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Determination and Isotope-Ratio Analysis of Different Forms of Nitrogen in Soils: 6. Mineralizable Nitrogen¹

ABSTRACT

An incubation method of estimating mineralizable soil nitrogen which does not require preliminary analyses to determine the amount of water required for incubation and is applicable without modification to soils having widely different textures is described. It is based on the finding that the amount of water required for maximal aerobic mineralization of nitrogen during incubation of soils is practically the same for different soils (ca. 0.6 ml/g of soil) if the soil sample is mixed with three times its weight of 30- to 60-mesh quartz sand before incubation. The method involves determination of the (exchangeable ammonium + nitrate + nitrite)-N produced when 10 g of soil mixed with 30 g of sand are treated with 6 ml of water and incubated at 30C for 14 days under conditions which ensure adequate aeration without loss of water. It is simple and precise, and evaluation tests indicate that it provides a good index of soil nitrogen availability.

¹ Journal Paper No. J-5383 of the Iowa Agr. and Home Econ. Exp. Sta., Ames. Projects 1070 and 1450.

² Research Associate and Professor, respectively, Department of Agronomy, Iowa State University, Ames.

Received for publication April 18, 1966. Accepted for publication September 12, 1966.