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Relationships Between Soil Cation-Exchange Capacity and the Toxicity of Ammonia to the Nitrification Process¹

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 NH₄⁺ at low CEC and the resulting increase in soil solution pH producing an NH₃ 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 NH₃ 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 NH₃ 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 NH₃ is not toxic decrease in size until most of the soil has reached the concentration of NH₃ toxic to nitrification.

¹ Contribution from the U.S. Soils Laboratory, Soil and Water Conservation Research Division, ARS, USDA, Beltsville, Md.

² Research Soil Scientist.

Received for publication January 13, 1964. Accepted for publication April 10, 1964.