Du Preez, C. C.Haumann, Eduard Johannes2023-08-152023-08-152023http://hdl.handle.net/11660/12109Thirteen randomised block nitrogen fertilization field trials were done in five irrigated cotton producing areas of South Africa: Bela Bela, Rustenburg, Vaalharts, Rietriver and Groblersdal, with the aim to refine the existing nitrogen fertilization guideline. At Rustenburg cotton was preceded by harvested oats, harvested soybean, ploughed in soybean and babala in order to create varying soil nitrogen contents. Soil samples (0–300, 300–600 and 600–900 mm soil depths) were taken at least two weeks after irrigation for land preparation to determine total nitrogen. Leaf petiole samples were collected at two-week intervals, starting one week before first flowering for measurement of total nitrogen. After ripening, seed cotton yield was determined. The yield was subjected to analysis of variance and correlated with the nitrogen application rate to quantify an optimal nitrogen fertilization rate per site for each trial year. The estimated optimal nitrogen fertilization levels were correlated with the residual soil nitrogen content for each depth resulting in four guidelines for either maximum yield or maximum profit. Therefore, the cotton grower has eight different options for the calculation of an appropriate nitrogen application level based on the residual nitrogen content of the soil.enThesis (Ph.D. (Soil Sciences))--University of the Free State, 2023Irrigated cottonnitrogen fertilization guidelinesresidual soil nitrogenleaf petiole nitrogenmaximum yieldmaximum profitThesisUniversity of the Free State