Residual value of phosphorus from superphosphate for wheat grown on soils of contrasting texture near Esperance, Western Australia.

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Abstract
The residual value for wheat of phosphorus (P) from superphosphate was measured in field experiments on two texture-contrast (duplex) soils near Esperance, Western Australia. Superphosphate was applied to previously untreated plots once only, in 1980, 198 1, 1982 or 1983. The residual value of this P was measured in 1983 relative to P applied in 1983. Results were similar for both soils. Superphosphate applied in previous years did not produce the same yield as superphosphate applied in the current year. As calculated from yield response, relative effectiveness was 65, 42 or 32% after 1, 2 and 3 years, respectively. Yield depended on P content of plant tops, and this relationship was independent of time of P application. As the period of contact of P with the soil increased, less P was taken up by the plants, and this limited yield. As calculated from the P content of plant tops, relative effectiveness was 60, 30 or 23% after 1, 2 and 3 years, respectively. The amount of P extracted from the soil by 0.5M sodium bicarbonate decreased by about 54% from day 210 to day 575 after application of superphosphate, by a further 35% from day 575 to day 940, and by 15% from day 940 to day 1305. Bicarbonate-extractable P determined on soil samples collected mid January 1983 gave a good prediction of yields measured in the spring of that year