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Predicting Nitrogen Fertilizer Needs of Iowa Soils: I. Evaluation and Control of Factors in Nitrate Production and Analysis¹

ABSTRACT

In order to adapt nitrate production for use in soil testing laboratories for the purpose of evaluating the capacity of soils to supply nitrogen to crops, a procedure was evolved which is accurate yet inexpensive and can be determined on a mass production basis. The influence of environmental factors on nitrate production during incubation in the laboratory was studied along with methods of sample manipulation to increase the time efficiency in analysis.

Ten-gram samples of soil were found to give accuracy comparable to 25- and 100-gram samples provided moisture was controlled during incubation. Moisture losses can be closely controlled without creating suboptimum aeration by stoppering the sample containers with one-hole rubber stoppers and incubating in an atmosphere nearly saturated with water. High humidity was maintained by placing flat open ceramic vessels filled with water in the top and bottom of the incubator.

Nitrate production took place over a fairly wide range of soil moisture. Adjustment of moisture by tension methods proved more satisfactory than by the addition of constant quantities per sample. Of the tensions studied, 100 cm. water provided the most optimum moisture for the production of nitrates. This resulted in 25 to 35% moisture, depending on the texture of the samples and was a little above the field capacity.

Automatic pipettes for adding the leaching solution, a specially constructed shaker for dispersing the soil which accommodated from one to four sample trays, simultaneous filtration of all samples in a tray and adding the solutions for color development by the use of automatic pipettes all served to increase the time efficiency of the nitrate production method and adapt it to soil testing needs.

¹ Journal paper No. J-2544 of the Iowa Agr. Exp. Sta., Ames, Iowa, Project No. 1190.

² Assistant Professor, Associate Professor and Technician in Soils, respectively. Senior author is now Director, Soil Testing Division, North Carolina Department of Agriculture and Professor of Agronomy, North Carolina State College, Raleigh, N. C.

Received for publication December 29, 1953.