Mineral solidsair-filled pores organic solidswater-filled pores.

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G eneral C om position ofS oil mineral solids air-filled pores organic solids water-filled pores

Transcript of Mineral solidsair-filled pores organic solidswater-filled pores.

General Composition of Soil

mineral solids air-filled pores

organic solids water-filled pores

Figure indicates ½ total volume is pore space,or 0.50 porosity. This varies from soil to soil.

Figure shows ½ of pore space filled with water.This depends on recent rain, internal drainageand evapotranspiration.

This is a mineral soil. There are organic soils.For a mineral soil, the indicated amount oforganic matter is higher than typical.

Problem

Assume the average density of solid particles = 2.60 g / cm3 and porosity = 0.50.What is the density of this soil when it is dry (zero water content)?

bulk density = 1.30 g / cm3

High bulk density is bad for plants because it impedes root development.

A dense zone therefore restricts the volume of soil occupied by roots, increasingsusceptibility to drought.

You can determine bulk density by taking a core sample (known volume), drying in oven to evaporate water, then weighing.

Often people assume a value of 2.65 g / cm3 for the density of solid particles, determine bulk density, then calculate the porosity from,

porosity = 1 – (BD / PD)

The water content of the core sample can determined by weighing fresh from thefield besides when oven-dried.

gravimetric water content = mass of water / mass of dry soil

volumetric water content = volume of water / volume of soil (core)

The mineral particles range in size. Excluding particles gravel-size and larger, thefine-earth particles include sand, silt and clay sizes.

Sand 2.000 mm – 0.050 mmSilt 0.050 mm – 0.002 mmClay < 0.002 mm

Texture is the proportion of sand, silt and clay. Crudely speaking, there are sandysoils, loamy soils and clay soils. However, since texture strongly affects manyother properties, people split textural classification further into 12 textural classes.

The main effect of texture on other soil properties is through particle surface area.Decreasing size of particles (sand → clay), greatly increases surface area. Greatersurface area means greater chemical and physical reactivity, like retention ofnutrients and capacity to hold water.

What’s thetextural class of a soil that’s

50 % sand10 % silt

Adsorption and Exchange of Ions

The clay-size particles typically have formed in the soil from prior minerals bychemical weathering. Besides just large surface area, the clay minerals havethe specific capacity to adsorb positively (mostly) and negatively charged nutrientand other ions because they carry electrostatic charge.

The adsorbed ions are in chemical equilibrium with ions in the soil water (solution),meaning that there is more or less free exchange.

The humus fraction of soil organic matter behaves similarly.

The cations shown are typically themost common ones in soil.

Of these, Ca2+, Mg2+ and K+ areimportant nutrients and are calledbase cations to distinguish them fromacid cations (e.g., H+). Al3+ is alsoan acid cation because when it reactswith water, H+ is produced.

Within a volume of soil, the amount of cations, bases or acids, adsorbed onto clayminerals and humus, is many times greater than the amount these cations in thesoil solution.

Adsorbed nutrients are a reservoir –when nutrients in the soil solution are depletedby plant uptake, they are replenished by release from clay minerals and humus.

An important concept pertaining to adsorbed cations is cation exchange capacity,CEC. This is the amount of adsorbed cation charge per mass of soil (cmol(+) / kg). It includes charge due to both acid and base cations.

Soil Reaction, Acidity, Alkalinity and pH Adjustment

Intuitively, if a soil is dominated by acid cations, H+ and Al3+ (and others), it is acid, has relatively little nutrient base cations, Ca2+, Mg2+, K+ (and others) and is chemically infertile.

Percentage base saturation, %BS = (charge due to bases / CEC) x 100

The level of acidity, as measured by soil solution pH, is related to the %BS inwhat is called a buffer curve.

Shape varies from soilto soil but pH alwaysincreases as %BSincreases.

To refresh memory:

pH = -log [H+]

So, low pH meanslarge [H+] and highacidity.

The below data are for an acid sandy loam in North Louisiana:

        Base Cations                                           Acid Cations ----------mmol (+) / 100 g or cmol (+) / kg ----------

    Ca     Mg     K     Na                                        Al        H     2.0     0.6   0.3    0.1                                      6.4      0.6

CEC = ? and % BS = ?

Soil A Soil B cmol (+) / kg

Basic cations 90 5Acidic cations 10 5

Which soil has the lower pH?Which soil is more fertile?

USDA NRCS categories

pH

< 4.5 extremely acid4.5 – 5.0 very strongly acid5.1 – 5.5 strongly acid5.6 – 6.0 medium acid6.1 – 6.5 slightly acid6.6 – 7.3 neutral7.4 – 7.8 mildly alkaline7.9 – 8.4 moderately alkaline8.5 – 9.0 strongly alkaline > 9.0 very strongly alkaline

Most plant do best in pH range5.5 – 7.0, though some do betterunder more acidic or basic conditions.

