Soil Acidity and pH Causes, remediation, and measurement

Soil pH alteration (naturally and manmade) Management and land use –Fertilizers, organic matter, and other amendments –Submergence and subsequent uplift of land exposing reduced sediments to oxidation processes Pollution –Acid rain –Mining Climate –Weathering and leaching –Rainfall leaching –Plant growth: uptake of cations and release of protons Metal hydrolysis

Climatic effects Excessive rainfall: Leaching of cations through the soil profile by rain, weathering of the soil –Carbonation; hydrolysis; hydration… Excessive irrigation: unlikely cause of acidity since most irrigation occurs in arid or semi-arid regions with accumulated salts, carbonates, etc (buffer pH) –Most irrigated regions are neutral to alkaline (they are irrigated because there isn’t enough rain to support crops, therefore the salts and cations don’t leach out of soils)

Carbonic acid formation forms in rainwater or soil water CO 2 + H 2 O  H 2 CO 3 H 2 CO 3  H + + HCO CO 2 + H 2 O  H + + HCO 3 - [CO 2 ] is higher in soils than aboveground Most unpolluted rainfall is slightly acidic

As CO 2 concentration increases, proton (H + ) production increases and pH decreases Soda pop or carbonated beverages have pH 3 - 4

Metal hydrolysis Polyvalent metals go through several hydrolysis steps releasing protons Alum (KAl(SO 4 ) 2 ) is a commercial product for lowering pH

Hydrolysis of Al +3 H 2 O  OH - + H + Al +3 + H 2 O  Al(OH) +2 + H + Al(OH) 2+ + H 2 O  Al(OH) H + Al(OH) H 2 O  Al(OH) 3 + H + Al(OH) 3 + H 2 O  Al(OH) H +

Increasing soil pH Burning plant residues or adding ashes –Wood ashes are a source of K, Ca, Mg CO 3 ’s Liming materials (consume H + and provide Ca 2+ ) Gypsum (calcium sulfate) is not a liming agent and has very little effect on soil pH. Gypsum may improve structure in soils that have extremely high sodium contents

More ways to increase soil pH Denitrification (reduction under anaerobic conditions) 5CH 2 O + 4NO H +  5CO 2 + 2N 2 + 7H 2 O Sulfate reduction SO CH 2 O + H +  2CO 2 + HS - + H 2 O

Factors affecting soil pH measurement 1. Soil:Water ratio –Saturated paste, 1:1, 1:2, 1:5, 1:10 Adding more water dilutes the [H + ] and usually causes a slight rise in pH due to “Dilution effect” –The amount of increase is a function of soil texture and porosity –Not linear (More water = more dissociation of H + from soil surfaces and more hydrolysis of Al +3 ) Important to report how pH was measured (both instrument and method) –Some pH measurements shouldn’t be compared directly e.g., 1:1 ≠ 1:10 or pH water ≠ pH KCl

pH Soil:water ratio 1:11:5 1: Brazito Sandy Loam pH CaCl KCl 7.1

pH determines the soil's "active" acidity. The ESTL uses 1:2 (Soil:water) volume ratio for determining soil pH. This is the acidity that affects the plant roots.

1. Salts in suspension to measure pH –Dissolved salts from saline soils –Fertilizers –CaCl 2 or KCl added for pH measurement (pH CaCl2 or pH KCl ) As salt concentration increases, pH decreases (but not necessarily linearly) –Displace Al +3 or H + from exchange sites –Hydrolysis

3. Time of sampling and season Before or after summer rains, irrigation, or leaching Before or after fertilization During warming or cooling trends Processing and storage of soil –The pH of a soil that has been dried out and sitting around will be different from a fresh sample.

Accuracy of pH measurements A measured soil pH can not really be much more accurate than +/- 0.5 pH unit. This means worrying about the last decimal point in a soil pH of 6.35 is not worth it when the same soil might vary seasonally between 5.8 and 6.8 It's not the pH being wrong, it's just the nature of soil.

4. CO 2 concentration and pH As [CO 2 ] increases, the pH decreases Formation of carbonic acid Stirring of sample may decrease pH Soil [CO 2 ] is MUCH higher than atmospheric [CO 2 ] Affects in-situ measurement of soil pH in calcareous soils. –Measure pH immediately or –Minimize headspace in sample containers and put on ice

5. Buffering capacity Soils high in SOM and clay minerals are more resistant to change in pH Sandy soils and highly weathered soils are least buffered Base Saturation =  exchangeable bases CEC BS = (exch Ca + Mg + Na + K) (exch Ca + Mg + Na + K + Al + H)

6. Lime Requirement Amount of CaCO 3 needed to increase the pH of the soil to an optimum pH Depends on soil mineralogy, % clay fraction, % OM, cultivation practices (leaching, fertilization, etc) Variety of liming materials Only practical to raise pH to ~6 (KCl- extractable acidity is ~0)

Lime material CaCO 3 calcic limestone CaMg(CO 3 ) 2 Dolomite CaO: Quick lime CaOH calcium hydroxide Byproducts: ground shells, cement factory waste

Lime characteristics –cost –purity –speed of effect (fine ground vs coarse) –ease of handling Lime requirement –depends on pH, CEC and buffer capacity of the soil Lime Application: small amounts split and incorporated into the soil Liming to increase soil pH

To increase pH from 6 to 7 requires more lime than from 4 to 5