Soil Acidity

Overview

Exchangeable Acidity

Aluminum Chemistry

Redox Effects

Neutralization of Soil Acidity

Overview

Humid region soils tend to be acidic due to biological activity and net drainage of water in humid regions.

A measure of acidification is the fraction of the CEC made up of basic (no hydrolysis) cations, principally Ca2+, Mg2+, K+ and Na+ --the higher thisfraction, the less acidic and more basic is the soil.

Base Saturation (BS) = (Σ [Na+]ads + … ) / CEC

Since a soil (polypedon) is an open system, absolute and relative amounts ofbasic cations and acidic cations (H+ and those that undergo hydrolysis, e.g.,Al3+, AlOH2+, etc.) change over time depending on inputs to the system.

Therefore, BS changes and pH changes.

Inputs (Natural)

Atmospheric deposition and gas absorption / dissolution

Plant residue

Lateral sub-surface flow

Internal sources

Outputs

Leaching and lateral sub-surface flow

To qualitatively show acidification of humid region soils, ignore lateral movement. Focus on internal production of H+ and bases.

Internal sources of H+ and basic cations

H+

H2CO3 = H+ + HCO3-

Not set by atmospheric PCO2 but by higher PCO2 in soil due to respiration.This is a source of H+ in soil solution.

Mineralization of organic matter in the soil releases organic acids and oxidizable N and S. Consider deamination of R-NH2 to NH4

+ then

NH4+ + O2 = NO3

- + H2O + 2H+

Net production of H+ (2 – 1).

H2S may be released and it is quickly oxidized as

H2S + 3/2 O2 + H2O = SO42- + 2H+

Another natural process affecting acidity is differential uptake of cationsand anions by plants. Excess uptake of cations balanced by release of H+.

The purpose of using a chemical extractant in soil tests of nutrient availabilityis to mimic the chemical environment of the rhizosphere. A widely usedextractant is the Mehlich 3 which is

0.2N CH3COOH + 0.25N NH4NO3 + 0.013N HNO3 +

0.015N NH4F + 0.001M EDTA

which is acidic ~ pH 2.5

Bases

Mineral weathering releases basic cations and consumes H+.

Effect

If internal production of H+ > internal production of bases, the equilibria

Bx+ads + XH+ = XH+

ads + Bx+

is favored. Concurrently, leaching tends to deplete the soil of Bx+, favoringreplacement of exchangeable Bx+ and reducing the BS.

Off-setting processes

This is partially off-set by return of Bx+ from plant residue and anaerobicrespiration (if important), e.g.,

¼ CH2O + ½ MnO2 + H+ = ½ Mn2+ + ¼ CO2 + ¾ H2O (Table 11.4)

Depending on atmospheric deposition rates of acidic versus basic components, soil acidification may be further retarded or accelerated. Regardless, high biological activity and leaching under humid climateforces acidification of soil, whether relatively fast of slow.

Atmospheric deposition of S in western Pennsylvania is about 20 kg ha-1 yr-1.Assuming complete replacement of bases by H+ and bulk density = 1.5 g / cm3, what is the annual decrease in BS in the upper 20 cm of soil?

masssoil = 1.5 kg dm-3 x (10002 dm2 x 2 dm) = 3 x 106 kg

cmol(+)H+ kg-1 yr-1 = (20,000 g / 32 g mol-1) x 2 x 100 cmol mol-1 / 3 x 106 kg

= 0.042

Which may seem small, however, if the CEC is low, say 8 cmol(+) kg-1, the soil was acidic 100 years ago, and H+ loading had been continuoussince then,

4.2 / 8.0 > 50 % reduction in BS for soil that was initially already acidic.

Management Effects

Use of NH4+ fertilizers –H+ generated from nitrification

Addition of organic fertilizers –low content of N, therefore high rates applied

H2CO3, organic acids, HNO3 and H2SO4 produced

How much poultry litter @ 2 % N must be applied to supply 200 kg N ha-1?

