ON-FARM EVALUATION OF NITRATE-NITROGEN DYNAMICS UNDER MAIZE IN THE NORTHERN GUINEA SAVANNA OF NIGERIA

Nitrate-nitrogen was analysed over two cropping seasons in 57 farmers' fields in the northern Guinea savanna of Nigeria. Differences between fields were at least five-fold and often ten-fold irrespective of fer tilization rates. Average concentrations were highest at the beginning of the rainy season. Loamy soils had a later peak for nitrate release and maintained high concentrations for a longer period after the star t of the rains than the more sandy soils. Nitrate-N was not correlated with soil organic carbon or total nitrogen content. Grain yield of ma ize was closely associated with nitrate-N in the soil but not with org anic carbon or total N. Four patterns of nitrate-N release over the se ason could be differentiated using cluster analysis. Soil texture, soi l pH, soil organic carbon, stover management and cropping history cont ributed most to a differentiation of the four cluster groups in a disc riminant model. There was wide variability in the inherent soil-nitrat e level and in its importance in explaining differences in yield among farmers' fields. The development of technologies resulting in improve d nitrogen availability or better nitrogen utilization should be based on research of inherent soil processes. For the transfer of such tech nologies, recommendation domains should be defined based on different native patterns of soil-nitrogen availability.