Influence of long-term wheat residue management on some fertility indicators of an avalon soil at Bethlehem
Awareness of the environmental aspects of soil quality and crop production has been increasing in recent years, which has led to renewed interest in crop residues as a source of soil organic matter and nutrients for crops. Crop residue management is known to both directly or indirectly affect soil quality and therefore soil fertility. Some residue management practices have been tested since 1979 in a long-term wheat trial at the ARC-Small Grain Institute near Bethlehem in the Eastern Free State on an Avalon soil. This trial offered an opportunity to study the influences of wheat residue management practices on some soil fertility indicators and to establish whether differences in wheat grain yield could be attributed to changes in the soil fertility indicators. The treatments that were applied are two methods of straw disposal (burned and unburned) x three methods of tillage (ploughing, stubble mulch and no tillage) x two methods of weed control (mechanical and chemical). Soil samples were collected in 1999 at depth intervals of 0-50, 50-100, 100-150, 150-250, 250-350 and 350-450 mm and analyzed for various soil fertility indicators, viz. organic C and total N as indices of organic matter. In addition the pH, P, K, Ca, Mg, Na, Cu, Fe, Mn and Zn were also determined. The different tillage practices had a larger effect on organic matter than either straw burning or weeding method, especially in the upper 100 mm soil. No tillage and to a lesser extent mulch tillage, especially when combined with chemical weeding were more beneficial to soil organic matter than when ploughing was combined with mechanical weeding. Soil acidification seems to be retarded by mulch or no tillage when combined with chemical weeding. The burning of wheat residues increased pH significantly compared to no burning. It was found that the content of P, K, Cu, Fe, Mn and Zn were increased with straw burning when compared to no burning. No tillage and to a lesser extent also mulch tillage resulted in an accumulation of P, K, Ca, Mg, Cu, Fe, Mn and Zn in the upper 150 mm soil compared to mouldboard ploughing. Grain yield does not coincide with the higher organic matter and lower acidity resulting from mulch and no tillage. A reason for this may be that the nutrients accumulated in the upper 150 mm soil with these two tillage practices, are not always available for plant uptake. This aspect warrants further investigation.