Abstract

                                Soil testing for available N is not comparatively as simple as in the case of soil test for availabe P or K. Many indices of N availability have been followed by different workers. These indices mainly fall under three major categories, namely, measurement of the potential source of mineralisable N, measurement of N mineralised by some chemical treatment and the measurement of N mineralised during incubation. Attempts have been made in the past to correlate the N values obtained by one or the other method with crop yield and N uptake. The alkaline permanganate oxidisable N is being used as an index of available N in Tamil Nadu. The present investi- gation was conducted to evaluate the merits of this soil N availability index and also of the four other indices, namely, total soil N, organic carbon content of the soil, nitrate N and ammoniacal N. The relationships between the alkaline permanganate oxidisable N and the other four indices were also studied in order to find out whether the former index could be substituted by the latter indices.

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