Nitrogen Fertilization of Cotton Based on Inorganic Nitrogen Analysis of the Soil
Sep 20, 2018
Cotton is produced on widely differing soils under irrigated or rainfed conditions in South Africa. Under these circumstances seed cotton yields can vary greatly and so do their nitrogen demands. This model was the culmination of nitrogen field trials carried out over three seasons at four locations scattered over South Africa. In these trials, six levels of nitrogen were applied, with four replications at each site. Pre-plant soil samples were taken at 0-300mm, 300-600mm and 600-900mm. These samples were analysed immediately for inorganic nitrogen. For assessment of the amount of nitrogen that can be mineralised, these samples were incubated at 15/26C over a 14-week period. Petiole samples were taken for analysis at one and three weeks after the appearance of the first flower. The higher the initial nitrogen plus mineralised complement in the soil, the lower the percentage of nitrogen utilized by the cotton crop. A utilization factor of 66.7% seems to be a good average. The same results were obtained with applied nitrogen, where a utilization factor of 80% seems to be a good average. The nitrogen required to achieve a certain target cotton yield is taken as the point of departure in this model. Subtract from this 66.7% of the total inorganic N-content of the top 900mm soil plus the amount of nitrogen mineralised during the growth period. This leaves the amount of nitrogen to be applied for production of the target yield. The Nitrogen content of the irrigation water should be taken into account but it was negligibly low during these trials. Working with a utilization factor of 80% for the applied nitrogen, the actual amount can be calculated. Tests of the calculated nitrogen requirement against actual yields achieved in this and other trials gave a very good correlation of r2 = 0.977 between calculated and actual nitrogen needs.