Long-term effects of wheat residue management on some fertility indicators of a semi-arid Plinthosol

This study was done to establish how soil fertility indicators such as pH, Ca, Mg, K, Na, P and Zn were influenced by the application of different residue management practices in a field trial near Bethlehem in South Africa. The trial consisted of 36 treatments, including all combinations of straw burning (burnt or unburnt), three primary tillage methods (ploughing, stubble mulch or no tillage), two weed control methods (mechanical or chemical) and three levels of nitrogen fertilization (20, 30 and 40 kg N ha−1). Representative soil samples were collected at 0–50, 50–150, 150–250, 250–450 and 450–650 mm depth intervals from the 30 kg N ha−1 plots for analysis, reducing the number of treatments to 12. After 11–12 years, only pH, K, P and Zn were significantly influenced by the residue management practices. Straw burning and conservational tillage increased the levels of those four indicators when compared with no burning and conventional tillage. The prominence of these effects declined with depth and phased out below 250 mm, viz. ploughing depth. A thorough investigation into the plant availability of the K, P and Zn that accumulated in the upper 50 mm soil as a result of either straw burning or conservational tillage is suggested.