A Simple Available Soil Nitrogen Index

Summary Mineralizable‐N (Δ mineral‐N) was correlated more with total‐N than with organic‐C in topsoils from maize fertilizer trial sites. In the absence of N fertilizers, Δ mineral‐N was correlated with maize yield and per cent N in the leaf, in both seasons. Tests showed that a single regression equation calculated from data for two seasons, could be derived to express both these relationships. Maize yield on plots treated with fertilizer‐N was not correlated with mineralizable‐N in the first season and was correlated only at the lower rate of application (45 kg N/ha) in the second season. But maize‐yield response to both rates of fertilizer‐N was correlated with mineralizable‐N in both seasons. Tests on the regression equations showed that yield responses were subject to seasonal variation. The Δ mineral‐N value determined on conditioned soil samples (see Part 1) provided a good index of available N. It may be used to predict maize yield without N fertilizer and to rank soils, in terms of likely response to fertilizer‐N, for advisory purposes.