Superphosphate requirements of clover-ley farming. II.* The residual effects of topdressing

Abstract

The residual effects of topdressing the grazed clover-ley were evaluated at four sites in southern New South Wales. The significance (for the initial crop) of changes in soil fertility induced by topdressing were studied in crop experiments (five superphosphate drill rates x three nitrogen rates) immediately following the ley phase.

Topdressing influenced the level of available phosphate at all sites: heavy applications induced a large increase in available phosphate, light applications induced a moderate increase, and omission of topdressing led to a slight decline. The rate of soil nitrogen accretion was affected by topdressing only at sites that were initially of medium soil phosphate status.

Topdressing exerted residual effects on crop growth, grain yield, and grain composition; crops following topdressed leys showed little response to superphosphate drilled with the crop at sowing. By contrast, crops following untopdressed leys showed a consistent but low requirement for superphosphate; grain production was maximized at low application rates (< 100 kg superphosphate ha-1) and the (economic) optimal rate was usually in the range 30-60 kg superphosphate ha-1. For the initial crop, low drill application rates were apparently equivalent to large amounts of superphosphate applied by topdressing the ley.

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*Part I, Aust. J. Agric. Res., 28: 269 (1977).