Only 200 kg N ha-1 / 0.02 = 10,000 kg

Biomass removal –base cation cycling reduced

Drainage of wetlands with high content of reduced forms of S –high H2SO4

When a large fraction of the biomass of a crop is removed in harvest (like with hay), removal of base cations by harvest reduces the soil base saturation and contributes to increased soil acidity. If 40 kg of Ca2+ is removed per hectare in this way, what is the reduction in BS if the CEC of a soil is 10 cmol (+) / kg? Assume 2,000,000 kg / HFS.

That’s 2 x 105 cmol(+)Ca2+ / 2 x 106 kg = 0.1 cmol(+)Ca2+ kg-1

or 0.1 cmol(+) kg-1 / 10 cmol(+) kg-1 = 0.01

which does not seem like much but there are other bases besides Caand this is done year after year.  

Define acid neutralizing capacity (ANC) as moles of H+ per unit volume or mass needed to change the pH of the solution to the pH at which the net charge from ions that do not react with OH- or H+ is zero.

This means cations of strong bases and anions of strong acids.

ANC = [Na+] + [K+] + 2[Ca2+] + 2[Mg2+] – [Cl-] – [NO3-] - 2[SO4

2-]

This definition makes sense because if the solution was basic, ANC > 0,and it would take acid to titrate it, increasing the concentration of anions.

If the solution was acidic, ANC < 0, and titration with base would increasethe concentration of cations.

Concept is applicable to water bodies but may be considered with respectto the soil solution even though the composition of the soil solution is largelycontrolled by soil solids (very low solution volume to solids mass ratio compared to a lake, for example).

Including dissolved CO2, carbonate equilibria would exist with [H+], [OH-], [HCO3

-] and [CO32-] present. Charge balance requires

[H+] + [Na+] + [K+] + 2[Ca2+] + 2[Mg2+]

- [HCO3-] - 2[CO3

2-] - [OH-] - [Cl-] - [NO3-] - 2[SO4

2-] = 0

So that [H+] - [HCO3-] - 2[CO3

2-] - [OH-] + ANC = 0 or

ANC = - [H+] + [HCO3-] + 2[CO3

2-] + [OH-]

Other species such as Al3+, AlOH2+, Al(OH)2+ and L- (generalized organicligand) may be included.

ANC = - [H+] - 3[Al3+] - 2[AlOH2+] - [Al(OH)2+] + [HCO3

-] + 2[CO32-] + [OH-] + [L-]

Obviously, ANC for the soil solution depends on sorption / desorption surface reactions of soil solids.

Buffer intensity, β is defined as derivative of ANC with respect to pH, i.e.,moles of H+ released from the soil or adsorbed by the soil when the pH ofthe soil solution changes by one pH unit. Organic matter in topsoil may dominate β, particularly for sandy texture soil, because of its high CEC.

Max β topsoil about 0.1 – 1.5 molc kgom-1 pH-1

Add 1mmole of H+ to 1 kg of a soil with β = 0.2 and 2 % organic matter

ΔpH = 0.001 mol / (0.2 molc kgom-1 pH-1 x 0.020 kgom) = 0.25 pH unit

From

ΔpH = ΔnH / β = ΔnH / fomβom

However, β depends on pH as previously indicated as well as type of colloid.

pH-dependent

Besides pH buffering by adsorption and desorption of H+ by organic matterand soil minerals, reactions involvingAl affect β.

Exchangeable Acidity

Due to high affinity of H+ and Al(OH)x(3-x)+ adsorption, these acidic species

are not quantitatively displaced by even Ba2+. A portion remains bound andso is measured by displacement with Ba2+ in OH- background. This allowstitration of residual OH- to quantify total extracted acidic species.

2H+ads + Ba2+ = 2H+ + Ba2+

ads

H+ + OH- = H2O

Similar equation for Al(OH)x(3-x)+ with reaction driven to completion by

precipitation of the Al-hydroxy ion with OH-.

This quantity is called the total acidity (TA).

Together with bases extracted by NH4OAc, TA gives the CEC by sum of cations.