Especially high or low pH bad for plant directly and indirectly through effects on thesolubility and plant-availability of other nutrients besides Ca2+, Mg2+ and K+, like P, B, Mo, Fe etc. Also, high or low pH adversely affects beneficial soil microorganismsinvolved in nutrient cycling (release of nutrients bound in organic matter) and others that carry out N-fixation.

Therefore, an important soil management practice is pH adjustment.

Just how much base or acid that needs to be applied depends on initial and target pHs, shape of the buffer curve and the CEC.

In practice, different amounts of base or acid are added to a set of soil samplesand the pH measured to develop a response curve.

Different lime materials may be used to neutralize soil acidity and raise pH.

CaCO3 calcitic limestoneCaMg(CO3)2 dolomitic limestoneCaO burned lime (quicklime) These latter two are more solubleCa(OH)2 hydrated lime so react faster.

To lower pH, sulfur (S) is typically added. Although it is not itself an acid, it isoxidized in the soil (primarily by certain microorganisms) to produce sulfuric acid, H2SO4.

Fertilizer Recommendations

The nutrients most commonly applied are N, P and K because these are the most commonly deficient with respect to crop needs.

Some states have recommendations for N based on soil tests for N. Louisianadoes not. The problem with N is that the overwhelming amount of soil N is bound in organic matter. Release of this N depends on microbial activity, which is highly variable.

In Louisiana and some other states, N recommendations are based on long-termdata for crop response to different rates of N fertilization.

Some states have found that the concentration of soil nitrate-N (NO3-) can be

used to make recommendations.

Soil Test P (ppm)

Based on the relationshipbetween yield and soil test,you need to add sufficientfertilizer P to raise the soiltest from 3 to 18 if you wantto increase expected yieldfrom 50% to 90% of max.

The recommendation isin terms of pounds P / acre.

When soil test concentrations of a nutrient are used, crop growth/yield are plottedwith respect to soil test value and modeled.

Assuming you wanted to apply 120 lbs of N per acre and used a 12-12-12 fertilizer that cost $ 400 per ton (2000 lbs), how much would it cost to fertilize a 40 acre tract? You need  40 ac x (120 # N ac-1) / (0.12 x 2000 # N ton fertilizer

-1) = 20 ton fertilizer

 costing  20 ton x $ 400 ton-1 = $8000

Things you need to know about fertilizer grade, e.g., 12-12-12. In order, thenumbers refer to % N, % P2O5 and % K2O in it. However, there is no P2O5 nor % K2O in it! It does, of course, contain P and K but not in these forms.

This representation is highly misleading because these oxides weigh much more than the elements they represent. You have to use conversion factors,0.44 and 0.83 for P and K, respectively.

Continuing with the above scenario, how much P and K are applied along with the 120 lbs of N per acre? Conversion factors for P2O5 and K2O are 0.44 and 0.83, respectively. You apply the 12-12-12 fertilizer at a rate of 0.50 ton per acre, so you apply, # P ac-1 = 1/2 ton fertilizer x (0.12 x 2000 # P2O5 ton fertilizer

-1) x 0.44 = 53 # P ac-1

 # K ac-1 = 1/2 ton fertilizer x (0.12 x 2000 # K2O ton fertilizer

-1) x 0.83 = 100 # K ac-1

Some Points on N

Taken up as nitrate (NO3-) or ammonium (NH4

+). Little of either in soil atany one time.

Concentrations naturally depend on complex, interrelated microbial processes.

Deficiency seen as chlorosis.Growth is stunted.

N is mobile within the plant,therefore, when deficiency existsN is translocated from older tissueto younger tissue so chlorosis seenon older tissue.

Ammonia or ammonium is oxidized to nitrate fairly quickly by a microbialprocess called nitrification. This negatively charged ion is not retained byadsorption on soil particle surfaces (since they are negatively charged), soit will be leached if not taken up.

Can use a chemical inhibitor to stop nitrification.

Nitrification produces H+, so ammonia / ammonium fertilizers are acid-forming.

Besides likely to leach, if the soil is anaerobic, the nitrate is converted to nitrous oxide or nitrogen gas and lost from the soil in these forms. This isalso a microbial process (called denitrication).

Nitrate fertilizers are used with rice (True / False).

A very important process involving N is biological N fixation, in whichatmospheric N is converted into organic N by certain soil microbes, particularlytypes living symbiotically with certain plants (including legumes).

The plants are called N-fixing plants but it is the microbes that do it.

A

B

If the soil does not have the right bacteria, you needto use seed innoculated with them.

Some Points on P

Soils are naturally very low in P, the solubility of P in the soil is low and there islittle atmospheric deposition of P. Therefore, P is limiting to plant growth.

Taken up as the phosphate ions, H2PO4- and HPO4

2-.

