

degrees Fahrenheit.

Environmental Considerations

Nitrate is generally a very mobile in most soils. Excessive amounts of nitrate that are not taken up by plants is subject to

leaching. Nitrate leaching can have an adverse effect on the environment.

Nitrate, which moves through the soil profile during periods of rain, pollutes ground water reserves.

Surface runoff of nitrate is a source of eutrophication, or algal blooms, in lakes and estuaries.

To learn more about the possible negative effects associated with nitrate/nitrite and other elemental leaching

on our water systems, click on the links below:

o http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm1.pdf

o http://ga.water.usgs.gov/edu/earthgwquality.html

GASEOUS LOSSES OF N ITROGEN

Denitrification

Denitrification is the biological process in which nitrate is converted to atmospheric N2.

It is one source of N loss from the soil.

Like other N processes, denitrification is a biological process that is mediated by denitrifying bacteria.

Soil conditions that lead to denitrification:

Waterlogged soils: In waterlogged soils, the flow of air is poor. Even in aerated soils, small, localized areas in

the soil (microsites) can lack oxygen. Any microenvironment within the soil that lacks oxygen is referred to as

anaerobic. In contrast, nitrification requires oxygen and occurs under aerobic conditions.

Presence of nitrate: Nitrate must be present for denitrification to occur. Nitrification provides a source of

nitrate, as well as certain synthetic fertilizers.

The greater the amount of nitrate present, the greater the denitrification potential.

Presence of decomposable organic matter: Decomposing organic matter yields is a source of carbon. In

return, carbon is the source of energy for denitrifying microorganisms.

Oxygen: As stated earlier, denitrification occurs only when oxygen is absent. In aerated soils, denitrification

can occur but is limited to those microsites that lack xygen.

Soil pH: Denitrifying microorganisms are sensitive to low pH.

Generally, denitrification is severely reduced at pH less than 5.0.

The optimal range for denitrification is between 6.0 and 6.5.

Soil Temperature: Denitrification will occur between 35 and 77 degrees Fahrenheit.

Volatilization

A second loss of nitrogen to the atmosphere is due to volatilization. By definition, volatilization is the loss of gaseous

ammonia to the atmosphere.

Note the distinction between ammonia and ammonium. Although similar in form, ammonia is a gas that can escape

from the soil into the atmosphere. You may be familiar with ammonia, which is characterized by its pungent smell.

Factors affecting volatilization

There are several factors that affect volatilization:

Soil pH: At a soil pH of 9.3, half of the ammonium in the soil is converted to ammonia and subject to

volatilization loss. Generally, a pH greater than 7.5 allows for considerable loss of ammonia due to

volatilization.

Type of fertilizer: Urea fertilizers experience greater losses due to volatilization than ammonium fertilizers.

However, if an ammonium fertilizer forms insoluble calcium compounds in the soil, the ammonium

fertilizer will have greater volatilization losses than urea.

Method of fertilizer placement: Broadcasting the fertilizer over the surface of the soil increases the losses

due to volatilization. Incorporation into the soil reduces losses.

Soil Temperature: The occurrence of volatilization increases as soil temperatures increase to 113 degrees

Fahrenheit

Soil Moisture: Evaporation promotes volatilization. Thus, volatilization is greatest as the soil dries after

reaching field capacity.

Buffering Capacity: Volatilization is less in well-buffered soils.

Crop Residues: Crop residues that are not incorporated into the soil may increase the rate of volatilization.

Manure: If not incorporated, nitrogen from manure sources can undergo volatilization.

NITROGEN EXCHANGE AND N ITRATE LEACHING

In our discussion on cation and anion exchange, we mentioned that cations, such as ammonium, are attracted to soil

particles that have a cation exchange capacity. Since most surface soils have a cation exchange capacity, ammonium

is retained by soil particles.

Ammonium is largely immobile

Losses of ammonium due to leaching are minimal

In contrast, nitrate is not retained by cation exchange capacity.

Nitrate is highly immobile

Losses of nitrate due to leaching can potentially be high

Not only is nitrate leaching an economic loss to the farmer, it is also an environmental concern. The conditions that lead

to nitrate leaching follow:

High rainfall intensity and distribution

Highly irrigated fields

Coarsely textured soils

Nitrate Leaching in Maui soils

In Hawaii, there are soils that can have a high anion exchange capacity under acidic conditions, which reduces nitrate

leaching. This phenomenon primarily occurs in the acidic subsurface soil layers that have the greatest anion exchange

capacity. As a result, researchers have found that subsurface soil layers in Hawaii can retain nitrate and may prevent it

from leaching into the ground water.

