Nitrogen is the most widely needed and the most widely used of the fertilizer elements. Yet, in spite of the voluminous research work of evaluating the nitrogen status of soils, laboratory tests for the purpose of predicting need of nitrogen fertilizer are not very widely used. In general, laboratory tests for nitrogen supplying power have not given satisfactory results. Methods currently employed by a few laboratories can be classified either as biological or chemical. In the former method, the soil is incubated under optimum conditions and the amount of nitrate released over a given period of time is measured and used as an index to the nitrogen supplying power of the soil. In the latter method a portion of the total nitrogen is released by chemical means and measured. An attempt is made using chemical methods to release the same amount or a constant portion of the amount of nitrogen that normally would be furnished to a growing plant by the soil during one season.