Deficiency symptom is purplish colorGrowth is stunted.

Reasons why phosphate is not soluble:

Reacts with Al and Fe in acid soils to form insoluble mineralsBonds to the surface of soil mineral particlesReacts with Ca in alkaline soils to form insoluble minerals

Solubility of P is greatest in the pH range of 5.5 – 7.0.

Without fertilizer input, natural systems are very efficientin cycling P –mineralize organic P to phosphate.

Some Points on K

Take up as K+.

Very abundant in soils but plants take up a lot of it and although abundant, most K is in the structure of soil minerals (unavailable).

K+ adsorbed on negatively charged particlesis the main reservoir but it is depleted.

Deficiency symptom is chlorosis andnecrosis about leaf margins.

Plants have a tendency to take up moreK than they need for normal growth anddevelopment. Called luxury consumption.Accelerates depletion.

K is known for luxury consumption but it happens with most nutrients.

Some Points on S

Taken up as sulfate, SO42-.

Deficiency seen as chlorosis but,different from N, sulfur is relativelyimmobile so chlorosis is seen onyounger tissue. Growth is stunted.

Reasonably abundant in soils butcan be depleted. There is atmosphericdeposition of S as an input. Some addedas an impurity in P fertilizers.

Like N and P, S exists in a cycle involving organic S. Depending on the formof organically-bound S and whether the soil is aerobic or anaerobic, the Sthat is mineralized in organic matter decomposition will be either SO4

2- or S2-.The latter is toxic.

Some Points on Micronutrients

Macronutrients C H O N P K S Ca Mg

Micronutrients B Cl Co Cu Fe Mo Mn Ni Zn

Micros are essential, only called micro because they are needed in muchsmaller amounts. Weathering of soil minerals, nutrient cycling and deposition usually adequate to supply micronutrients.

However, deficiencies can occur where biomass removal is high, high analysis(i.e., low impurities) fertilizers are used, and on soils with few weatherable minerals (sands and organic soils).

High pH can also produce deficiencies because it greatly reduces the solubility of micronutrients except Cl and Mo.

High pH favors Mo solubility and availability.

primary nutrients, the otherssecondary in terms of howmuch is typically added

Mn deficiency cotton

The opposite situation can also occur –toxicity.

This may occur from unmindful over-applicationas with biosolids or arise from soil conditions.

Low pH increases the solubility of most of themicronutrients.

Under anaerobic + acid conditions, Fe and Mnsolubilities are especially high.

Crinkle-leaf in cotton due to Mn toxicity.

Soil likely too acidic and wet?

What to do?

Soil testing and plant analysiswould provide key information.

Soil Testing and Plant Analysis

Unbiased soil sampling is essential. Must take steps to account for variability.

Divide field into homogeneous areasTake many random samples, combine, mix and take subsampleAvoid atypical areas.

Lab analyses

Based on soil test levels that are calibrated to field tests.

Best to use lab in state becausetypes of tests and field dataare appropriate for local soil andenvironmental conditions.

Plant analysis is based on the relationship between concentration of elementsin tissue and growth.

Also known as hidden hunger. Plants not so deficient as to show symptoms,only stunted. Can’t see this because all plants around are equally stunted.

90 %max

Salinity and Sodium Problems

Lab analyses can indicate or confirm problems to too high salt and Na.

Salinity may be directly toxic but indirectly a serious problem because high saltsreduce the capacity of plants to uptake water.

Plants take up water because the water potential (energy status of water) in roots is less than the water potential in the soil. Salts in the soil solution reduce the water potential in the soil, causing drought stress when the water content of the soil is relatively high.

This problem is morecommon in arid regions,however can occuranywhere irrigation isused and drainage isinadequate.

Drainage is important because it allows salts to leach below the root zone.

Avoid irrigation induced salinity by applying a bit more water than the cropneeds to force salt leaching. Called leaching requirement.

To reclaim saline soil, improve drainage and leach salts.

High Na is a more difficult problem. High Na leads to very high soil pH and very poor hydraulic conductivity. The latter means you can’t fix the problemby leaching excess Na.

To remediate a sodic soil, add gypsum, CaSO4. It works in a two-fold way:

The Ca2+ will form calcite, CaCO3, which will lower the pH to the equilibriumValue set by calcite solubility, about 8.4, high but much better.

Second, whereas Na+ is a dispersing cation (tends to break up soil aggregatesand clog pores, reducing conductivity), Ca2+ does the opposite. So, gypsumrestores freer water flow so that the excess Na can be leached.

How do lab analysis show salinity and sodium problems?

Salts are indicated by the electrical conductivity (EC) of a saturated soil paste.

A soil is saline if the EC > 4 dS / m

Sodium problem is indicated if the concentration of Na, relative to Ca + Mg, in a water extract of soil is above a certain threshold. This parameter iscalled the sodium adsorption ratio, SAR.

