Soil Reaction

Chapter 9

Sources of H+ and OH-

Pools of soil acidity

Buffering

Why pH changes

Managing pH

pH affects

Nutrient availabilityPlant growthMicrobial activity

Here are some relations and termsyou need:

H2O = H+ + OH-

Water dissociates as above and theEquilibrium constant for it is

Kw = [H+][OH-] = 10-14

So,

log[H+] + log[OH-] = -14

-log[H+] – log[OH-] =14

pH + pOH = 14

Keep in mind, low pH means high [H+] and visa versa.

Plant growth

Most grow best in pH range 5.5 to 7.0

Some prefer extremes

Acid pH AzaleasBlueberries

Alkaline Alfalfa

Sources of H+ and OH-

Strongly acid soil pH < 5

Exchangeable H+ and Al3+

Remember why Al3+, Fe3+, etc. are acidic, they hydrolyze,

Al3+ + H2O = AlOH2+ + H+, AlOH2+ + H2O = Al(OH)2+ + H+,

Al(OH)2+ + H2O = Al(OH)3 + H+

Sources of H+ and OH-

Moderately acid soil 5 < pH < 6.5

Exchangeable H+ and Al(OH)x(3-x)+

Higher %BS so fewer acid cations

This is called a buffer curve. Notice that as%BS increases, pH increases.

Sources of H+ and OH-

Neutral to alkaline soils

pH > 6.5

High %BS so few acidic cations

Remaining adsorbed H+ and Al(OH)x(3-x)+ are

considered boundNegligible exchangeable H+ and Al(OH)x

(3-x)+ but some acids remain that arestrongly adsorbed.

Sources of H+ and OH-

Carbonate and bicarbonate salts arepresent at alkaline pH

CaCO3 → Ca2+ + CO32-

CO32- + H2O → H2CO3 + 2OH-

Pools of Soil Acidity

ActiveExchangeableResidual (bound)

Total

Active acidity is H+ in solution

Exchangeable is adsorbed H+ and Al3+

Can be displaced by addition of extractingcation such as K+

Pools of Acidity

Pools of Acidity

Residual is adsorbed H+ and Al(OH)x(3-x)+

that is unextractable

Can be determined by extraction at alkaline pH

█2H+ + Ba2+ → █Ba2+ + 2H+

2H+ + 2OH- → 2H2O

Buffering

Tendency of soils to resist a change is pHupon addition of acid or base

What happens when you add acid toan acid soil?

Buffering

Well, H+ is a very reactive chemical species and it reacts, consuming it. Thus,the concentration of H+ is less than would otherwise be the case. Sure, addingacid at a concentration greater than exists in the soil solution increases the concentration of H+ in the soil solution but not nearly a much as would be thecase without these various reactions. See next slide.

Some H+

Adsorbed on colloids

Can react with soil minerals

Al(OH)3 + 3H+ Al3+ + 3H2O

Buffering

Al(OH)3 + 3H+ Al3+ + 3H2O

What happens when you add acid to analkaline soil?

Buffering

At high pH carbonates react with H+

CO32- + 2H+ H2CO3

H2CO3 H2O + CO2

Buffering

CO32- + 2H+ H2CO3

H2CO3 H2O + CO2 ↑So what happens? It depends on how much acid you add and how muchbase is present. If you add more acid than base, then the base is consumedand you make the soil acidic.

Usually, you are interested in adding base to an acid soil but in some cases you may wish to decrease the basicity.

And if you add OH- to an acid soil

Reacts with H+ or Al3+

Buffering

Magnitude of buffering quantifiedin the buffer curve

Quantity of acid or base required for agiven change in pH depends on

Shape of buffer curveCEC

Buffering

Consider the matter of raising the pH by adding a base, like CaCO3, calcite.Two things are going on: 1) reaction of the base, CO3

2-, with H+, consumingthe latter and producing H2O and CO2; and 2) Ca2+ replaces acidic cationsthat are adsorbed on soil colloids, forcing them into solution where they reactwith CO3

2- or HCO3- or OH- (the latter two produced from CO3

2- reactionwith water). The soil solution pH increases ([H+] decreases) and the fractionof the CEC made up of acidic cations decreases, i.e., the %BS increases.

