Soil acidity

Understanding

Soil Acidity

Brady and Weil (2002)

Neutral

In humid regions, most soils

are naturally acidic but the

following factors contribute to

greater acidity:

- parent material w/ low ANC

- forest vegetation

- ↑duration and intensity of

chemical weathering

ANC = acid neutralizing capacity Natu

ral

pH

im

pacts

op

tim

al

pH

Optimum pH ranges have been proposed for many crops Do plant roots really care about the H+ concentration in soil?

The acid infertility complex

Collective term for the challenges

faced by crops growing in acid soils

Nutrient

availability

varies with pH

For most soils, nutrient

availability is optimized

between pH 5.5 and 7.

http://www.farmtested.com/research_pp.html

most

^

Molybdenum becomes more available as pH goes up !

Understanding aluminum toxicity

Toxic forms

of Al are

bioavailable

at lower pHs

Aluminum toxicity

is minimal above

a water pH of 5.5

http://www2.ctahr.hawaii.edu/tpss/research_extension/rxsoil/alroot.gif

Fe and Mn toxicities also

occur at lower pHs

Many biological processes

are sensitive to aluminum toxicity

Brady and Weil, 2002

Crop varieties differ in their sensitivity to Al toxicity

Multiple forms of soil acidity

Soil pH is

primarily a

measure of

active acidity

Reserve acidity Active

acidity


Sometimes called residual acidity

H+

H+

H+

H+ H+

H+

Al+3


Understanding pH

pH = -log(H+)


neutral

So what is the H+ ion concentration when the pH = 6?

Understanding pH

pH = -log(H+) X = -log(10-x)


neutral

So what is the pH if the H+ ion concentration is 10 x higher?

Understanding pH

pH = -log(H+) X = -log(10-x)

Understanding reserve acidity

Reserve

acidity

Reserve

acidity

Active

acidity Active

acidity

Very little lime is needed to neutralize the active acidity in soils

High CEC soil Low CEC soil

Both soils

initially have

the same pH

(i.e., the same

amount of

activity

acidity)


Reserve

acidity

Reserve

acidity

Active

acidity Active

acidity

If only enough lime is added to deplete the active acidity,

reserve acidity will quickly begin resupplying the active acidity



Reserve

acidity

Reserve

acidity

Active

acidity Active

acidity

More lime is needed to bring about persistent pH change

in soil with more reserve acidity


ΔpH

ΔpH Effect of

adding the

same rate

of lime to

soils with

different

amounts of

reserve

acidity

exchangeable

cations

soil

solution

Humus

Clay

-

-

- -

-

-

- -

- -

Al+3

Ca+2

H+

K+

Ca+2

Mg+2

K+

H20 H20

H20 H20

H20

+ SO4

-2 + - -

H20

What is the “base”

saturation of this soil ?

Exchangeable

acidity

Each charge depicted on this diagram represents 1

centimol of charge per kg of soil

+ H2O ↔ Al(OH)3 + 3H+

Sum of non-acid cations

_____________________

Sum of all cations

* 100 %BS =

Is pH related to base saturation ?

100 80 60 40 20 0

Acid Saturation, %

Is pH related to base saturation ?

100 80 60 40 20 0

Acid Saturation, %

It is probably more accurate to say that active

acidity is related to acid saturation

pH dependent charge

The dominant clay minerals in

IL have mostly permanent

charge created by isomorphic

substitution

In contrast, the

charge on humus is

higher at higher pHs

Brady and Weil (2002)

H+ ions dissociate when the soil pH increases

and reassociate when the pH drops.

The charge on humic

substances (and low

activity clays) is very pH

dependent

H+

H+

H+

H+

Soil acidity increases when H+ producing

processes exceed H+ consuming

processes.

Many processes add H+ ions to soils

1) Carbonic acid forms when carbon dioxide dissolves in water.

H+ ions are released when carbonic acid dissociates:

H2CO3 → HCO3- + H+

2) Organic acids form during the decomposition of organic matter.

H+ ions are released when these organic acids dissociate.

