Factors affecting nitrogen utilization by sugarcane in South Africa.

Abstract

TIhe response of sugarcane to applied N in South Africa varies considerably from one soil to another, particularly in the plant crop. Responses to fertilizer N by ratoon cane are generally much greater than those given by plant cane. Where irrigation is practised yield response per unit of N is significantly higher than that obtained under rain grown conditions. Response of cane to N can be influenced by various factors, some of which are able to bring about differences in yield as great or greater than those obtained from the N fertilizer itself. These include seasonal effects, time and method of N application, the form of N applied, and the nature of the soil . The N cycle in relation to sugarcane has been examined, as several factors affecting response of cane to N are concerned with the transformations which N undergoes in the soil - plant system. The potential of different soil series within the sugar belt to mineralize N, greatly influences the response of plant cane to applied N. The N supplying power of sugar belt soils is also dependent upon how recently they were opened for cultivation, and the length of time they remain dry prior to replanting. However, accurate assessment of soil N available to cane remains difficult, and it is probable that N recommendations will continue to be made largely on an empirical basis of management and yield. Incorporation of cane trash in the soil, and the C/N ratio of cane roots may affect efficiency of N fertilizer usage by the crop, particularly in the sandier soils of the industry low in N, due to the immobilization of applied N. Apart from the soil pH effect as such, specific N carriers are able to influence r ates of nitrification and thus susceptibility to leaching, especially in the more weakly buffered soils which constitute over 30% of the industry. It appears likely that utilization of N by cane grown in these soils, could be enhanced by the use of the nitrification inhibitor N-Serve. Application of all the N to the furrow at time of planting can cause severe leaching losses even in heavily textured soils. Top-dressing some weeks after planting, results in more efficient recovery of fertilizer N. Even so, only 25%-30% of N applied in the widely used ammonium form is recovered by the above ground parts of the cane crop.