The lower and upper threshold values, biological optimum and mineralization of nitrogen in the main maize producing soils of South Africa

Abstract

There is still no consensus in South Africa on the parameters used to advise maize farmers on the quantity of N needed for optimum yields. In order to determine the lower and upper threshold values for nitrogen (N), the biological optimum and the mineralization of organic N in the main maize growing areas of South Africa, field trials were carried out at seven localities over three years. The treatments were full fertilization as reference (Ref), with the comparative treatment being where N was omitted (0N). Relative maize yields (RY) were determined and composite soil samples were taken. Bulk density (t m−3) and the NO3-N extracted with K2SO4-solution were determined in the laboratory and the kg NO3-N ha−1 calculated. The RY responded to N with a lower threshold value in the soil stratum (0–600 mm) of 70 kg NO3-N ha−1 (8 mg NO3-N kg−1), an upper threshold value of 161 kg NO3-N ha−1, a biological optimum of 280 kg NO3-N ha−1 and an estimated N mineralization during the growing season of 40 kg NO3-N ha−1.