Acidification: Management of Nitrogen Fertilizers

The fertilizer formulation can alter the pH of soil. Generally, fertilizers with high proportions of total nitrogen and are

derived from ammonium sources (such as urea, ammonium sulfate, ammonium phosphate or ammonium nitrate) can

acidify soils with repeated applications. Most fertilizers provide the “Lime Equivalent” on the bag’s label. The lime

equivalent is the amount of limestone (calcium carbonate) it takes to neutralize the acidifying effects of using one ton of

a particular fertilizer.

In contrast, other fertilizers can increase soil pH. These fertilizers are usually low in ammonium, but high in nitrate.

Additionally, these fertilizers sometimes contain calcium from calcium nitrate. The lime equivalent is also given for these

fertilizers, but it indicates the equivalent liming effect rather than the lime needed to offset acidity.

The lime equivalent is only an estimate. The actual acidifying effect of the fertilizer is influenced, to some degree, by soil

conditions that affect the transformation of the ammonium to nitrate and also by how much ammonium the plant

assimilates before this transformation occurs. If growers know the history of their fertilizer applications over time, they

can use the lime equivalent to predict when lime additions are likely needed.

See the table below for some fertilizer formulations and note the relationship between the percentage of nitrogen in the

fertilizer that is derived from ammonium and the lime equivalent needed to counter this acidifying effect.

Table 8. Fertilizer composition can change the pH of soil

Fertilizer Ammonium nitrogen Fertilizer’s Potential:

Acidity Basicity

  % lbs lime/ton fertilizer

2-7-7 90 1700  

24-9-9 50 822  

20-20-20 69 583  

20–10-20 38 393  

20–0-20 25 40  

15-5-15 28   135

--data derived from various commercial fertilizer labels

Essential Nutrients

There are 15 essential elements that plants must have in order to grow properly.

18 Essential Nutrients

Nutrient elements obtained from atmosphere through photosynthesis

o Hydrogen

o Carbon

o Oxygen

Nutrient elements obtained from the soil

o Nitrogen

o Phosphorus

o Potassium

o Sulfur

o Magnesium

o Calcium

o Iron

o Boron

o Manganese

o Zinc

o Molybdenum

o Copper

Out of the 15 essential elements that come from the soil, we deal with only the 12 that are generally managed by the

growers. These 12 elements are ‘mineral nutrients’ and are obtained from the soil. We further divide mineral nutrients

into 3 groups: primary, intermediate, and micronutrients. Our presentation will exclude cobalt, chlorine, and nickel from

our discussion on the management of essential mineral nutrients, though are included by many as essential nutrients.

The primary nutrients are nitrogen, phosphorus and potassium. You may be most familiar with these three

nutrients because they are required in larger quantities than other nutrients. These three elements form the

basis of the N-P-K label on commercial fertilizer bags. As a result, the management of these nutrients is very

important. However, the primary nutrients are no more important than the other essential elements since all

essential elements are required for plant growth. Remember that the ‘Law of the Minimum’ tells us that if

deficient, any essential nutrient can become the controlling force in crop yield.

The intermediate nutrients are sulfur, magnesium, and calcium. Together, primary and intermediate

nutrients are referred to as macronutrients. Macronutrients are expressed as a certain percentage (%) of the

total plant uptake. Although sulfur, magnesium, and calcium are called intermediate, these elements are not

necessarily needed by plants in smaller quantities. In fact, phosphorus is required in the same amount as the

intermediate nutrients, despite being a primary nutrient. Phosphorus is referred to as a primary nutrient

because of the high frequency of soils that are deficient of this nutrient, rather than the amount of phosphorus

that plants actually use for growth.

The remaining essential elements are the micronutrients and are required in very small quantities. In

comparison with macronutrients, the uptake of micronutrients is expressed in parts per million (ppm, where

10,000 ppm = 1.0%), rather than on a percentage basis. Again, this does not infer that micronutrients are of

lesser importance. If any micronutrient is deficient, the growth of the entire plant will not reach maximum yield

(Law of the Minimum).

Since the soil provides most essential nutrients, it is crucial that we understand the soil processes that determine the

availability of each essential nutrient for plant uptake.

Table 4. Forms of Essential Elements Taken up by Plants

Element Abbreviation Form absorbed

Nitrogen N NH4+ (ammonium) and NO3

- (nitrate)

Phosphorus P H2PO4- and HPO4

-2 (orthophosphate)

Potassium K K+

Sulfur S SO4-2(sulfate)

Calcium Ca Ca+2

Magnesium Mg Mg+2

Iron Fe Fe+2 (ferrous) and Fe+3 (ferric)

Zinc Zn Zn+2

Manganese Mn Mn+2

Molybdenum Mo MoO4-2 (molybdate)

Copper Cu Cu+2

Boron B H3BO3 (boric acid) and H2BO3- (borate)

In this website, we will discuss major factors that affect the availability of the essential nutrients.

