Buffer pH Methods : Ongoing Iowa Laboratory and Field Studies

Soil acidity can affect plant growth directly and indirectly by affecting the plantavailability of nutrients, levels of phytotoxic elements, and microbial activity. Soils may become acidic in the long term as a result of several natural processes. In the short term, however, soil acidity develops mainly due to application of N fertilizers or manure, primarily those having high concentrations of ammonium or urea because nitrification releases hydrogen (H) ions. Soil pH decreases as the acidity increases because pH expresses acidity as the negative logarithm of concentration (activity) of H ions. Changing soil pH requires changing the balance of acids and bases in both the soil water and on the cation exchange capacity of the soil. Soils have a capacity to release additional H ions (often referred to as reserve acidity) which depends largely on the concentrations of clay and organic matter. Therefore, soil pH is used to determine whether or not to lime a soil but not to indicate the quantity of acid that should be neutralized. Direct titration of soil acidity seldom is used in the U.S., and a buffer test often is used to determine the lime requirement. Buffer solutions have a large capacity to resist a change in pH. The amount of lime needed to increase pH to a desirable level can be estimated by mixing a buffer of known pH with soil and measuring the resulting pH decrease. Low buffer pH values indicate high reserve acidity and lime requirement. Laboratory tests estimate acidity released over a short period of time, however, and at the field often additional acidity is released over time.