Understanding pH
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Understanding pH. pH = - log (H + ion concentration). ?. pH = 7. neutral. As pH decreases…. As pH increases…. Brady and Weil, 2002. Optimum pH ranges have been identified for many crops. Native species also have pH preferences. http://asecular.com/forests/graphics.jpg.

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Understanding pH

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Understanding ph

Understanding pH


Understanding ph

pH = - log (H+ ion concentration)

?

pH = 7

neutral

As pH decreases…

As pH increases…

Brady and Weil, 2002


Understanding ph

Optimum pH ranges have been identified for many crops


Understanding ph

Native species also have pH preferences

http://asecular.com/forests/graphics.jpg


Understanding ph

Understanding

Soil pH

Neutral

Brady and Weil (2002)


Understanding ph

Do plants actually care about soil pH?

The acid infertility complex

Problems related to nutrient availability

and metal toxicity in acid soils


Understanding ph

For most soils and crops,

total nutrient availability is optimized between

pH 5.5 and 7.

Nutrient availability

varies with pH


Understanding ph

Molybdenum is more available at high pHs.

most

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


Understanding ph

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


Understanding ph

Crop varieties differ in their sensitivity to Al toxicity

Brady and Weil, 2002


Understanding ph

Reserve vs. active acidity

Soil pH is primarily a measure of active acidity

Reserve acidity

Active acidity

Brady and Weil, 2002


Understanding ph

Understanding reserve acidity

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

Reserve acidity resupplies the active acidity

BIG

pH change

Small pH change

Reserve

acidity

Active

acidity

Reserve

acidity

Active

acidity

Low CEC soil

High CEC soil


Understanding ph

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

K+

-

-

Ca+2

What is the CEC of this soil ?

-

-

Mg+2

-

-

H+

Humus

H20

H20

H20

Exchangeable

acidity

exchangeable

ions

soil

solution

H20

H20

H20

-

Clay

-

-

Al+3

-

+ 3H2O  Al(OH)3 + 3H+

+

-

+

-

K+

SO4-2

What is the “base” saturation ?

Ca+2


Understanding ph

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

K+

Many people refer to non-acid cations as base cations

-

-

Ca+2

-

-

Mg+2

-

-

H+

Humus

H20

H20

H20

Exchangeable

acidity

exchangeable

cations

soil

solution

H20

H20

H20

-

Clay

-

-

Al+3

+ 3H2O ↔ Al(OH)3 + 3H+

-

+

-

+

-

K+

SO4-2

What is the “base” saturation of this soil?

Ca+2


Understanding ph

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, %


Understanding ph

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

H+ consuming processes

H+ producing processes


Understanding ph

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 -> H+ + HCO3-

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).

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.


Understanding ph

  • 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)


Understanding ph

Sources of pH buffering

Carbonates

Lime (CaCO3)

fountain of soil youth?

Young soil

Old soil

Chadwick and Chorover ( 2001)


Understanding ph

Acid inputs promote leaching of non-acid cations

Brady and Weil, 2002


Understanding ph

S and N oxides cause acid precipitation

Brady and Weil, 2002


Understanding ph

plant

uptake

mineralization

nitrification


Understanding ph

Alfalfa field with dead strip where lime was not applied

How much lime should be applied ?


Understanding ph

Lime requirements should be guided by soil testing


Understanding ph

Pocket pH meters can be very useful but require regular calibration !!!


Understanding ph

Salt pH vs. water pH

Why do labs in arid regions use a salt solution?

Brady and Weil, 2002


Understanding ph

Sources of variation in soil pH measurements

1. The nature and type of inorganic and organic constituents that contribute to soil acidity.

2. The soil to solution ratio used in measuring pH.

3. The salt content of the diluting solution used to achieve the desired soil to solution ratio.

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

5. Errors associated with standardization of the equipment used to measure pH.


Understanding ph

The amount of lime needed to bring about a 1 unit change in pH varies widely between soils


Understanding ph

“Illinois method” of determining lime requirement

How do you know which line to use ?

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


Understanding ph

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)


Understanding ph

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


Understanding ph

It takes time for lime to react in soil


Understanding ph

Soil pH and lime requirement can vary widely within fields


Understanding ph

Both past management and inherent

soil properties affect soil pH and lime requirement


Understanding ph

Insufficient lime is applied in IL to neutralize the acidity from N fertilizers

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


Understanding ph

Have you ever seen a stream look like this ?


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