Superphosphate utilization in clover ley farming. II.* Residual effects of pasture topdressing in the cropping phase

Abstract

A grazing experiment was carried out at the Agricultural Research Institute, Wagga Wagga, over the period 1962-1966. The treatments involved three stocking rates (5, 10 and 15 sheep per ha) and four superphosphate topdressing rates (0, 125, 250 and 375 kg ha^-1) with three replications.

After the grazing phase three successive crops were grown. Superphosphate was applied with the crops at 0, 63, 125 and 250 kg ha^-1. Current and residual effects of this superphosphate were measured. Nitrogen (as urea at 78 kg ha^-l) was topdressed on half of each plot. The low stocking rate plots were further tested for response to sulphur.

Superphosphate applied to one crop had no residual effect on subsequent crops, regardless of the previous pasture treatment.

On the average the yield of the first crop sown without superphosphate increased about 1/3 kg ha^-1 for each kilogram of superphosphate per hectare applied annually in the pasture phase, that of the second crop 1 kg and the third crop I-1/3 kg.

First crops did not respond to superphosphate applied with the crop, but second crops on plots that were not topdressed in the pasture phase and all third crops did respond to current superphosphate.

Yields of third crops receiving superphosphate at 250 kg ha^-1 on land that was not topdressed in the pasture phase exceeded those of similar crops on land that had been topdressed in the pasture phase (five out of six comparisons were significant at P = 0.01). A similar effect was found in the second crop in one replicate.

The effect of nitrogen varied with the climate. In the drought year of 1967 there was one negative response to nitrogen. In 1968 yields were high and nitrogen decreased yield by increasing lodging. In the following 3 years nitrogen increased yield and the increase was generally greater as the rate of pasture topdressing increased.

The stocking rate of the pasture did not affect the yield of the following wheat crops.

There was no response to sulphur by the crops.

It is suggested that superphosphate can be more efficiently used by omitting pasture topdressing and applying 125 kg ha^-I or more with crops. Farmers in the clover ley farming areas could considerably reduce their annual superphosphate requirements.

*Part I, Aust. J. Agric. Res., 25: 525 (1974).