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 Ca 2+, Mg 2+, 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., Al 3+, AlOH 2+, 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 + H 2 CO 3 = H + + HCO 3 - Not set by atmospheric P CO2 but by higher P CO2 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-NH 2 to NH 4 + then NH O 2 = NO H 2 O + 2H + Net production of H + (2 – 1). H 2 S may be released and it is quickly oxidized as H 2 S + 3/2 O 2 + H 2 O = SO H +

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 CH 3 COOH N NH 4 NO N HNO N NH 4 F M 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 B x+ ads + XH + = XH + ads + B x+ is favored. Concurrently, leaching tends to deplete the soil of B x+, favoring replacement of exchangeable B x+ and reducing the BS. Off-setting processes This is partially off-set by return of B x+ from plant residue and anaerobic respiration (if important), e.g., ¼ CH 2 O + ½ MnO 2 + H + = ½ Mn 2+ + ¼ CO 2 + ¾ H 2 O(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 / cm 3, what is the annual decrease in BS in the upper 20 cm of soil? mass soil = 1.5 kg dm -3 x ( dm 2 x 2 dm) = 3 x 10 6 kg cmol(+) H+ kg -1 yr -1 = (20,000 g / 32 g mol -1 ) x 2 x 100 cmol mol -1 / 3 x 10 6 kg = 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 NH 4 + fertilizers –H + generated from nitrification Addition of organic fertilizers –low content of N, therefore high rates applied H 2 CO 3, organic acids, HNO 3 and H 2 SO 4 produced How much poultry 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 H 2 SO 4

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 Ca 2+ 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 10 5 cmol(+)Ca 2+ / 2 x 10 6 kg = 0.1 cmol(+)Ca 2+ 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[Ca 2+ ] + 2[Mg 2+ ] – [Cl - ] – [NO 3 - ] - 2[SO 4 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 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 CO 2, carbonate equilibria would exist with [H + ], [OH - ], [HCO 3 - ] and [CO 3 2- ] present. Charge balance requires [H + ] + [Na + ] + [K + ] + 2[Ca 2+ ] + 2[Mg 2+ ] - [HCO 3 - ] - 2[CO 3 2- ] - [OH - ] - [Cl - ] - [NO 3 - ] - 2[SO 4 2- ] = 0 So that [H + ] - [HCO 3 - ] - 2[CO 3 2- ] - [OH - ] + ANC = 0or ANC = - [H + ] + [HCO 3 - ] + 2[CO 3 2- ] + [OH - ] Other species such as Al 3+, AlOH 2+, Al(OH) 2+ and L - (generalized organic ligand) may be included. ANC = - [H + ] - 3[Al 3+ ] - 2[AlOH 2+ ] - [Al(OH) 2 + ] + [HCO 3 - ] + 2[CO 3 2- ] + [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 mol c kg om -1 pH -1 Add 1mmole of H + to 1 kg of a soil with β = 0.2 and 2 % organic matter ΔpH = mol / (0.2 mol c kg om -1 pH -1 x kg om ) = 0.25 pH unit

From ΔpH = Δn H / β = Δn H / f om β om 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 matter and soil minerals, reactions involving Al 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 Ba 2+. A portion remains bound and so is measured by displacement with Ba 2+ in OH - background. This allows titration of residual OH - to quantify total extracted acidic species. 2H + ads + Ba 2+ = 2H + + Ba 2+ ads H + + OH - = H 2 O 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 NH 4 OAc, 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 Al 3+ forms are exchangeable (exchangeable acidity). Exchangeable acidity is negligible above pH ~ 6.

Comments on Al Chemistry in Acid Soils Clearly, various monomeric Al 3+ 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 Al 2 (OH) 2 4+ Al 6 (OH) [AlO 4 Al 12 (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 Al 3+ 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) / (Al 3+ )] 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, (Al 3+ ) 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 NO 3 - 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 CaCO 3 / MgCO 3, 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 Al 3+ 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 Ca 2+, favors replacement of H + ads and Al(OH) (3-x)+ ads with Ca 2+. The acidic species in solution are then neutralized by solution base.

Using CaCO 3 as the example lime material, 2Al 3+ ads + 3Ca 2+ = 2Al Ca 2+ ads 3CO H 2 O + 2Al 3+ = 3CO 2 + 2Al(OH) 3 ____________________________________ 2Al 3+ ads + 3Ca CO H 2 O = 2Al(OH) 3 + 3CO 2 + 3Ca 2+ 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, CaSO 4 2H 2 O can be used to reduce subsoil Al 3+. The cation exchange reaction is as shown above and displaced Al 3+ is then subject to leaching loss.

To a small extent, adsorption of SO 4 2- increases pH by -Al-OH + SO 4 2- = -Al-OSO OH - or -Al-OH + H + = -Al-OH 2 + -Al-OH SO 4 2- = -Al-OSO H 2 O The inner sphere complex may bond to an adjacent Al-OH or Al-OH 2 + site to form a bi-nuclear bridging complex, increasing the strength of adsorption. These reactions with SO 4 2- tended to off-set acidification by H 2 SO 4 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 H 2 SO 4 deposition lingers. Assigned problems: 11 and 12