In the tropics, the management of nitrogen and phosphorus can be problematic. Thus, it is appropriate that we

discuss management issues of each nutrient separately.

In a second section, we will collectively discuss the availability of potassium, calcium, and magnesium in

Hawaii soils.

Lastly, we will address issues of micronutrient management in the tropics.

We will omit a discussion on sulfur, since it is seldom deficient in Hawaii soils.

Soil acidity and liming

Soil pH is a useful indicator of the relative acidity or alkalinity of a soil. The pH scale ranges from 0 to 14, and the soil is

assigned a value from the pH scale to describe the acidity or alkalinity. Since pH 7 falls midway along the scale, pH

values that are equal to 7 are said to be neutral. However, pH values that fall below 7 are acidic, while pH values above

7 are alkaline.

By definition, the pH of a soil is the measurement of the concentration of hydrogen ions in soil water. Recall that the

hydrogen ion is an acid cation. The greater the concentration of hydrogen ions in the soil water solution, the lower the

pH. In return, the lower the pH value, the greater the acidity of the soil will be. The concentration of hydrogen ions in the

soil solution is directly proportionate to and in equilibrium with the hydrogen ions retained on the soil’s cation exchange

complex. Thus, the hydrogen ions retained by clay particles replenish, or buffer, the hydrogen ions in soil water.

Table 1. pH of some common items.

Item pH Item pH

Most acid soils 4.0 - 6.0 Lemon juice 2.2 - 2.4

Orange juice 3.4 - 4.0 Vinegar 4.0 - 4.5

Acid rain 3.0 - 5.0 Clean rain water 5.5 - 5.7

Fresh milk 6.3 - 6.6 Blood plasma 7.2 - 7.4

Mild soap solution 8.5 - 10.0    

Source: Hue, N.V. and Ikawa, I. Acid Soils in Hawaii: Problems and Management. CTAHR.

http://www.ctahr.hawaii.edu/huen/hue_soilacidity.htm

Figure 3. pH values of common substances.

Soil pH is an important soil property, because it affects the chemical, biological, and physical processes of the soil.

Thus, pH is often considered the “master variable” of soil. Its importance in nutrient management cannot be

understated. To understand the significance of pH, its effects are listed below:

Effects of soil pH:

NUTRIENT AVAILAB IL ITY

Controls the availability of the essential nutrients

Availability of nitrogen, phosphorus, sulfur, calcium, magnesium, sodium, and molybdenum is limited under

acidic conditions

Figure 4. The effect of soil pH on the availability of essential plant elements. Greater nutrient availability is indicated by

thickened lines, whereas narrow lines indicate a decrease in availability.

BIOLOGICAL ACT IV ITY AND PROCESSES

Determines the abundance of soil microorganism

Determines which plant species will grow

Low soil pH slows the biological transformation of ammonium to nitrate

PHYSICALLY

Indirectly, high pH can disrupt soil structure, or aggregation.

Origins of acidity

There are a multiple origins of soil acidity. The following is a list of causes which are common in Hawaii:

Release of hydrogen atoms under natural chemical processes in the soil

o Atmospheric carbon dioxide reacts with water to form carbonic acid

o Organic molecules react with water and cause acid dissociation

o Oxidation of ammonium nitrogen, sulfur, and iron

Accumulation of organic matter and subsequent release of fulvic and humic acid, products of decomposition

Reaction of aluminum cations with water (a process known as hydrolysis)

Natural Deposition

o Lightning deposits acidic HNO3

o Volcanic activity deposits acidic H2SO4

Human Factors

o Oxidation of applied synthetic ammonium based fertilizers

o Oxidation of nitrogen compounds in applied animal manures and/or sewage sludge

o Deposition of acid rain (HNO3 and H2SO4) caused by industrial pollution

Pools of Soil Acidity

There are three general pools, or sources, of acidity: active, exchangeable or residual.

Active acidity is the quantity of hydrogen ions that are present in the soil water solution. The active pool of

hydrogen ions is in equilibrium with the exchangeable hydrogen ions that are held on the soil’s cation

exchange complex. This pool most readily affects plant growth. Active acidity may be directly determined using

a pH meter, such as an electron probe.

The second pool, exchangeable acidity, refers to the amount of acid cations, aluminum and hydrogen,

occupied on the CEC. When the CEC of a soil is high but has a low base saturation, the soil becomes more

resistant to pH changes. As a result, it will require larger additions of lime to neutralize the acidity. The soil is

then buffered against pH change. (See base saturation discussion.)

