Threshold values for sulphur in soils of the main maize-producing areas of South Africa

Abstract

Fertilizers introduced to South Africa after World War 2 contained sufficient sulphur (S) to satisfy the demands of all crops. However, dramatic shifts to high-analysis NPK fertilizers containing little or no S and restriction on sulphur dioxide emissions by industries lowered the coincidental input of S. At the same time continuous crop- ping and higher-yielding cultivars removed more S which led to low S levels in cultivated soils. The aim of this study was to determine the threshold value for S and to evaluate the S-levels in soils of the main maize-growing areas of South Africa. In order to determine the critical sulphur content in these soils, field trials were carried out at ten localities during four consecutive seasons. The treatments were full fertilization as reference, with the comparative treatment being where S was omitted. Relative yields were calculated as the yield of the (-S) plot expressed as a percentage of the yield of the plot (in the same replicate) which received all the nutrients including S. Composite soil samples were taken and S was extracted with monocalcium phosphate which includes SO₄ and some mineralizable organic S. The results indicate that maize will respond to S fertilization when the S value is lower than the threshold value of 9 mg S kg^-1 in the topsoil. A survey of soil samples submitted by farmers highlighted the importance of supplementing S in the main maize-growing areas of South Africa.

Keywords:

• Critical levels
• maize
• relative yields
• soil
• sulphur