Cover crops and controlled-release urea decrease need for mineral nitrogen fertilizer for cotton in sandy soil

Cover crops can affect nitrogen fertilization efficiency, soil nitrogen content, and cotton yield. However, the interaction effects of controlled-release nitrogen fertilizers, applied N rate, and cover crops on cotton growth, yield and fiber quality remains unknown, especially in sandy soils with low N content. Soil nitrogen and cotton growth, yield and fiber quality were evaluated as a function of different N rates and N sources in five rotation systems with or without cover crops, over four years. The treatments were: (1) cotton rotation crops (i.e., fallow, single grass, grass + grass, grass + legume and a mixture of cover crops), (2) nitrogen rates (70, 100 and 130 kg N ha⁻¹), and (3) nitrogen sources (urea and controlled-release urea). Under a mixture of cover crops, fiber yields were 12% and 21% higher compared to the single grass and rotation fallow, respectively. Cotton in the fallow rotation required a higher rate of N (130 kg ha⁻¹) than in rotation systems with cover crops (100 kg ha⁻¹), in systems with cover crops to achieve the highest yields. Controlled-release urea reduced the demand for N fertilizer by 30% in the fallow. It also increased the content of soil N by 16% (mean of two seasons). Micronaire and fiber strength were lower with 130 kg ha⁻¹ of N. There is no need to increase nitrogen fertilization in cotton, as long as controlled-release urea and cover crops are employed, preferably mixtures of grasses and legumes.