Residual acidity comprises of all bound aluminum and hydrogen in soil minerals. Out of all pools, residual

acidity is least available.

Buffering capacity

The quantity of aluminum and hydrogen in each of the 3 pools of acidity is not permanently fixed. Instead, the relative

amounts of aluminum and hydrogen can change, as aluminum and hydrogen moves from pool to pool. Thus, the soil is

said to have a buffering capacity. Buffering capacity is the ability of the soil to resist change. In the case of acidity, it is

the ability of the soil to resist change in pH. Thus, aluminum and hydrogen of one pool will replenish the aluminum and

hydrogen of another pool as these acid cations are removed.

For example, as aluminum and hydrogen are removed from soil solution, the acid cations of the CEC replenish the soil

solution. Likewise, minerals containing aluminum and hydrogen dissolve and release these cations as they are removed

from the exchangeable pool.

OUTL INE OF BUFFER ING REACT IONS:

Exchangeable acidity will buffer changes in active acidity

Residual acidity will buffer changes in exchangeable and active acidity

Each soil has a unique buffering capacity. As a rule of thumb, finely-textured clay soils tend to have greater buffering

capacities than coarse-textured soils.

Rule of Thumb

Finely-textured clay soils tend to have greater buffering capacities than coarse-textured soils

Recall that 90% of Hawaii’s soils fall into this category. As a result, most Hawaii soils largely buffer soil acidity. This has

great implications on nutrient management since buffering capacity determines the amount of resources, such as

lime, that must be added to correct soil acidity. Soils that have high buffering capacities require larger amounts of liming

resources to raise the pH to a target value than soils with low buffering capacities.

Problems associated with acidity in Maui County

The primary problems related to soil acidity in Hawaii are aluminum and manganese toxicities. Both toxicities, if not

prevented, may cause severe damage to the crop and crop yield.

ALUMINUM TOXIC ITY

Aluminum toxicity can occur in soils that have large amounts of aluminum containing minerals. In such soils, aluminum

can dissolve into the soil solution as the soil pH drops below 5.4. In contrast, aluminum solubility decreases dramatically

as the soil pH increases above 5.4. As a result, proper management of soil pH can prevent problems associated with

aluminum toxicity.

Excessive amounts of aluminum can inhibit root development and limit crop growth.

Aluminum saturation is an expression which describes the relative abundance of aluminum in the soil.

o Like base saturation, aluminum saturation is the percentage of the CEC occupies by aluminum. Like

all cations, aluminum held by the cation exchange complex is in equilibrium with aluminum in the soil

solution.

o Although the tolerance to aluminum varies among plant species, most plants do not tolerate greater

than 15% aluminum saturation.

o However, certain crops grown in Hawaii, such as sugarcane, pineapple, corn and ti, can tolerant

relatively high levels of aluminum saturation.

Conditions that cause aluminum toxicity

Aluminum toxicity occurs readily under acidic conditions, especially when pH values are equal to or less than 5.4. In the

acidic soils of the tropics, aluminum toxicity may become a serious problem and limit crop yield. Management of soil pH

is the key factor in avoiding aluminum toxicities. Aluminum toxicity may be ameliorated by liming your fields.

MANGANESE TOXIC ITY

Manganese toxicity can become a problem in soils with manganese-containing minerals. When these minerals dissolve,

manganese ions are released into the soil solution. Although manganese is an essential plant nutrient, excessive

quantities of manganese may be detrimental to plant growth.

Manganese toxicity symptoms include yellowing of leaves (chlorosis) of older leaves, which darken into small, brown

spots. Although crop tolerance of manganese toxicity varies, most crops are sensitive to high levels of manganese. For

example, manganese toxicity will result in the “sudden crash” syndrome of watermelon, in which plants suddenly wilt

and die. Cases of “sudden crash” have been reported on O’ahu.

Conditions that cause manganese toxicity

Manganese toxicity can develop in soils that contain manganese-minerals. Moist, organic soils under acidic conditions

are especially susceptible. Like aluminum toxicity, management of pH is very important. When the soil pH drops below

5.2, manganese minerals become highly soluble and perhaps toxic. Farmers may reduce manganese toxicity by liming

and aerating fields. Aeration may be accomplished by irrigating less or draining water from fields.

Table 2. The effect of soil pH on aluminum and manganese in selected Oahu sugarcane fields.

