Wheat (Triticum aestivum) varieties ‘Nugaines’ and ‘Lemhi’ were grown on Portneuf silt loam soil with three nitrogen and three irrigation treatments in 1967. After harvest, 84 kg N/ha was applied to half of each plot before plowing. Uniform straw samples were enclosed in fiberglass cloth bags and buried in the plots September 7. The straw placed in N‐treated plots received sufficient N in solution to increase the straw N from about 0.29 to 1.5%. Straw samples were recovered November 15, after the soil had cooled below 4C, and at three later sampling dates to October 3, 1968, after a bean crop (Phaseolus spp.) was harvested. Weight loss and total N were determined on all samplings and total C on the first sampling. Decomposition was greater with N than without it for the November sampling and the March sampling for Nugaines but not at other samplings for both varieties. The weight of N in both N‐treated straw varieties decreased 55% by November 15, while in the non‐N‐treated Nugaines and Lemhi straw, N weight increased 12 and 32%, respectively, by March 22. Later, N moved out of all the straw samples when the N percentages were much lower than the theoretical equilibrium value.

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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.