Assessing Nitrogen Fertigation Strategies for Drip Irrigated Sugarcane in Southern Africa

Standard nutritional advice for irrigated sugarcane in South Africa and Swaziland is based on soil sample analysis and threshold values that have been determined experimentally over many years with surface-applied fertilisers. Fertilisers are normally applied in solid form during the early stages of crop development and watered in by the next irrigation cycle. The increasing popularity of subsurface drip irrigation in the Southern African sugar industries raises opportunities for improved fertiliser management. However, without appropriate recommendations there could be a tendency towards leaching and denitrification associated with poor timing and excessive application of fertilisers by fertigation. While little active research has been carried out on the fertigation requirements of sugar cane in Southern Africa, the results of studies of nutrient uptake patterns in irrigated sugarcane have been used to formulate monthly fertigation programmes that account for seasonal variation in nutrient demand within a single crop cycle (“growth curve nutrition”). This paper reports the initial results of two experiments established in Swaziland to test the benefits of using such an approach against more conventional principles of fertiliser application, with specific reference to nitrogen. Cumulative nitrogen uptake curves were tested for a summer cycle plant crop and for a winter cycle first ratoon crop. Results indicated that the winter nitrogen uptake curve correctly predicted the proportional monthly nitrogen demand of a winter ratoon crop but did not account for the effect of late nitrogen application on cane quality. The summer nitrogen uptake curve appeared to underestimate the nitrogen demand of a summer plant crop between January and April. Splitting nitrogen applications evenly over the first four months of crop development led to more efficient and productive use of nitrogen than the growth curve nutrition approach.