Source: Hue, N.V., J.A. Silva, G. Uehara, R.T. Hamasaki, R. Uchida, and P. Bunn. 1998. Managing manganese toxicity

in

former sugarcane soils on Oahu. University of Hawaii at Manoa, College of Tropical Agriculture and Human

Resources, publication SCM-1. p. 7

For an excellent discussion of manganese toxicity, click on the following link. This publication includes a discussion of

the effects of manganese toxicity on yield and provides management advice, as well as images of manganese toxicity

symptoms:

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/SCM-1.pdf

Management of soil acidity

Land managers can manage soil acidity by raising the pH to a desired value through several methods:

Flooding: In lowlands systems, flooding may be an effective technique in raising the pH of the soil. However,

this effect is only good for the time for which the soil is flooded. Flooded or paddy mineral soils are ‘self-liming’.

When they are flooded and become anaerobic (lack of oxygen in the soil atmosphere) for a period of time, the

pH rises toward neutrality even when the soil pH was originally acidic. If the soil is subsequently drained and

becomes more aerobic (more oxygen in the soil atmosphere), the pH will return to an acidic state

However, care must be taken if the soil contains manganese-oxide minerals, since flooding conditions

may lead to manganese toxicity.

Crop consideration is also required. Flooding conditions reduces the oxygen within the soil, which is

needed for plant life. As a result, crops that do not tolerate high amounts of water and low oxygen

levels are not be suited for flooded conditions. Taro and rice are examples of crops that grow well in

flooded lowlands.

Additions of organic matter: Additions of organic matter is a viable option to manage problems associated

with soil acidity.

Organic matter increases the cation exchange capacity of the soil. As the base saturation increases,

the relative amount of “acid cations” decreases.

In addition, organic matter forms strong bonds, known as “chelates,” with aluminum. Chelation

reduces the solubility of aluminum and soil acidity. Again, if your soil is prone to manganese toxicity, it

is not suggested that you add organic matter.

Additions of wood ash: Like organic mater, wood ash increases base saturation and forms chelates with

aluminum.

Conventional Liming: Various liming materials may be added to the soil that neutralize, or counteract, soil

acidity. Liming materials are bases that react with hydrogen ions in the soil solution to form water?

Examples of common liming materials are limestone (calcium carbonate), dolomite

(calcium/magnesium carbonate), hydrated lime (calcium hydroxide), and quicklime (calcium oxide).

Calcium and magnesium silicates are also used as liming agents.

Liming

If you make the decision to apply lime to your soil, how much lime should you apply? Like other soil properties, the lime

requirement will various depending upon the soil.

There are four guidelines that help us determined the lime requirement: the desired change in pH, buffering capacity

of the specific soil, type of liming material, and the fineness or texture of the liming material.

Should you lime your soil? The optimal pH range for most plant is between 6.0-6.5. To avoid aluminum and

manganese toxicity problems, a soil should be limed if the pH is less than 5.4.

What is you soil type? Recall the rule of thumb governing buffering capacity. Since finer textured soils have

greater buffering capacity than coarser textured soils, more lime must be added to the finer textured soil to

achieve the same effect and reach your target pH.

What type of liming material should you use? Due to differences in chemical composition and purity, liming

materials have varying neutralizing strengths. See Table 5 below. As a result, you must know the neutralizing

strength of your liming material before you can determine how much to add to the soil to reach a target pH.

What is the texture of your liming material? In general, the finer the liming material, the greater the

neutralizing activity. However, application of extremely fine lime may be difficult, especially under windy

conditions.

Calcium carbonate equivalent of liming materials

The calcium carbonate equivalent (CCE) is an important measurement in determining how much lime should be applied

to your soil.

By definition, CCE is the capacity of the liming material to neutralize acidity.

CCE is expressed as a percentage of calcium carbonate, which serves as the standard (100%).

Table 3. Various liming materials and their relative neutralizing strengths

Source: Uchida, R.S., and N.V. Hue. Soil acidity and liming. p.101-111. In: Silva, J.A. and Uchida, R.S. (eds.) Plant

Nutrient Management in Hawaii’s Soils: Approaches for Tropical and Subtropical Agriculture. College of Tropical

Agriculture and Human Resources, University of Hawaii at Manoa, Honolulu.

Neutralizing Exchangeable Aluminum

A direct benefit of liming your soil is to reduce aluminum saturation. Most plants suffer when aluminum saturation is

greater than 15%. You can first determine the aluminum saturation through soil testing, and then determine how much

lime is required to neutralize the exchangeable Al. This is particularly important in the tropics, where aluminum

saturation can become high.

Liming curves

Easy to read and interpret, liming curves are very useful tools in determining the calcium carbonate equivalent that

should be added to a particular soil to reach a target pH. Liming curves can be experimentally determined and

calibrated for specific soils. CTAHR has published liming curves for selected soils of Hawaii and are available on the

web.