Alternatively, one may wish to consider the effective CEC with consists of extracted bases and acids quantified by titration after incomplete extraction using a neutral salt (KCl). By methodological definition, the H+ and Al3+ formsare exchangeable (exchangeable acidity).

Exchangeable acidity is negligible above pH ~ 6.

Comments on Al Chemistry in Acid Soils

Clearly, various monomeric Al3+ species are important sources of H+. Someare exchangeable whereas a portion remains bound under extraction withneutral salt. Besides monomeric forms, there are polynuclear Al-hydroxy species, including

Al2(OH)24+ Al6(OH)12

6+ [AlO4Al12(OH)24]+7

which may exist in solution (or suspension) and be adsorbed onto mineraland organic surfaces.

Mineral phases that, together with cation exchange, control the solubility of Al3+

in acid soils are:

Gibbsite and higher solubility forms, e.g., soil- and microcrystalline-gibbsiteKaolinite and higher solubility formsSmectite (beidellite)Hydroxy-interlayered vermiculite (HIV)

Activity ratio diagrams [(Al-mineral) / (Al3+)] may be constructed.

For (Al-mineral) = 1, the ordinate is the same as pAl and the below figureshows that data for three soil orders (broad classification groups) of acidicsoils, (Al3+) generally falls within stability ranges for gibbsite, kaolinite, beidellite and HIV.

Redox Effects

Anaerobic respiration with oxidized species as electron acceptors tendsto consume H+, raising the pH of acid soils towards neutral pH.

Transitory effect, therefore,lower pE / Eh reactionsnot important.

However, pockets of anoxic conditions may exist in otherwise oxicsoil.

pE OK forNO3

- reductionat interior ofaggregate.

Micro-electrode.

Neutralization of Soil Acidity

For most plants, optimal pH range is ~ 5.5 – 7.0.

The solubility of several metal micronutrients is very low at higher pHs, limiting plant growth / health.

At more acidic pHs, availability of Ca, Mg and K is limited. Conversely, Al solubility may reach a toxic level. If the soil is wet sufficiently long to give anaerobic conditions, increased solubility of Fe(II) and Mn(II) at acid pH may be toxic.

Activity of beneficial microbes is highest at near neutral pH.

Therefore, must adjust pH up, using CaCO3 / MgCO3, Ca(OH)2 / Mg(OH)2, orCaO / MgO. Advantage of the hydroxide or oxide forms is faster dissolutionand reaction with H+ consuming it and precipitating Al3+ as Al(OH)3.

While reaction occurs in the solution phase, decreased activities of H+ andAl(OH)x

(3-x)+ in solution, coupled with increased concentration of Ca2+, favors replacement of H+

ads and Al(OH)(3-x)+ads with Ca2+. The acidic species in

solution are then neutralized by solution base.

Using CaCO3 as the example lime material,

2Al3+ads + 3Ca2+ = 2Al3+ + 3Ca2+

ads

3CO32- + 6H2O + 2Al3+ = 3CO2 + 2Al(OH)3

____________________________________

2Al3+ads + 3Ca2+ + 3CO3

2- + 6H2O =

2Al(OH)3 + 3CO2 + 3Ca2+ads

The amount of lime needed to adjust soil pH from an initial value to a targethigher value is the lime requirement. Determined by test.

Although not a lime material, gypsum, CaSO4 2H2O can be used to reducesubsoil Al3+. The cation exchange reaction is as shown above and displacedAl3+ is then subject to leaching loss.

To a small extent, adsorption of SO42- increases pH by

-Al-OH + SO42- = -Al-OSO3

- + OH- or

-Al-OH + H+ = -Al-OH2+

-Al-OH2+ + SO4

2- = -Al-OSO3- + H2O

The inner sphere complex may bond to an adjacent Al-OH or Al-OH2+ site to

form a bi-nuclear bridging complex, increasing the strength of adsorption.

These reactions with SO42- tended to off-set acidification by H2SO4 deposition,

however, where levels of atmospheric deposition have been greatly reducedin recent times, the reverse of these reactions is thought to be occurring so that the effect of H2SO4 deposition lingers.

Assigned problems: 11 and 12