Long-term effects of conservation practices on the nitrogen fertility of a soil cropped annually to wheat

Abstract

Sustainable farming in the semi-arid interior of South Africa requires the adoption of conservation practices to maintain and, if possible, increase soil fertility. The effects of some such practices on the nitrogen fertility of a hydromorphic sandy clay loam soil were examined in a field trial at Bethlehem, in the eastern Orange Free State. Soil from selected plots was analysed, after this trial had been run for 11 and 12 years, for organic carbon and total nitrogen to use as measures of soil organic matter, and for residual inorganic and mineralizable nitrogen to use as measures of plant-available nitrogen. The trial includes 36 treatments, namely, all combinations of straw burning (burnt or unburnt), three primary cultivation methods (ploughing, stubble mulch or no cultivation), two weed control methods (mechanical or chemical) and three levels of nitrogen fertilization (20, 30 or 40 kg N ha−1). The 30 kg N ha−1 level was selected, reducing the number of treatments to 12. Similar measurements were made...