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 this
fraction, 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 of
basic 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+ + HCO3Not 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 cations
and anions by plants. Excess uptake of cations balanced by release of H+.
The purpose of using a chemical extractant in soil tests of nutrient availability
is to mimic the chemical environment of the rhizosphere. A widely used
extractant 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+, favoring
replacement of exchangeable Bx+ and reducing the BS.
Off-setting processes
This is partially off-set by return of Bx+ from plant residue and anaerobic
respiration (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 climate
forces 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 continuous
since 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 Ca
and 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[SO42-]
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 increase
the concentration of cations.
Concept is applicable to water bodies but may be considered with respect
to the soil solution even though the composition of the soil solution is largely
controlled 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[CO32-] - [OH-] - [Cl-] - [NO3-] - 2[SO42-] = 0
So that [H+] - [HCO3-] - 2[CO32-] - [OH-] + ANC = 0
or
ANC = - [H+] + [HCO3-] + 2[CO32-] + [OH-]
Other species such as Al3+, AlOH2+, Al(OH)2+ and L- (generalized organic
ligand) 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 of
the 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.
Besides pH buffering by adsorption
and desorption of H+ by organic matter
and soil minerals, reactions involving
Al affect β.
pH-dependent
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 and
so is measured by displacement with Ba2+ in OH- background. This allows
titration 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+ forms
are 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+. Some
are exchangeable whereas a portion remains bound under extraction with
neutral salt. Besides monomeric forms, there are polynuclear Al-hydroxy
species, including
Al2(OH)24+
Al6(OH)126+
[AlO4Al12(OH)24]+7
which may exist in solution (or suspension) and be adsorbed onto mineral
and 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-gibbsite
Kaolinite and higher solubility forms
Smectite (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 figure
shows that data for three soil orders (broad classification groups) of acidic
soils, (Al3+) generally falls within stability ranges for gibbsite, kaolinite,
beidellite and HIV.
Redox Effects
Anaerobic respiration with oxidized species as electron acceptors tends
to consume H+, raising the pH of acid soils towards neutral pH.
Transitory effect, therefore,
lower pE / Eh reactions
not important.
However, pockets of
anoxic conditions may
exist in otherwise oxic
soil.
pE OK for
NO3- reduction
at interior of
aggregate.
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, or
CaO / MgO. Advantage of the hydroxide or oxide forms is faster dissolution
and reaction with H+ consuming it and precipitating Al3+ as Al(OH)3.
While reaction occurs in the solution phase, decreased activities of H+ and
Al(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+ + 3CO32- + 6H2O =
2Al(OH)3 + 3CO2 + 3Ca2+ads
The amount of lime needed to adjust soil pH from an initial value to a target
higher value is the lime requirement. Determined by test.
Although not a lime material, gypsum, CaSO4 2H2O can be used to reduce
subsoil Al3+. The cation exchange reaction is as shown above and displaced
Al3+ 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+ + SO42- = -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 reduced
in 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