Mineralizable Nitrogen in Soils in Relation to the Response of Oats to Nitrogen Fertilization

D the importance of nitrogen as a factor limiting crop yields, the estimation of nitrogen fertilizer needs by means of soil analysis is a subject that has received relatively little attention. Recourse has been had to plant-physiological methods rather than to methods of soil analysis. Among the plantphysiological methods may be mentioned those of Mitscherlich (io) who determined the yield in pot experiments with different increments of nitrogen, v. Doby (4) who determined the invertase activity of rye seedlings, Gassner and Goeze (5) who determined the chlorophyll content of plants, and Macy (9) and Pfeiffer, Simmermacher, and Rippel (13) who determined the nitrogen content of plants. The earliest methods of soil analysis for available nitrogen involved the determination of ammoniacal and nitrate nitrogen. These methods proved unsatisfactory, however, because the quantities of ammoniacal and nitrate nitrogen in soils are ordinarily small and rather variable and do not necessarily reflect the total quantity of nitrogen that will be at the disposal of the crop during the growing season. Methods were needed that would extract a quantity of nitrogen more nearly like that utilized by the crop. Several methods of this type have been proposed. Konig, Coppenrath, and Hasenbaumer (8) heated soils in an autoclave at 5 atmospheres pressure for 5 hours and determined the quantity of nitrogen extracted. Results obtained by this method did not correlate well with the yield or the nitrogen content of plants grown in pot experiments. The possibilities of the method were not adequately tested, however, because the crops appeared to have ample nitrogen in all cases. Tyurin (15) and Tyurin and Kononova (16, 17) determined the quantity of nitrogen liberated from the soil organic matter upon hydrolysis with dilute sulfuric acid and claimed that this procedure distinguished organic matter decomposable by microorganisms from organic matter stable in that respect. The quantity of nitrogen in the acid hydrolysate was correlated with the nitrogen needs of crops in field experiments. The method was tested further. by Gracie and Khalil (6) who found likewise that the acid-hydrolyzable nitrogen was related to the nitrogen needs of crops in field experiments. Several investigators ha*'e. used a different approach, .determining the quantity of nitrogen mineralized in soils during a period of incubation under conditions of moisture and temperature favorable for microbiological activity. This method has the advantage of being less artificial than are the steam-heating or acid-hydrolysis methods mentioned, but has the disadvantage, as ordinarily used, that more time is required. One of the first to use the incubation method was Bogdanov (2, 3) who incubated soils for 48 hours at 30° C and then determined the quantity 'of ammonia and nitrate nitrogen present in the soils. The results obtained correlated favorably with the yield of crops in pot experiments. Varallyay (18) and Varallyay and Fejer (19) recommended that an incubation period of at least a month should be used. Varallyay (18) reported that the incubation method for determining available nitrogen furnished a satisfactory estimate of the nitrogen needs of soils in Hungary. Hardy (7) found that the quantity of nitrate nitrogen produced in soil samples during incubation for 4 weeks at 30° C was inversely related to the responsiveness of cotton to nitrogen fertilization in field experiments in the British West Indies. In a previous paper (i), the authors gave the results of wheat fertilization experiments conducted during 1944 to 1946 in western Iowa in which the mineralizable soil nitrogen was determined by incubating the soils for 3 weeks at 30° C. In these experiments the quantity of nitrogen mineralized in the soils during incubation was directly related to the yield of unfertilized wheat on the plots that received no nitrogen fertilizer, and was inversely related to the increase in yield produced by the application of 20 pounds of nitrogen per acre. A considerable number of nitrogen fertilization experiments on oats have been conducted on the more important soil types in Iowa during 1944 to 1947. In the present paper the results of analyses for mineralizable nitrogen made on soil samples from these experiments are summarized, together with data on the yields and responses of oats to nitrogen fertilization.