Type of colloids present affect the shape ofthe buffer curve

Two soils have the same shape buffer curve,they have the same pH (= 5.5) but the CEC of soil A is 10 cmole(+) / kg and the CEC of soil Bis 20 cmole(+) / kg.

Which soil would require more CaCO3 (lime)to raise its pH to 6.5?

How much more?

∆ % BS = (90 % – 50 %) = 40 %

40 % of 10 cmol (+) / kg = 4 cmol (+) Ca / kg

40 % of 20 cmol (+) / kg = 8 cmol (+) Ca / kg

Causes of Soil pH Changes

Natural acidificationNH4

+ fertilizersBiomass removalOrganic wastesS oxidation in drained wetlandsAcid depositionNa addition

Excessive Na

Natural acidification

Carbonic acidOrganic acidsH+ produced by N and S oxidation

Why pH Changes

Respiration in the soil produces, CO2, which reacts with water to form H2CO3,which dissociates giving H+. Carbonic acid is weak but it’s more or less constantly produced. Further, when microbes decompose organic matter, theN and S that it contains is effectively released as nitric and sulfuric acids, HNO3

and H2SO4, which are strong acids. Various organic acids (-COOH groups)are also released. Of course, these acids yield H+ to the soil solution.

Acid cations replace basic cations

Basic cations leach

% BS decreases and pH decreases

Why pH Changes

And this is what happens due to this natural production of H+ in the soil --

Why pH Changes

Acidifying fertilizers

Microbial oxidation of NH4+

NH4+ + 2O2 → NO3

- + 2H+ + H2O

Fertilizers that contain ammonium or from which ammonium is derived(like, NH3 + H+ = NH4

+ or urea, (H2N)2CO + H2O = 2NH3 + CO2), areoxidized by certain soil microorganisms, generating H+. You can counton the above reaction to occur, and can figure how much acid is generated based on how much ammonium-containing fertilizer is added.

Removal of a lot of biomass

Let’s say 10 Mg (= 10000 kg) of biomass is removed from1 ha and it contains 1 % Ca. Therefore, 100 kg Ca removed.

100 kg = 100000 g AWCa = 40 g / mole

So, 100000 g / (40 g / mole) = 2500 mole Ca removed

Or 2500 x (100 cmole / mole) x 2 = 500,000 cmol (+) removed

1 HFS = 2000 Mg or 2,000,000 kg

500,000 cmole (+) / 2,000,000 kg = 0.25 cmol (+) / kg

The previous calculation may not look like much, 0.25 cmol(+) kg-1, however,figure that other base cations are removed in harvested biomass and sumthat over several harvests –the effect can be substantial, particularly if the CEC is low, i.e., the effect on decreasing %BS is more pronounced.

Disposal of organic wastes

Organic and mineral acids produced bymicrobial oxidation

Why pH Changes

The phenomenon is like natural acidification but by adding organic matterrespiration and decomposition of organic matter are substantially increased.

Drainage of wet soils with high level ofreduced S

Oxidation of reduced S produces H2SO4

Very low pHs

Why pH Changes

Acid deposition

Burning fuels that contain N and S leads toatmospheric HNO3 and H2SO4

Problem for forest soils where

AcidicLow buffer capacity

Is it easy to get a lime spreader through thewoods?

Why pH Changes

High Na irrigation water

High concentration of Na+ raises pH

Why pH Changes

Ca Na ↓ ↓

█ █ ↓ ↓

Ca Na H Ca Na H

Ca-saturated Na-saturated

Ca Na ↓ ↓

█ █ ↓ ↓

Ca Na H Ca Na H

Ca-saturated Na-saturated

% BS and pH increase in both cases but

If dominated by Ca, there will also exist a CaCO3 phasewhich controls pH at ≈ 8.4

Na2CO3 is soluble so there is no pH control

Unlike all the foregoing,this effect raises the pH.

