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 on soil from adjacent undisturbed natural pasture sites both upslope and downslope of the trial. There was less organic carbon and total, mineralizable and available nitrogen but more residual inorganic nitrogen within than outside the cultivated area. Soil depth to the bedrock increased down the slope: if it can be assumed, in the absence of initial samples, that soil organic matter content originally increased in the same direction, cultivation before and during the trial had considerably reduced it. Mouldboard ploughing depleted organic matter and available nitrogen more than stubble mulch, compared with no cultivation. Mechanical weed control, compared with chemical weeding, reduced available nitrogen in the surface layer of plots that had received no primary cultivation. Straw burning, compared with no burning, increased residual inorganic nitrogen at the surface in one of the two years. All the practices tested showed beneficial effects on soil fertility expected from experience elsewhere but had reduced grain yield, probably because they increased the incidence of soil-borne fungal pathogens. It is suggested that wheat grown in this area be rotated with annual legumes to conserve fertility and control diseases. © 1993 Taylor & Francis Group, LLC.