Influence of long-term wheat residue management on organic matter in an Avalon soil

Abstract

This study was initiated to determine the influence of different wheat residue management practices, that were sustained for about 20 years, on the organic matter content of an Avalon soil in a long-term trial near Bethlehem in the Eastern Free State. The treatments were two methods of straw disposal (burned and unburned) × three methods of tillage (ploughing, stubble mulch and no tillage) × two methods of weed control (mechanical and chemical). Soil samples were collected at various depth intervals and organic C and total N, as indices of organic matter, were determined on these samples. The effect of either straw burning or weeding method on organic matter was small compared to that of tillage practice. A slightly higher organic C and lower total N content were measured in the unburned than burned plots to a depth of 450 mm. The tillage practices affected organic C and total N significantly in a similar manner to 100 mm depth with no-tilled plots having the highest organic matter content, followed in decreasing order by the mulched and ploughed plots. The organic matter content of the chemically-weeded plots was significantly higher than that of the mechanically-weeded plots to 100 mm depth, as indicated by organic C and total N. Significant interactions between the treatments on either organic C or total N were virtually absent. However, no tillage combined with chemical weeding resulted in the highest organic matter content. The latter combination is therefore recommended to maintain and even increase the organic matter content of this Avalon soil when cropped annually with wheat.