3) Sulfuric and nitric acids form during the oxidation of reduced forms

of N and S (e.g., NH4+ from fertilizer).

NH4+ + 2O2 → NO3

- + 2H+ + H2O

4) Sulfuric and nitric acids form when sulfur oxides and nitric oxides

(released into the atmosphere by automobile emissions, industry

smoke stacks, volcanoes, forest fires) dissolve in precipitation.

H2SO4 and HNO3 are strong acids and fully dissociate in water.

5) Roots release H+ to balance internal charge when cation uptake

exceeds anion uptake.

VERY IMPORTANT PART OF SOIL FORMATION

Nitrification

K+

H+

NO3-

OH-

The pH of a plant’s

rhizosphere changes

as the plant regulates

its internal charge

balance.

http://departments.agri.huji.ac.il/plantscience/topics_irrigation/uzifert/4thmeet.htm

Which plant received nitrate (NO3-)?

Which plant received ammonium (NH4+)?

Many processes consume H+ ions in soils

1) Weathering of most minerals (e.g., silicates, carbonates…)

2) Decomposition of organic anions

3) Reduction of oxidized forms of N, S and Fe.

4) Roots release OH- or HCO3- to balance internal charge when anion

uptake exceeds cation uptake

5) Inner sphere adsorption of anions (especially sulfate) which displaces

hydroxyl (OH-) groups

What is liberated and what is left behind

when plant biomass is burned ?

Oxides of

Ca, Mg and K

Oxides of

C, N and S

Alkalinity

Acidity

Elements that

have traditionally

been called

“bases”

C, N and S oxides cause acid precipitation


Forest damaged by acid rain


Looks great but

may be devoid

of life if acid rain

has created Al

toxicity


Looks great but

may be devoid

of life if acid rain

has created Al

toxicity

Monument getting

dissolved by acid rain

Chadwick and Chorover ( 2001)

Carbonates

The effect of

added acidity on

soil pH

depends on the

soil’s buffer

capacity

Sliding down the acidity slope

Acid inputs promote leaching of non-acid cations


Why does

leaching of

these anions

cause soil

acidification ?

Nitric acid = HNO3 → NO3- + H+

NH3

1H+ consumed

1H+ consumed

released into

the soil

Nitrification is an acidifying process, right??

NH3

1H+ consumed

1H+ consumed

released into

the soil

Complete N cycle (no net acidification)

The 2 H+ produced during nitrification are balanced by 2 H+ consumed

during the formation of NH4+ and the uptake of NO3

- by plants

Very important in places where lime is expensive!

Nitrogen source Composition Lime required

(lb CaCO3 / lb N)

Anhydrous ammonia 82-0-0 1.8

Urea 46-0-0 1.8

Ammonium nitrate 34-0-0 1.8

Ammonium sulfate 21-0-0-24 5.4

Monoammonium

phosphate 10-52-0 5.4

Diammonium

phosphate 18-46-0 3.6

Standard values for the quantity of lime needed to

neutralize the acidity generated by specific N fertilizers

Assumes: 1) all ammonium-N is converted to nitrate-N and

2) half of the nitrate is leached.

Crop Cation : N ratio

in plant biomass

Lime required to

replace alkalinity

removed in harvest

(lb CaCO3 /100 lb of

N harvested)

Corn grain 0.14 25

Corn stover 0.73 131

Soybean 0.14 25

Oats grain 0.14 25

Oats straw 0.94 169

Alfalfa 1.41 254

Harvest of crop biomass removes alkalinity

from agricultural fields

http://www.ianrpubs.unl.edu/epublic/pages/publicationD.jsp?publicationId=111

Scenario Corn/soybean rotation

200 bu corn, 50 bu soybeans

All P supplied as DAP

N applied as DAP and AA

Acidity from N fertilizer

3.6 x 52 lbs of N in DAP required to

supply P removed in harvest

1.8 x 150 lbs of N in AA

Acidity from grain harvest

25 x 180 lbs of N harvested/100

25 x 200 lbs of N harvested/100

Projected lime requirement ~ 0.3 tons/rotation

~ 190 lbs of lime

~ 270 lbs of lime

~ 45 lbs of lime

~ 50 lbs of lime

In many parts of the world, notably the US Midwest and Europe, soils

are often limed to a near neutral pH 6.5–7.0. Because plants do not

directly respond to H+ concentration, it is pertinent to inquire why this

approach to liming has enjoyed such widespread popularity.

