The climate induced probability distribution of short-term responses of a tropical grass to nitrogen fertilizer

Abstract

The probability distribution of the dry matter response (DMR) of a tropical grass to nitrogen fertilizer was calculated by applying previously established relations between DMR and environmental conditions to long-term climate records. The analysis was made for setaria (Setaria anceps) growing on an alluvial soil following an application of 112 kg N ha-1. It included six application times between August 6 to December 24 at two sites on the north coast of New South Wales; Broadwater with a high (1396 mm) and Casino with a medium (1070 mm) mean annual rainfall. The DMR usually increased with later fertilizer application times because of increasing temperatures, but for any one application time variation around the median DMR was largely determined by variation in the water supply. Such variation was generally greater and more symmetrically distributed at Broadwater than at Casino. The lower rainfall at Casino reduced the response and the overall median DMR, 4 and 8 weeks after topdressing, was 64 and 38 per cent respectively of that at Broadwater. Both the soil water content at topdressing and the follow-up rain contributed to the overall water supply. Consequently there was a trend for increasing DMR with increasing soil water at topdressing.