Nitrification of ammoniacal fertilizers in some California soils

Because of the manner in which ammonium fertilizers are applied and the tendency of the ammonium ion as well as of free ammonia to be absorbed at the point of contact with soil colloids, localized high concentrations are the rule rather than the exception. A concentration gradient is produced with a maximum along the line of application. In experiments intended to simulate a cross section of a fertilizer band, it was shown that nitrification is rapid and complete at low ammonium concentrations such as would occur at the edge of a band. At higher concentrations, inhibition of nitrification may result from one or more causes: (1) excessively high pH resulting from application of alkaline materials; (2) excessively low pH resulting from formation of nitrous and nitric acids; (3) presence of free ammonia, exerting selective inhibition on the nitrate-forming bacteria; (4) salt effect, resulting in osmotic concentrations too high for optimal activity of nitrifying bacteria. The data of these experiments show that, under optimum conditions, nitrification is an extraordinarily rapid process. This fact should prove helpful in interpreting numerous field observations involving either response or lack of response to ammonium fertilizers. If moisture, pH, and temperature conditions of the soil are satisfactory, it may be assumed with confidence that nitrate will be produced from an added ammonium fertilizer in sufficient quantity to keep pace with the needs of growing plants in most agricultural soils.