The original near-neutral pH of many of the soils was no doubt a

consideration as was the use of acid-sensitive forage legumes to

supply N in rotations during the era when the original lime

experiments were conducted.

The introduction of the pH meter at about the same time as N

fertilizers found widespread popularity (replacing forage legumes in

rotations) facilitated measurement of soil acidity and removed the

focus from the real problems of soil acidity, namely, toxic levels of Al

and Mn and deficiencies of nutrients such as Ca, Mg, N, S, P and Mo.

Even after forage legumes disappeared from most rotations, high

target pH values were retained.

Liming experiments throughout the world reveal that, with very few

exceptions, all grain crops including legumes cease to respond to lime

above pH 5:5–5:8; provided that the nutrients (Ca, Mg, Mo, B, P, etc)

negatively impacted by soil acidity are optimized.

On highly weathered soils (e.g., NC and Brazil), liming to near

neutrality can have disastrous effects on yields of many crops.

Many examples are presented in the article of the few benefits of

liming to neutrality and the many benefits of farming with levels of

acidity somewhat more intense than has normally been the case.

Among the latter benefits are increased profitability from higher

nutrient efficiencies, reduced diseases and pests, slower

nitrification with less water pollution, improved soil tilth,

improved availability of many metals and P.

http://soil.scijournals.org/cgi/content/full/68/2/545/FIG4

Soil pH

% N

itri

ficati

on

Impact of pH and an inhibitor on % nitrification

w/ N serve

Do you remember this graph?

According to Sumner:

Ever since pH became an easily measured soil parameter

(invention of the pH meter), we have focused on an indicator of

soil acidity (pH) rather than on the actual plant limiting factors

associated with acidity (toxicities, deficiencies and

imbalances).

Alfalfa field with

dead strip where

lime was not

applied

How should

lime rates be

determined?

Lime rates should

be guided by soil

testing

Pocket pH meters can be very useful

but require regular calibration !!!

1. The soil to solution ratio used when measuring pH.

2. The salt content of the diluting solution used to

achieve the desired soil to solution ratio.

3. The carbon dioxide content of the soil and solution.

4. Errors associated with standardization of the

instrument used to measure pH.

Sources of variation in soil pH measurements

Water pH > Salt pH


Salt solutions

are normally

used when

measuring the

pH of soils in

arid regions

(i.e. places

where salinity

is often an

issue)

Soil pH depends on the method

used to measure it !!

As a result, the method of measurement

should be reported whenever soil pH

data is discussed.

The amount of lime needed to

bring about a 1 unit change in

pH varies widely between soils

http://bioengr.ag.utk.edu/SoilTestLab/images/samples.jpg

soil colloid + CaCO3 soil colloid + H2O + CO2

H+

H+

Ca+2

When a soil is limed, Ca+2 from the lime

displaces exchangeable acidity from the

soil colloids. The active acidity that is

generated reacts with the carbonate ions

from the lime, producing water and

carbon dioxide.

“Illinois method” of determining lime requirement

How do

you know

which line

to use ?

http://iah.aces.uiuc.edu/pdf/Agronomy_HB/11chapter.pdf

The lines represent

different levels of

reserve acidity

Steeper line = more reserve acidity

Line A: Dark colored silty clays and silty clay loams (CEC > 24)

Line B: Light and medium colored silty clays and silty clay loams,

dark colored silts and clay loams (CEC 15-24)

Line C: Light and medium colored silt and clay loams, dark and

medium colored loams, dark colored sandy loams (CEC 8-15)

Line D: Light colored loams, light and medium colored sandy

loams and all sands (CEC < 8)

Line E: Mucks and peat (organic soils).

