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 NO 3 -N extracted with K 2 SO 4 -solution were determined in the laboratory and the kg NO 3 -N ha -1 calculated. The RY responded to N with a lower threshold value in the soil stratum (0-600 mm) of 70 kg NO 3 -N ha -1 (8 mg NO 3 -N kg -1 ), an upper threshold value of 161 kg NO 3 -N ha -1 , a biological optimum of 280 kg NO 3 -N ha -1 and an estimated N mineralization during the growing season of 40 kg NO 3 -N ha -1 .

Impact of Commercial Organic Ameliorants on Nitrogen and Phosphorus Concentrations of Maize Biomass at Ninth Leaf and Silking Growth Stages

“…At Potchefstroom the pH of the soil was 6.61, almost similar to the threshold value. Compared to the upper inorganic N threshold concentration established by Van Biljon et al (2008), the soils' inorganic N concentrations were very low. The Bray 1 extractable P concentration at Ottosdal (16 mg•kg −1 ) was slightly lower and at Potchefstroom (56 mg•kg −1 ) much higher than the 20 mg•kg −1 the FSSA (2007) recommended, and at Bothaville more or less similar to the threshold.…”

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“…Soil testing, may provide information on the limiting nutrients. Often, low soil test values signify a positive crop response to fertilizer application [30,31]. Soil test values need to be calibrated to crop response before they can be interpreted accurately [32], but the lack of data to calibrate soil tests is a major setback in many developing countries [33,34].…”