To view liming curves for selected soils of Hawaii, click on the following two links below:

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/AS-1.pdf

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm10.pdf

Application and Overliming

After uniform application to the soil surface, lime should be thoroughly tilled into the soil for optimal neutralization of soil

acidity. Since the liming material can only react at the depth to which it is incorporated, subsurface tillage may be

necessary to ameliorate subsoil acidity. Liming soils that contain with perennial crops can also be accomplished with

surface application with little or no tillage. Irrigation and rainfall slowly leaches the lime, which is relatively insoluble,

from the surface into the soil profile where it can react to neutralize soil acidity. Using the proper fertilizer formulation is

an important way to manage soil acidification, particularly in perennial crop systems where liming is less easily

incorporated than in annual or short-term systems.

Despite the many benefits of lime, over-liming your soil has an adverse effect. It is important to carefully considered

how much to lime to add. If the pH of the soil is adjusted too high, it can induce nutrient deficiencies (such as

phosphorus and micronutrient deficiencies), as well as permit molybdenum toxicity.

Gypsum

Gypsum is a soil conditioner that may be used to correct aluminum problems in the subsurface soil layers. However, it

is important to make the distinction between lime and gypsum. Although both are sources of calcium, lime raises the pH

of the soil, while gypsum does not.

Toolbox

To facilitate your liming and fertilization calculation, we provide easy-to-use calculators. These calculators enable

immediate determination of the quantity of fertilizers or lime needed based upon your specific land dimensions and

target nutrient levels. Additionally, since these calculators were created in an excel format, you can conveniently save

your calculations in record archives.

Liming

Blend Fertilization

Fertigation

Soil-Nutrient Relationships

Cation exchange

The ‘soil cations’ essential for plant growth include ammonium, calcium, magnesium, and potassium. There are three

additional ‘soil cations,’ which are not essential plant elements but affect soil pH. The additional ‘soil cations’ include

sodium, aluminum and hydrogen.

Soil cations that are essential to plant growth

Ammonium

Calcium

Magnesium

Potassium

Soil cations that affect soil pH

Sodium

Aluminum

Hydrogen

The major distinguishing characteristic of cations is their positive charge. Just like a magnet, a positive charge is

strongly attracted to a negative charge. When soil particles have a negative charge, the particles attract and retain

cations. These soils are said the have a cation exchange capacity. Although most soils are negatively charged and

attract cations, some Hawaii soils are exceptions as we will see.

The ‘soil cations’ are further divided into two categories. Ammonium, calcium, magnesium, potassium, and sodium are

known as the ‘base cations,’ while aluminum and hydrogen are known ‘acid cations.’

Base Cations

Ammonium

Calcium

Magnesium

Potassium

Sodium*

* Unlike the other base cations, sodium is not an essential element for all plants. Soils that contain high levels of sodium

can develop salinity and sodicity problems.

Acid Cations

Aluminum

Hydrogen

The words ‘base’ and ‘acid’ refer to the particular cation’s influence on soil pH. As you might suspect, a soil with a lot of

acid cations held by soil particles will have a low pH. In contrast, a highly alkaline soil predominately consists of base

cations.

Cations in the soil compete with one another for a spot on the cation exchange capacity. However, some cations are

attracted and held more strongly than other cations. In decreasing holding strength, the order with which cations are

held by the soil particles follows: aluminum, hydrogen, calcium, potassium and nitrate, and sodium.

Figure 2. CEC values of various soil type, media, and minerals. Soils which have high amounts of organic matter and

moderately weathered clays tend to have high CECs. As soils become highly weathered, the CEC of the soil

decreases. Sandy soils, too, generally have lower CEC values. This is due to the lesser surface of sandy particles in

comparison with clay minerals, which decreases the ability of sand particles to hold and retain nutrients.

Source: Brady and Weil. 2002. Elements of the Nature and Properties of Soil. Prentice Hall, New Jersey.

Anion exchange

In the tropics, many highly weathered soils can have an anion exchange capacity. This means that the soil will attract

and retain anions, rather than cations. In contrast to cations, anions are negatively charged. The anions held and

retained by soil particles include phosphate, sulfate, nitrate and chlorine (in order of decreasing strength). In

comparison to soils with cation exchange capacity, soils with an anion capacity have net positive charge. Soils that

have an anion exchange capacity typically contain weathered kaolin minerals, iron and aluminum oxides, and

amorphous materials. Anion exchange capacity is dependent upon the pH of the soil and increases as the pH of the soil

decreases.

Base Saturation

Base saturation is a measurement that indicates the relative amounts of base cations in the soil. By definition, it is the

percentage of calcium, magnesium, potassium and sodium cations that make up the total cation exchange capacity. For

example, a base saturation of 25 % means that 25 % of the cation exchange capacity is occupied by the base cations.

