A SIMPLE AVAILABLE SOIL NITROGEN INDEX

Available soil-N values in commonly cultivated East African soils were determined in the laboratory by several methods, and correlated with one another. Mineralizable-N was correlated more with total-N than with organic-C. The values of mineralizable-N (Δ mineral-N) obtained on aerobic incubation were correlated with crop yield and nitrogen uptake in greenhouse experiments. The yield of dry matter and N uptake by the tomato crop in the absence of applied nitrogen were significantly correlated with Δ mineral-N in the soils; the degree of correlation decreased with increasing rates of fertilizer-N applied and Δ mineral-N and dry-matter yield were not correlated at the largest rate used. N uptake by the crop was significantly correlated with Δ mineral-N at all rates of applied nitrogen. The Δ mineral-N value determined on soil samples conditioned by 6 weeks incubation followed by 3 weeks air drying was considered to provide a good index to available nitrogen.