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Available Soil Nitrogen Measurements by Microbiological Techniques and Chemical Methods¹

ABSTRACT

Microbiological and chemical techniques for assaying available soil nitrogen were evaluated using 30 soil samples selected from Georgia, Iowa, New Jersey, and Pennsylvania. The nitrifying capacity of these soils, as measured by incubation under controlled conditions, was used as the standard for comparison.

A microbiological techniques was devised, using a strain of the proteinaceous bacterium Pseudomonas aeruginosa. Production of a pigment, pyocyanin, by this organism during growth in a medium in which soil served as the nitrogen source was used as the criterion for measuring available soil nitrogen. With this technique, a highly significant negative correlation was obtained between pigment production and nitrifying capacity for the 30 soil samples studied. The method has the advantage that only 4 days are required for the assay, in contrast to 2 to 8 weeks for the determination of nitrifying capacity.

The microbiological technique was compared with the chemical methods of Truog and Purvis. In the Truog procedure, ammonia is distilled from soils in the presence of alkaline permanganate. The Purvis method involves hydrolysis and mild oxidation with dilute H₂SO₄ followed by ammonium ion determination by Nesslerization. Both chemical methods yielded highly significant correlation with nitrifying capacity. The correlation between the two chemical methods also was highly significant. The Truog method and the microbiological technique exhibited a negative correlation of high significance.

¹ Authorized for publication on April 5, 1961, as Paper No. 2548 of the Journal Series of the Pennsylvania Agr. Exp. Sta., University Park. Presented before Div. III, Soil Science Society of America, Dec. 5, 1960, at Chicago, Ill.

² Former Graduate Assistant in Agronomy, The Pennsylvania State University, now Assistant Soil Chemist, Georgia Exp. Sta., Experiment, Ga.; Professor of Soil Technology and Assistant Professor of Bacteriology, The Pennsylvania State University, respectively.

Received for publication April 23, 1961. Accepted for publication August 28, 1961.