The below data are for an acid sandy loam:

        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 = ?

CEC = sum of all cationic charge, 10.0 cmol (+) kg-1

%BS = (sum of base cationic charge x 100%) / CEC, or 70%

pH Measurement

Name 2 ways to do it

You can do it with pH sensitive dyes (like a colorimetric titration or likewith Duplex indicator in lab) or you can do it with an ion (H+) selectiveelectrode, right?

Managing Soil pH

Sulfur lowers pH

2S + 3O2 + 2H2O → 2H2SO4

Lime raises pH

To a minor extent, the above reaction occurs, as they say, chemically.However, sulfur oxidizing microbes are principally responsible.

CaCO3 CalciteCaMg(CO3)2 Dolomite

CaO Burned lime (quicklime)Ca(OH)2 Hydrated lime

∆CaCO3 → CaO + CO2 ↑

These are the types of lime materials. The advantage of the non-limestones,the oxides and hydroxides, is that they are more soluble and so react fasterto consume H+. To get good results using limestone, it should be finely groundto increase surface area and rate of dissolution.

But acidification of soil continuesMore lime eventually needed

It’s worth pointing out that fixing topsoil acidity is an easy matter, just till in some lime. However, fixing subsoil acidity is more difficult because unlessthe lime is placed down there in the first place, its reactivity is largely confinedto where it was applied --it is mostly immobile.

Subsoil acidity can be a real problem in some cases if it curtails deep root development, thereby restricting the root system to a smaller portion of theprofile (access to fewer potential nutrients, and increased drought risk,especially. A way around the expense of deep tillage and liming is to useanother source of Ca2+ (replaces acidic cations on soil colloids, putting theminto solution where they may leach deeper and away) that is much moresoluble and mobile is gypsum, CaSO4. Besides the effect of Ca2+ from it,the sulfate tends to bond to soil colloids, and in doing so releases a tad ofOH-, a pseudo-liming effect, however, gypsum is not considered a limematerial.

Lime Requirement

Target pH changeBuffer capacity of soilType and purity of lime materialFineness of lime

Which will neutralize more acidity, 1 kg of 90 % purity CaCO3 or 0.74 kg of Ca(OH)2?

You have to have a common basis of comparison, like moles –one mole ofEither will neutralize two moles of H+,

Ca(OH)2 + 2H+ → 2H2O + Ca2+ or

CaCO3 + 2H+ → H2O + CO2 + Ca2+

MWCa(OH)2 = 74 g mole-1 and MWCaCO3 = 100 g mole-1

Therefore,

1000 g x 0.90 / 100 g mole-1 = 9 moles CaCO3

740 g / 74 g mole-1 = 10 moles Ca(OH)2

Based on a buffer curve for a certain soil, 1.0 millimoles of OH- are required to raise the pH of 10 g (oven-dry mass) of this soil from an initial value of 5.0 to 6.5.

How many Mg per hectare furrow slice (HFS) of CaCO3 are required to raise the pH of this soil from 5.0 to 6.5? 

One Mg = 1000 kg.  Assume 2000 Mg / HFS.

(1.0 mmoleOH / 10 gsoil) x (50 mgCaCO3 / 1 mmoleOH) x (0.001 g / 1 mg) =

0.005 gCaCO3 / 1 gsoil

just a ratio of mass to mass so multiply it by mass of soil, 2000 Mg / HFSto give, 10 MgCaCO3 / HFS

The key, besides seeing the sense in the multiplied ratios, is knowing whatmass of lime material is chemically equivalent to 1 mmole of OH-1. This meansfiguring molecular weight, MW (and milli-MW, mMW), and recognizing thata mmole of any lime, CaO, etc., will neutralize 2 mmoles H+.