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The effect of ammonium sulfate, ammonium chloride, potassium chloride, calcium chloride, aluminum chloride, and copper sulfate added in amounts corresponding to 0.2, 2, and 20 atm concentration on mineralization of soil nitrogen in three soils was measured over a 30‐day period. Significant increases were produced by the ammonium salts which were labeled with 15N to allow distinction between added and soil nitrogen. In Aiken clay loam nitrogen mineralization was also increased by solutions of potassium chloride, calcium chloride, and aluminum chloride, but in Altamont clay loam and Mormon clay increases were not consistent. Copper sulfate depressed nitrogen mineralization in all soils. It was found that the quantity of organic nitrogen brought into solution was related to the concentration of the salt solution.

Three ammonium‐tagged fertilizers, two nitrate‐tagged fertilizers, and N‐tagged urea were added to three soils in green house pots. Plant uptake of surface‐applied fertilizer N was measured by growing four crops over a period of 314 days. In another experiment, tagged ammonium sulfate, ammonium nitrate, and aqueous ammonia, were compared using band application. With surface application, uptake of fertilizer N in plant tops varied from 35 to 71%, with nitrate sources showing the higher values. Residual N in soil ranged from 18 to 41%, and its availability decreased with successive crops. All ammonium sources suffered some loss with surface application, especially aqueous ammonia, but losses were negligible with band application. Availability of soil N was influenced to some extent by the nature of the fertilizer applied. From 5 to 13% of the soil N was utilized by four crops.

The term nitrogen reversion is suggested as applying to the conversion of fertilizer nitrogen to organic forms, which become progressively less available as a function of time. In greenhouse pot experiments with several soils, nitrogen availability ratios were found to decrease with successive cuttings of sudangrass Sorghum vulgarevar. sudanensisThis ratio is defined as the percentage of tagged N in the crop divided by the percentage of tagged N in the so l at the onset of crop growth. Similarly, the time required for mineralization of 1% of the residual fertilizer N in soil was found to increase with successive cuttings of grass. Where straw was added to promote immobilization as much as two‐thirds of the fertilizer N remained in soil after nearly 1.5 years of continuous cropping. It is suggested that nitrogen reversion cannot be accounted for solely on the basis of biological interchange, and that a non‐biological mechanism of N stabilization may be involved.

A series of greenhouse pot experiments was conducted to measure plant uptake of fertilizer N in the presence and absence of straw. Soils were incubated with straw for periods up to 12 weeks to overcome the depressing effect of straw and then cropped. Two or more crops were evaluated for each treatment. In spite of preincubation, plant recovery of N was invariably lower where straw had been added, even when the level of fertilizer N added was equivalent to 2.5% of the weight of straw. The presence of growing plants on the soil during the period immediately following application of fertilizer N resulted in more efficient utilization of fertilizer than where planting was delayed several weeks after fertilizer application. This was true whether straw was added or not. A tracer experiment lasting nearly 2 years indicated progressive stabilization of fertilizer N after incorporation into the soil organic fraction.

Nitrogen immobilization in flooded soils amended with straw was measured by following incorporation of tagged inorganic N into the soil organic fraction. Increases in tagged organic N were found to be highly correlated with net immobilization as measured by decrease in mineral nitrogen in the soil. Since most of the soils used contained clay minerals capable of ammonium fixation, apparent immobilization figures were corrected by subtraction of clay‐fixed ammonium. In comparisons of nitrogen immobilized under different soil conditions it was found that values obtained in flooded soils were intermediate between those obtained under aerobic and anaerobic conditions. The nitrogen factor, or additional N immobilized, per unit weight of straw, was found to vary with quantity of straw added, nitrogen content of the straw, and nature of the soluble nitrogen supplied, but in no case was it negligible. Rice straw (Oryza sativaL.) containing 0.47% N immobilized an additional 0.51% N in a flooded soil, and another sample of rice straw containing 1.17% immobilized 0.43% N.