Comparison of soil phosphorus fractions after 37 years of wheat production under different management practices in a semi-arid climate

Crop residue management is important for enhancing soil fertility and productivity which leads to an increase in crop yields. A long-term trial, situated near Bethlehem in the Eastern Free State in South Africa, was established to determine the influence of various wheat residue management practices on soil fertility indicators such as phosphorus (P) fractions. Field treatments applied were two straw disposal methods (burned and unburned), three tillage methods (no-tillage, stubble mulch and ploughing), and two weed control methods (chemical and mechanical). Samples were randomly collected at four soil depth intervals and analysed for P fractions. Burning of wheat straw increased the inorganic labile (NaHCO3-Pi) and stable (HCl-Pi) P fractions with an average of 5.07 and 2.16 mg kg(-1), respectively, compared to the unburned plots. The no-till plots had greater NaHCO3-Pi and HCl-Pi fractions than the stubble mulched and ploughed plots in all four soil layers. The inorganic moderately labile (NaOH-Pi) and organic labile (NaHCO3-Po), moderately labile (NaOH-Po) and stable (HCl-Po) P fractions were only occasionally significantly influenced. The chemically weeded plots showed increases of 3.98 mg kg(-1)NaHCO(3)-Pi and 2.06 mg kg(-1)HCl-Pi compared to mechanically weeded plots. Residual P fractions were not affected by the treatments which is similar to the wheat response. Based on the results obtained in this study, the burning of residues, no-tillage and chemical weeding practices are recommended for wheat production in a semi-arid climate.