Relationships Between Soil Cation‐Exchange Capacity and the Toxicity of Ammonia to the Nitrification Process

Abstract

Abstract Nitrification was studied in Red Bay sandy loam and Iredell clay loam treated with increments of sand to produce a range of cation‐exchange capacities (CEC). The soils were treated with 1% additions of alfalfa particles of three particle sizes. Analyses were made at intervals for ammonia, nitrate, nitrite, and pH. The nitrification rate of ammonia from alfalfa particles decomposing in soil was decreased when the exchange capacity of the soil was decreased by dilution with sand. The mechanism involves less exchange sites for NH 4 + at low CEC and the resulting increase in soil solution pH producing an NH 3 concentration that is toxic to Nitrobacter . The same effect was produced by decreasing the distance between the alfalfa particles in the soil, thereby reducing the interparticle zones of low NH 3 concentration where nitrification could proceed. A steep ammonia concentration gradient develops around the alfalfa particles when adequate exchangeable hydrogen is present. In the areas between alfalfa particles, where NH 3 concentration is low, nitrification proceeds without inhibition. As the exchangeable hydrogen concentration is reduced or the alfalfa particles are placed closer together, the zones where NH 3 is not toxic decrease in size until most of the soil has reached the concentration of NH 3 toxic to nitrification.