Effect of soil tillage, crop rotation and nitrogen application rates on soil mineral-N levels in the Swartland wheat producing area of South Africa
Studies on nitrogen (N) levels in soil were conducted from 1997 to 2000 in a long-term tillage and crop rotation trial, which started in 1976. Three tillage methods namely conventional tillage (CT), tine tillage (TT), and no-tillage (NT) were compared. Crop rotation systems used were continuous wheat (WW) and wheat/lupin/wheat/canola (WLWC). Three rates of N-fertilisers (60, 100 and 140 kg N ha-1) were applied. All treatments received 60 kg N ha-1 at planting, while the 100 and 140 kg N ha-1 treatments received additional top-dressings of 40 kg N ha-1 at tillering and stem elongation stages respectively. Soil samplings were done at 14-day intervals, starting immediately before planting and continued till harvesting each year. Samplings at planting were done before N-fertiliser was applied and therefore regarded as pre-planting (Pp). The effect of tillage methods on mineral-N was variable and inconsistent among the soil samplings and years. During the four-year-period of this study, higher values of total mineral-N were found with conventional tillage at the pre-planting (Pp) samplings. Crop rotation did not significantly influence total mineral-N, which may be explained by the cultivation of a legume crop (lupins) once every four years only. In general the application of N-fertiliser resulted as expected in an initial increase in the mineral-N content of the soil. However, this was soon followed by a rapid decline as crops start to utilise the added nitrogen, resulting in values of less than 10 mg kg-1 soil during the grain filling stages with application rates of 60 and 100 kg N ha-1. These values may be regarded as deficient for grain yields of 4000 to 5000 kg ha-1.