If the soil does not exhibit an anion exchange capacity, the remainder 75 % of the CEC will be occupied by acid cations,

such as hydrogen and aluminum. Generally, the base saturation is relatively high in moderately weathered soils that

formed from basic igneous rocks, such as the basalts of Hawaii. The pH of soil increases as base saturation increases.

In contrast, highly weathered and/or acidic soils tend to have low base saturation.

Movement of nutrient from soil to root

There are three basic methods in which nutrients make contact with the root surface for plant uptake. They are root

interception, mass flow, and diffusion.

Root interception: Root interception occurs when a nutrient comes into physical contact with the root surface.

As a general rule, the occurrence of root interception increases as the root surface area and mass increases,

thus enabling the plant to explore a greater amount of soil. Root interception may be enhanced by mycorrhizal

fungi, which colonize roots and increases root exploration into the soil. Root interception is responsible for an

appreciable amount of calcium uptake, and some amounts of magnesium, zinc and manganese.

Mass flow: Mass flow occurs when nutrients are transported to the surface of roots by the movement of water

in the soil (i.e. percolation, transpiration, or evaporation). The rate of water flow governs the amount of

nutrients that are transported to the root surface. Therefore, mass flow decreases are soil water decreases.

Most of the nitrogen, calcium, magnesium, sulfur, copper, boron, manganese and molybdenum move to

the root by mass flow.

Diffusion: Diffusion is the movement of a particular nutrient along a concentration gradient. When there is a

difference in concentration of a particular nutrient within the soil solution, the nutrient will move from an area of

higher concentration to an area of lower concentration. You may have observed the phenomenon of diffusion

when adding sugar to water. As the sugar dissolves, it moves through parts of the water with lower sugar

concentration until it is evenly distributed, or uniformly concentrated. Diffusion delivers appreciable amounts of

phosphorus, potassium, zinc, and iron to the root surface. Diffusion is a relatively slow process compared to

the mass flow of nutrients with water movement toward the root.

Nutrient Uptake into the root and plant cells

Before both water and nutrients are incorporated into plants, both must first be absorbed by plant roots.

UPTAKE OF WATER AND NUTRIENTS BY ROOTS

Root hairs, along with the rest of the root surface, are the major sites of water and nutrient uptake.

Water moves into the root through osmosis and capillary action.

Soil water contains dissolved particles, such as plant nutrients. These dissolved particles within soil water are

referred to as solute. Osmosis is the movement of soil water from areas of low solute concentration to areas

of high solute concentration. Osmosis is essentially the diffusion of soil water.

Capillary action results from water’s adhesive (attraction to solid surfaces) and cohesion (attraction to other

water molecules). Capillary action enables water to move upwards, against the force of gravity, into the plant

water from the surrounding soil.

Nutrient ions move into the plant root by diffusion and cation exchange.

Diffusion is the movement of ions along a high to low concentration gradient.

Cation ion exchange occurs when nutrient cations are attracted to charged surface of cells within the root,

called cortex cells. When cation exchange occurs, the plant root releases a hydrogen ion. Thus, cation

exchange in the root causes the pH of the immediately surrounding soil to decrease.

Once water and nutrient ions enter the plant root, they move though spaces that exist within the root tissue

between neighboring cells.

Water and nutrients are then transported into the xylem, which conducts water and nutrients to all parts of the

plant.

Once water and nutrients enter the xylem, both can be transported to other parts in the plant where the water and

nutrients are needed. The basic outline of how nutrient ions are absorbed by plant cells follows.

ABSORPT ION OF NUTRIENTS INTO PLANT CELLS

Plant cells contain barriers (plasma membrane and tonoplast) that selectively regulate the movement of water

and nutrients into and out of the cell. These cell barriers are:

permeable to oxygen, carbon dioxide, as well as certain compounds.

semi-permeable to water.

selectively permeable to inorganic ions and organic compounds, such as amino acids and sugars.

Nutrient ions may move across these barriers actively or passively

Passive transport is the diffusion of an ion along a concentration gradient. When the interior of the cell has a

lower concentration of a specific nutrient than the outside of the cell, the nutrient can diffuse into the cell. This

type of transport requires no energy.

Active transport is the movement of a nutrient ion into the cell that occurs against a concentration gradient.

Unlike passive transport, this type of movement requires energy.

Nutrient Mobility

WITHIN PLANT

An important characteristic of some nutrients is the ability to move within the plant tissue. In general, when certain

nutrients are deficient in the plant tissue, that nutrient is able translocate from older leaves to younger leaves where that

nutrient is needed for growth. Nutrients with this ability are said to be mobile nutrients, and include nitrogen,

phosphorus, potassium, magnesium, and molybdenum. In contrast, immobile nutrients do not have the ability to

translocate from old to new growth. Immobile nutrients include calcium, sulfur, boron, copper, iron, manganese, and

zinc.

