Nitrogen and phosphorus requirements of wheat following subterranean clover pasture

Abstract

The effects of superphosphate and nitrogen fertilizer on wheat yield, grain nitrogen and phosphorus levels, total soil nitrogen and available soil phosphorus were studied over a 6-year period on plots which had received 718–3343 kg superphosphate ha^-1, during a preceding ley pasture-crop period.

Wheat yields generally declined with time on all treatments, fertilizer nitrogen reducing the rate of decline. From 125 to 2.50 kg superphosphate ha^-1 year^-1 was required to maintain maximum wheat yields when nitrogen fertilizer was also applied, but only 125 kg was required in its absence.

Grain nitrogen content varied from 1.47 to 2.11% and was not a good indicator of nitrogen fertilizer requirement. Nitrogen removal per crop varied from 27 to 41 kg ha^-1 in the presence of added nitrogen and from 21 to 31 kg ha^-1 without added nitrogen.

Soil nitrogen levels declined at rates varying from 8 to 40 kg ha^-1 year^-1.

Grain phosphorus levels varied from 0.21 to 0.44%. Phosphorus removal in the grain varied from 4 to 8 kg ha^-1 per crop.

Changes in available soil phosphorus on plots which had previously received 718, 1593, 2468 or 3343 kg superphosphate ha^-1 were closely correlated with the rate of superphosphate applied with the crop (r = 0.97, 0.97, 0.93 and 0.80).

The amount of superphosphate required to maintain soil phosphorus levels was calculated, and the following conclusions regarding superphosphate use in the pasture-crop rotation in southern New South Wales have been made. Where pastures are not topdressed, the amount of superphosphate needed with each crop varies with the number of years of cropping, the level of soil nitrogen, the number of years of untopdressed pasture relative to the years of crop, and the initial soil phosphorus level.