Net mineralization of N in 39 widely differing soils was determined over a 30‐week period at 35C, using incubation intervals of 2, 2, 4, 4, 4, 6, and 8 weeks. Mineral N was leached from the soils before the first incubation and following each of seven incubations by means of 0.01M CaCl2 and a minus‐N nutrient solution. Soil water contents were adjusted by applying suction (60 cm Hg), and losses of water during incubation under aerobic conditions were negligible. With most soils, cumulative net N mineralized was linearly related to the square root of time, t½. The pH of soils changed very little in the course of 30 weeks' incubation. Because of the generally consistent results, the data were employed in calculating the N mineralization potential, No, of each soil, based on the hypothesis that rate of N mineralization was proportional to the quantity of N comprising the mineralizable substrate. Values of No ranged from about 20 to over 300 ppm of air‐dry soil. The fraction of total N comprising No varied widely (5 to 40%) among soils. Mineralization rate constants did not differ significantly among most of the soils. The most reliable estimate of the rate constant, k was .054 ± .009 week‐1. The time required to mineralize one‐half of No, t½, was estimated to be 12.8 ± 2.2 weeks. Results suggest that the forms of organic N contributing to No were similar for most of the soils.

The relationships between soil N mineralization, soil water content, and matric suction were studied with nine soils of widely differing chemical and physical properties. Highest N mineralization rates occurred between matric suctions of ⅓ to 0.1 bar, in which range 80 to 90% of the total pore space was filled with water. In the range from optimum soil water content (⅓ to 0.1 bar) to 15 bars, a near‐linear relation generally existed between amounts of mineral N accumulated and soil water contents (percent of oven‐dry soil). With increasing dryness, N mineralization continued to decline. Water levels above optimum often reduced mineral N accumulations, presumably because of denitrification.Upon expressing the values for N mineralization (Y) and soil water content (X) for each soil on a relative basis with respect to maximum N mineralized (Y = 100) and associated optimum soil water content (X = 100), the regression of Y on X did not differ among soils. Regression coefficients for common (based on covariance) and total regressions, respectively, were 1.07 and 1.02. Corresponding Y‐intercepts were −2 and −4. A reasonable approximation of the relationships is expressed by Y = X. Possible application of the findings in predicting N mineralization under fluctuating soil water conditions is discussed.

During the past decade, the percentage of the corn (Zea mays L.) acreage in the USA receiving N fertilizer has risen steadily. By 1971, in the Corn Belt and five adjoining states, this proportion ranged from 93 to 100% of the total corn acreage. Between 1964 and 1970 the average rate of N applied to fertilized acres increased about 83% in the Corn Belt and 128% in adjacent states (Nebraska, Kansas, Michigan, Wisconsin, and Minnesota). Undoubtedly, there has been an accompanying increase in the proportion of the corn acreage receiving optimum to excessive amounts of N fertilizer. These trends emphasize the importance of developing improved procedures for achieving optimum fertilizer N use (i.e., adequate but not excessive rates, and proper timing for greater efficiency) consistent with the goal of minimizing the possibility of environmental pollution.The basic information required in predicting optimum use of N includes: (i) the internal N requirement of the crop for expected attainable yield, (ii) the amount of soil N mineralized during the cropping season, (iii) the amount of residual mineral N present the root zone early in the cropping season, and (iv) the expected efficiency of recovery of the plant-available N supply. With good management, efficiency, or % recovery of applied N by the grain and stover often is in the range of 50 to 70%. In this range, at nearoptimum rates of N, essentially all of the unrecovered fertilizer N is subject to immobilization during decomposition of plant residues (stover and roots). At higher than optimum rates, however, significant portion of the nitrate remains mobile and susceptible to loss by leaching or denitrification. Additional Index Words: denitrification, fertilizer N efficiency, ground water pollution.

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