The effects of strategic nitrogen fertiliser application during the cool season on perennial ryegrass-white clover pastures in the Western Cape Province 1. Soil nitrogen dynamics

Abstract

The dynamics of soil inorganic-N in reaction to a single, strategic application of fertiliser N were studied during the period 2000 to 2002 in an Oakleaf soil with a clay content increasing from 10 to 30.8% with depth. Soil inorganic-N was monitored to a soil depth of 500 mm (0–100, 200–300 and 400–500 mm layers) over a seven-week period (7, 21 and 49 days) following the application of 0, 50 and 150 kg N ha⁻¹ in the form of LAN (28) applied as a one-off application to a perennial ryegrass-white clover pasture during five different seasons (autumn, early winter, late winter, early spring or late spring). Inorganic-N content increased from 2000 to 2002 possibly as a result of biological N fixation by the clover. The application of 50 kg N ha⁻¹ did not result in significant differences in inorganic-N content compared to the 0 kg N ha⁻¹ treatments during almost the entire study and can therefore be regarded as a low risk treatment with regard to environmental pollution. The increasing effect of 150 kg N ha⁻¹ on inorganic-N lasted for at least 3 weeks in the 0–100 mm layer and 7 weeks in the 200–300 mm layer, with the highest inorganic-N content measured in autumn and early winter during years of average rainfall (2000 and 2002). Leaching of fertiliser N within one week after application to the 400–500 mm soil layers during 2000 and 2001 emphasised the possible negative effect of application rates exceeding the absorption capacity of the pasture. Nitrogen application rates exceeding the N absorption capacity of the pasture may therefore result in reduced nitrogen response efficiencies (kg additional dry matter per kg N) and inorganic-N contamination of natural resources especially in winter when the inorganic N-uptake capacity of the pasture is low as a result of low pasture productivity.