Light colored (< 2.5% OM)

Medium colored (2.5-4.5% OM)

Dark colored (4.5% OM)

Choosing the right line

“Buffer pH” is a measure of reserve acidity

Not all limestone is the same !

Pure calcium carbonate has a calcium carbonate

equivalency (CCE) of 100 and is the standard against

which all liming materials are compared. A ton of material

with a CCE of 90 % can neutralize 10% less acid than a ton

of pure calcium carbonate.

Liming materials that are finely ground, have more surface

area in contact with the soil solution than coarser ground

materials and thus will neutralize soil acidity more rapidly.

Fineness of grind is rated according to the percentage of

material that will pass through 8-, 30-, and 60-mesh

screens.

http://www.agr.state.il.us/news/pub/2007LimeBook.pdf

Page from the 2008 IL Lime book

Multiply by these factors

Adjusting for differences in lime particle size distribution

Lime requirements determined using the “Illinois

method” assume the following:

A. A 9-inch tillage depth. If tillage is less than 9 inches, reduce the

amount of limestone; if more than 9 inches, increase the lime rate

proportionately. In no-till systems, use a 3-inch depth for calculations

(one-third the amount suggested for soil moldboard-plowed 9 inches

deep).

B. Typical fineness of limestone. Ten percent of the particles are

greater than 8-mesh; 30 percent pass an 8-mesh and are held on 30-

mesh; 30 percent pass a 30-mesh and are held on 60-mesh; and 30

percent pass a 60-mesh.

C. A calcium carbonate equivalent (total neutralizing power) of 90

percent. The rate of application may be adjusted according to the

deviation from 90.

Lime rates should be adjusted if any of these

assumptions are not accurate

It takes time for lime to react in soil

Soil pH often

varies widely

within fields

pH measurements on the fly

Don’t forget that some measure of OM, CEC or clay

content is also needed to make a variable rate lime map.

Both past management and inherent

soil properties affect soil pH and lime requirement

Why is variable rate lime

more likely to pay than

variable rate N, P or K?

Both past management and inherent

soil properties affect soil pH and lime requirement

Why is variable rate lime

more likely to pay than

variable rate N, P or K?

Over-liming and under-

liming have negative

effects on yield

Insufficient lime is applied to neutralize

total acid inputs to IL soils

http://iah.aces.uiuc.edu/pdf/Agronomy_HB/11chapter.pdf

South eastern IL

has fewer quarries

and the greatest

lime deficit

Barak P, Jobe BO, Krueger AR, Peterson LA, Laird DA 1997. Effects of long-

term soil acidification due to nitrogen fertilizer inputs in Wisconsin.

PLANT AND SOIL. 197(1): 61-69

Abstract:

Agroecosystems are domesticated ecosystems intermediate between natural

ecosystems and fabricated ecosystems, and occupy nearly one-third of the

land areas of the earth. Chemical perturbations as a result of human activity

are particularly likely in agroecosystems because of the intensity of that

activity, which include nutrient inputs intended to supplement native nutrient

pools and to support greater biomass production and removal. At a long-term

fertility trial in South-Central Wisconsin, USA, application of ammoniacal N

fertilizer resulted in significant increases in exchangeable acidity accompanied

by decreases in cation exchange capacity (CEC), base saturation, and

exchangeable Ca2+ and Mg2+ . Plant analysis shows that a considerable

portion of the alkalinity generated by assimilation of N (and to a lesser extent

by S) is sequestered in the above-ground plant parts as organic anions and is

not returned to the soil if harvested. Elemental analysis of soil clays

indicates a loss of 16% of the CEC. The reversibility of this change is

doubtful if the changes are due to weathering of soil minerals.

pH < 5.5 pH > 7.0

Al toxicity to plant roots Fe deficiency

Mn toxicity to plant roots Mn deficiency

Ca and Mg deficiency Zn deficiency

Mo deficiency in legumes *Osmotic stress from salts

P tied up by Fe and Al P tied up by Ca and Mg

Slow N transformations Potato scab

Summary of common soil fertility problems that

rarely occur in soils with pHs between 5.5 and 7

Soil acidity

Education

Transcript of Soil acidity