Nutrient mobility, or immobility, provides us with special clues when diagnosing deficiency symptoms. If the deficiency

symptom appears first in the old growth, we know that the deficient nutrient is mobile. On the other hand, if the

symptom appears in new growth, the deficient nutrient is immobile.

WITHIN THE SOIL

Mobility of a nutrient within the soil is closely related to the chemical properties of the soil, such as CEC and AEC, as

well as the soil conditions, such as moisture. When there is sufficient moisture in the soil for leaching to occur, the

percolating water can carry dissolved nutrients which will be subsequently lost from the soil profile. The nutrients which

are easily leached are usually those nutrients that are less strongly held by soil particles. For instance, in a soil with a

high CEC and low AEC, nitrate (an anion) will leach much more readily than calcium (a cation). Additionally, in such a

soil, potassium (a monovalent cation) will leach more readily than calcium (divalent cation) since calcium is more

strongly held to the soil particles than potassium.

Silica from minerals also dissolves and leaches from the soil profile during the processes of weathering. It is this

dissolution and leaching that transforms primary minerals to the more weathered, secondary minerals that make up the

finely-textured soils of Maui.

Goals of Modern Nutrient Management

The objective of this web resource is to assist you in making decisions, as well as taking action, in accordance with the

goals of modern nutrient management. Furthermore, we aim to help you identify, prevent, minimize, and solve the most

crucial nutrient management problems on Maui that may lead to economic and environmental loss.

Goals of modern nutrient management

Apply nutrients in a way that optimizes profitability

Minimizes the risk of environmental harm

Researchers in the College of Tropical Agriculture and Human Resources (CTAHR) have stated that the goal of a

sound nutrient management plan is “to ensure the availability of adequate nutrients for crop production with minimal

nutrient loss in runoff or leaching from the root zone” (Silva and Uchida, 2000).

Nutrient Management plan recommended by CTAHR

According to this plan, the farmer would take action in four ways:

Assess environmental concerns of your field/property

Evaluate the current status of plant-available nutrients

o One such method is soil testing

Calculate appropriate rates of applied nutrients after considering available soil nutrients and practical yields

Implement the correct method of nutrient applicatio

Law of the Minimum

The Law of the Minimum is an important concept in any nutrient management strategy. It states that crop yield will be

limited by any essential component of plant growth which is insufficiently present. Since plant nutrients are essential to

plant growth, any deficiencies in any one nutrient can limit crop yield. For example, even if nitrogen is sufficiently

supplied, a phosphorous deficiency will cause yields to be much less than if both were adequately present.

Critical Nutrient Levels

There are 12 essential elements which plants obtain from the soil that are commonly managed by growers. In addition,

plants require carbon, hydrogen, and oxygen to grow.

What makes an element essential to plant growth?

An element is essential if the plant cannot complete its life cycle without the element.

It is essential if the element is directly or indirectly involved in the metabolic processes of the plant (i.e.

photosynthesis or respiration).

A deficiency in an essential nutrient will result in the development of a characteristic, visual symptom.

Table 1. The essential elements for plant growth

Element

Abbreviation Source

Carbon C Air

Hydrogen H Water

Oxygen O Air/water

Nitrogen N Air/Soil

Phosphorus P Soil

Potassium K Soil

Sulfur S Soil

Calcium Ca Soil

Magnesium Mg Soil

Iron Fe Soil

Zinc Zn Soil

Manganese Mn Soil

Molybdenum Mo Soil

Boron B Soil

Copper Cu Soil

Figure 1. Periodic Table

Source: http://can-do.com/uci/lessons98/index.html

The relative nutrient status of a plant may be easily described using the following terms:

Deficiency range

Nutrient deficiencies occur in plants when an essential element is not taken up by the plant in sufficient amounts. As a

result, yield will be limited by the element which is deficient. While slight to moderate deficiencies do not always result in

visual deficiency symptoms, distinct visual symptoms appear in severe cases.

Critical range

Below the critical range of nutrients, an addition of the essential element will trigger an increase in yield. Above the

critical range, the levels of essential nutrients are considered sufficient.

Sufficiency range

Within this range, additions of the essential nutrient will not result in any increase in yield. However, uptake of the

nutrient may continue. Thus, the concentration of that essential nutrient in plant tissue will also increase. We refer to the

uptake of an essential nutrient within the sufficiency range as luxury consumption.

Toxicity range

Toxicities occur when an essential (or nonessential) element is taken up in great enough quantities to actually reduce

plant growth. As a result, toxicities can severely limit yield.