A Laboratory Method for Evaluation of Available Nitrogen in Soil

Abstract

A simple and rapid laboratory method of evaluating the N status in soil was proposed. Four grams of air-dried soil or the equivalent amount of air-dried soil or the equivalent amount of fresh soil are heated for 20 h at 80.degree. C in 40 ml 2 M KCl. The soil extract contains the original inorganic N in the soil plus the ammonium (.DELTA. NH4-N) released from the organic matter. This amount of N is nearly the same as inorganic N (NO3-N) in fresh soil after aerobic incubation for 14 days at 30.degree. C. The correlation between these 2 N-indexes for 43 soil samples, with great variations in organic matter, was very good (r = 0.98). This indicates that the analytical results from the proposed method might provide good information about available N in the soil. The content of NH4-N found by the proposed method and .DELTA. mineral N found by aerobic incubation were also of the same order of magnitude and the correlation between them was good (r = 0.92). The analytical results referred to dry matter were the same for fresh and air-dried samples. Variation in soil:KCl ratio between 1:5 and 1:40, and shaking during the heating period had only small influence on the results. The NO3-N content of the soil was not affected by heating in 2 M KCl. The content of NO3-N increased during storage of fresh soil while the released NH4-N after the proposed method was not affected by storage at all. Nitrate and ammonium were determined colorimetrically in 2 M KCl extracts using automatic methods. The precision of the method was good. The variation coefficient for 43 soil samples was 3.6%.