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

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.

Ion Gradients and Nitrification Associated with Decomposition of a Plant Material Layer in Soil

“…2 support the hypothesis made in a previous paper (8) concerning the effect of CEC on the toxicity of ammonia from decomposing particles of plant material in soil. The presence of extensive toxic zones assodated with ammonia released from decomposing layers of plant material make it reasonable to assume that toxic zones may also exist around individual particles of decomposing plant material in soil.…”

“…For example, in the no-lime treatment the pH varied with distance from the plant material layer from 8.9 to 5.6. This alone would change the relative proportions of NH 3 in solution from almost 50% of the total ammonia to < 0.1% (8,10) and could thereby change an extremely toxic condition near the plant material layer to a nontoxic condition at a greater distance. The same p1-1 change could also markedly alter the availability of essential plant nutrients.…”

“…Ammonia accumulation can be toxic to nitrification (3,4,5,9). In a previous paper (8) the toxicity of ammonia from particles of plant material in soil was related to the cationexchange capacity (CEC) of the soil. A decrease in CEC by sand dilution of the soil decreased the capacity of the soil to adsorb ammonia and increased ammonia toxicity.…”

“…All the other soil layers were divided into two parts. Ammonia was extracted from the first with 20 ml of acidic KaSO4 (8) and analyzed by the Conway microdiffusion technique in a manner similar to that described by Bremner and Shaw (2). The second part was used for pH, NO2-, NO2-, and nonwater-extractable K` determinations.…”

Nitrogen Gradients and Nitrification Associated with Decomposing Corn Plants and Barley Straw in Soil

“…All the other layers were mixed and divided into two parts. Ammonia was extracted from the first part of the soil layer with 20 ml of acidic [(,SO4 (6) analyses for ammonia, NO2-, NO3-, and pH were made on the remaining part of the soil layer. Analyses were made on moist samples and converted to dry soil equivalent values.…”