Nitrogen fertilizer guidelines for small-scale farming on an irrigated duplex soil based on soil analyses

Reported nitrogen fertilizer research results were inadequate to meet the needs of small-scale farmers in the Bushbuck Ridge area. The objectives of this study were to establish inorganic N optima for maize production and to calibrate soil responses to N applications under high rainfall plus supplemental flood irrigation conditions. During the 1998/99 season a site situated near Dingleydale (24°42'S, 31°07'E) on a Sepane soil form with 9 and 20% clay content in the 0–300 and 300–600 mm soil layers, respectively, was identified for the establishment of a N×K fertilizer trial. Five N application rates (0, 52,104, 156 and 208 kg N ha-1) and four K application rates (0, 63, 126 and 189 kg N ha-1) were factorially combined and replicated twice according to a randomised block design. Treatments were repeated for four successive seasons on the same plots. Over the seasons, relative yields of 100, 90, 80, 70, 60 and 50% respectively were obtained at inorganic N quantities of 139, 105, 85, 72, 61 and 52 kg N ha-1respectively six weeks after planting in the 0–600 mm soil layer. An average N requirement factor of 1.4 kg N applied per kg N measured was calculated for the top 600 mm soil according to a linear model, but a quadratic model explained more variation and should therefore be preferred. Farmers should preferably be encouraged to manage inorganic levels in the soil to obtain a certain percentage of the expected yield. Optimum soil N levels can be extrapolated to similar soils varying in inorganic N content. Farmers with difficulties in accessing analytical laboratories may also rely on the established relationship between N rates and relative grain yield, provided that inorganic N residues are near 40 kg N ha-1during the season, as was the case in this study, that soil and cultivation practices are comparable.