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Understanding Soil Acidity. Neutral. Brady and Weil (2002). pH = - log (H + ion concentration). pH = 7. neutral. As pH increases…. As pH decreases…. Brady and Weil, 2002. Optimum pH ranges have been identified for many crops.

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slide1

Understanding

Soil

Acidity

Neutral

Brady and Weil (2002)

slide2

pH = - log (H+ ion concentration)

pH = 7

neutral

As pH increases…

As pH decreases…

Brady and Weil, 2002

slide5

For most soils, nutrient availability is optimized between pH 5.5 and 7.

Nutrient availability

varies with pH

slide6

Molybdenum becomes more available as pH goes up !

most

^

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

slide7

Understanding aluminum toxicity

Toxic forms

of Al are

bioavailable

at low pHs

Aluminum toxicity is minimal above

pH 5.5

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

slide8

Multiple forms of soil acidity

Soil pH is primarily a measure of active acidity

Reserve acidity

Active acidity

Brady and Weil, 2002

slide9

Understanding reserve acidity

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

Reserve acidity resupplies the active acidity

ΔpH

ΔpH

Reserve

acidity

Active

acidity

Reserve

acidity

Active

acidity

Low CEC soil

High CEC soil

slide10

Each charge depicted on this diagram represents 1 centimol of charge per kg of soil

K+

-

-

Ca+2

-

-

Mg+2

-

-

H+

Humus

H20

H20

H20

Exchangeable

acidity

exchangeable

cations

soil

solution

H20

H20

H20

-

Clay

-

-

Al+3

+ H2O ↔ Al(OH)3 + 3H+

-

+

-

+

-

K+

SO4-2

What is the “base” saturation ?

Ca+2

slide11

Is pH related to base saturation ?

It is probably more accurate to say that pH is related to acid saturation

100 80 60 40 20 0

Acid Saturation, %

slide12

pH dependent charge

The dominant clay minerals in IL have mostly permanent charge

slide13

The charge on humic substances (and low activity clays) is very pH dependent

pH dependent

charge

H+ ions dissociate when the soil pH increases

and reassociate when the pH drops.

Brady and Weil (2002)

slide14

Soil acidity increases when H+ producing processes exceed H+ consuming processes.

H+ consuming processes

H+ producing processes

slide15

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, elemental S).

NH4+ + O2 → NO3- + 2H+ S0 + O2 → SO4-2 + 2H+

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 fullydissociate in water.

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

VERY IMPORTANT PART OF SOIL FORMATION

slide16

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
slide17

Acidity

What is liberated and what is left behind

when plant biomass is burned ?

Oxides of

C, N and S

Elements that have traditionally been called “bases”

Oxides of

Ca, Mg and K

Alkalinity

slide19

Sources of pH buffering

in soils

Carbonates

Chadwick and Chorover ( 2001)

slide20

K+

H+

The pH of a plant’s rhizosphere changes as the plant regulates its internal charge balance.

NO3-

?

slide21

Which plant received nitrate ?

Which plant received ammonium ?

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

slide22

Acid inputs promote leaching of non-acid cations

Why does leaching of these anions cause soil acidification ?

Brady and Weil, 2002

slide23

Complete N cycle (no net acidification)

released into

the soil

1H+consumed

Nitrification is an acidifying process, right??

1H+consumed

NH3

The 2 H+ produced during nitrification are balanced by 2 H+ consumed during the formation of NH4+ and the uptake of NO3- by plants

slide24

Excellent but focused

on Australian soils

slide26

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.

slide27

Harvest of crop biomass removes alkalinity

from agricultural fields

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

slide28

Scenario

Corn/soybean rotation

200 bushels, 50 bushels

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

~ 190 lbs of lime

~ 270 lbs of lime

~ 45 lbs of lime

~ 50 lbs of lime

Projected lime requirement ~ 0.3 tons/rotation

slide32

Sources of variation in soil pH measurements

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.

slide33

Why measure soil pH

using a salt solution ?

Water pH > Salt pH

Brady and Weil, 2002

slide34

Soil pH depends on method used to measure it !!

As a result, the method of measurement should be reported whenever soil pH data is discussed.

slide36

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.

H+

Ca+2

soil colloid + CaCO3 soil colloid + H2O + CO2

H+

slide37

“Illinois method” of determining lime requirement

How do you know which line to use ?

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

slide38

Choosing the right line

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)

slide39

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.

slide41

Page from the 2008 IL Lime book

Multiply by these factors

slide43

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.

Rates of lime should be adjusted if any of these assumptions are not accurate

slide45

pH measurements on the fly

Soil pH and lime requirement can vary widely within fields

slide46

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?

slide47

Insufficient lime is applied to neutralize total acid inputs to IL soils

South eastern IL has few quarries and the greatest lime deficit

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

slide48

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): 61-69Abstract: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.