A soil is sodic if the SAR > 13.

There can be situations when the soil is saline and sodic, saline-sodic.

Remediation of saline-sodic soils is the same as for sodic soils, gypsum.

Comments on Soil Water

There is a relationship between soil water potential and soil water content.This is called the soil moisture characteristic curve. It is soil-specific.

This is the general shape.

As the water potential decreases (left to rightin figure) water contentdecreases. This occursbecause as a soil drainsor dries, the water it contains is held ever moretightly by soil solids –thetension in the water greatly increases.

This water drains so quickly from the root zone that little is available for plant use.

This water isheld at too greattension for plants to extractit from the soil.

Field Capacity

Factors affecting plant-available water

Texture Organic Matter

Which soil is more droughty, a silt loam with high organic matter or a sandwith low organic matter?

Field hydrologic cycle

Want to conserve water by:

Increasing infiltration /reducing runoff

Reduce evaporation

Reduce transpiration ofcompeting weeds

But have adequate surface and internaldrainage so aeration isgood in root zone.

Surface drainage is a matter of smoothing, grading and making channels. Subsurface drainage is through a network of parallel buried pipe.

Non-point Source Water Pollution from Agriculture

Growing issue with potential regulation of operations.

Main effect is on downstream water quality, particularly suspended solids andnutrients (N, P and C), that affect levels of dissolved oxygen and / or changethe ecology of the water bodies.

Suspended solids is largely a matter of soil erosion and its control.

N and P lost in surface runoff or internal drainage to some outlet enrich downstream water (eutrophy), leading the ecological changes, including reduced oxygen. Gulf of Mexico hypoxia is thought to be caused by N and P.

A problem with using animal waste as a fertilizer is that the nutrient content isnot optimal. In particular, if you base the rate of application on N content, thenmuch more P is added than can be used by the crop so soil P builds-up withrepeated applications, increasing the likelihood of loss to surface waters.

Numerous best management practices (BMPs) can be used to reducesoil erosion and nutrient losses.

Buffer or filter strips trap suspendedsolids and reduce nutrient losses.

Cover crops reduce runoff and conservenutrients or reduce fertilizer needed.

Vetch

Conservation tillage leaves the soilprotected with residue. Less erosion,greater infiltration and build-up oforganic matter in surface soil.

Water and Wind Erosion and Their Control

The two are conceptually similar. They involve detachment and movement ofsurface soil by fluids. Consequently, the factors affecting them are similar.

Factor Water Erosion Wind Erosion

Climate Heavy rainstorms Dry and windy

Soil Fine-sandy to silty Fine-sandy to siltymost erodible most erodible

Topography More erosion on More erosion on smooth,long, steep grades wide-open areas

Plant cover Good cover protects Good cover protectssoil and limits erosion soil and limits erosion

Control practice Contour rows, strip- Rough-up surface, wind-cropping, terracing breaks

Soil Survey

Spatial map of soil types in county with chemical and physical data for eachand interpretation of their relative suitability for different uses.

Available in hardcopy, CD and web forms (Web Soil Survey).

Soil types referred to as mapping units.

Locate soil type at a site by using index map to find correct areal photo map sheet, then go from there based on landmarks or legal description.

Physical and chemical data are given for each horizon.

Particularly relevant table is land capability classification (classes I to VIII).

Class Severity of Limitation

I None. Can be intensively farmed.II Minor. Major limitation given as lower case letter, i.e., e = erodible, w = wetV Major. Only perennial vegetation, i.e., pasture OKVIII Severe. Only recreation.

Soil Morphology and Classification

There are 5 master horizons (vertical zones) in soils. Some soils have all, somehave only 2. Except for the lowermost horizon, these developed as the soilaged in place.

OA

E

B

C

O = organic material above mineral soilA = upper mineral, enriched in organic matter

E = little organic material or clay (or salts)

B = clay (or salts) accumulated from above

C = close to presumed original geologic material

Besides master horizons, there are numeroussecondary horizon designations and transitionalhorizon designations, e.g., Ap = plowed, Bt = clay

Classification

US system is called Soil Taxonomy. It is analogous to biological classificationSystem, including soil order (parallel to phylum) as the most general level, and series (parallel to species) as the most specific.

The simplest of the 12soil orders is the Entisol.It only has A and Chorizons. Thought to beyoung.

The order Alfisol is commonin soils formed under forest.It shows A, E, Bt and Chorizons and is consideredto have been developingin place for a long time.

Mollisols are prairie This is an Oxisol from Similar to the Oxisol is thissoils with typically the humid tropics. The Ultisol from Louisiana. Thesehigh organic matter. red color is from oxidized forest soils are less highlyPrized agriculturaliron. These are highly weathered and more fertilesoils. weathered, infertile soils. than Oxisols.