Long-term wheat production management effects on soil fertility indicators in the semi-arid eastern Free State, South Africa

This study evaluated temporal effects of wheat production management practices on soil fertility under semi-arid conditions. Treatments applied for 37 years included straw management (unburned and burned), tillage practices (no-tillage, stubble mulch, and mouldboard ploughing) and weed control methods (chemical and mechanical). Soil samples were collected from 0–50, 50–150 and 150–250 mm layers, in 1990, 1999, 2010 and 2016, and analysed for organic carbon (OC), total nitrogen, pH, phosphorus (P) and potassium (K). Supplied grain yield data were used to estimate straw yield. Uptake of N, P and K by grain (removal) and straw (recycle) were estimated across sampling periods. Results revealed that OC, C:N ratio and P generally increased from 1990 to 1999 or 2010, and that conservation tillage increased soil fertility as compared with mouldboard ploughing. The only year × treatment interaction effects were on pH and K in the soil layers 0–50 and 50–150 mm in some treatments. However, all soil fertility indicators increased from 1990 to 1999 or 2010, then declined or stabilised across all treatments. The results suggest that adoption of conservation tillage with associated straw and nutrient recycling and additions cannot enable farmers to maintain or improve soil fertility and yields over time.