NITROGEN DYNAMICS AND MAIZE GROWTH IN A ZIMBABWEAN SANDY SOIL UNDER MANURE FERTILIZATION

This study was conducted to determine the relationships between nitrogen (N) release and plant uptake patterns in a sandy soil amended with aerobically decomposed and dried cattle manure, and inorganic N. Six treatments, a control, manure, manure+N applied at pre-planting, manure+N applied six weeks after planting, N applied at pre-planting, and N applied six weeks after planting, were used. The manure was applied at a rate equivalent to 10 t/ha and N at 100 kg/ha. Nitrogen mineralisation, uptake, and leaching in the field was estimated by an in situ incubation technique. Estimated N uptake was compared to the actual N uptake measured from maize plants. All treatments which received manure showed an initial immobilisation of N in the first eight weeks. Early application of N resulted in significantly more mineralisation of N in the manure compared to application at six weeks after planting. The estimates of N uptake from the in situ incubation technique were similar to the actual N uptake in all treatments except for the control, manure, and the treatment that received N six weeks after planting. Estimated values of N leaching ranged between 0.67 g/m2 and 4.22 g/m2 which is 3% to 42% of applied N. Highest amounts were leached from the treatment receiving N only at preplanting. Apparent N recovery was from less than 0% to 69% of applied N. Synchrony between N release and uptake was best achieved in treatments receiving combinations of manure and fertiliser N, especially late applied N. Though it is not entirely reliable, the in situ technique can be used to make estimates of N mineralisation, uptake, and leaching in conditions similar to those used in the present study.