Environmental effects on the short-term response of tropical grasses to nitrogen fertilizer

Abstract

The short term response of tropical grasses during spring to ammonium nitrate and urea fertilizers (1 12 kg N ha -1) was measured on kikuyu (Pennisetum clandestinum) at one site, and on setaria (Setaria anceps cv. Kazungula) at two sites. Fertilizers were applied at weekly intervals on 16 occasions at each site during the August-December period. Treatments were assessed in terms of the dry matter response (DMR) and apparent recovery of fertilizer nitrogen (NR), where DMR is the differences in dry matter yield of topdressed plots over the control and NR is the corresponding difference in nitrogen yield expressed as a percentage of the amount of nitrogen applied as fertilizer. These responses were measured at four, and sometimes at eight weeks, and were related to environmental variables by regression analysis. The DMR was more responsive than NR to changes in temperature. Both DMR and NR were very sensitive to changes in soil moisture potential, especially on wet soils, and to changes in effective rainfall (rainfall less run-off). If low temperature limited growth during the first four weeks after topdressing, both DMR and NR increased further when growth during 5-8 weeks extended into warmer periods. However, when moisture supply limited the response at four weeks, the NR remained depressed at eight weeks even with an improved moisture supply. Environmental factors influenced the response more than the type of nitrogen fertilizer and at four weeks, the respective overall DMR and NR from urea were 90 and 86 per cent of those from ammonium nitrate.