Phosphate applied to soil increases the effectiveness of subsequent applications of phosphate for growing wheat shoots

Amounts of phosphorus ranging from 0 to 599 kg P/ha were applied as single (ordinary) superphosphate once only in 1976 to plots of a field experiment on a lateritic ironstone gravel sand. Subterranean clover (Trifolium subter raneum) and cereals (wheat, Triticum aestivum, and barley, Hordeum vulgare) were grown in rotation on the plots for 20 years after phosphorus applicat ion. In 1996, samples of the <2 mm fraction of the top 10 cm of soil were c ollected from the field plots to measure phosphorus retention by soil and f or a glasshouse experiment. The phosphorus retention index, a measure of th e capacity of the soil to sorb phosphorus, decreased from 35 to 2 mL/g as t he amount of phosphorus applied 20 years previously increased from 0 to 599 kg/ha. In the glasshouse experiment, wheat was grown for 35 days in soil to which 9 amounts of freshly-applied powdered single superphosphate (0, 0.0125, 0.0 25, 0.05, 0.1, 0.2, 0.4, 0.8 and 1.6 g P/pot, for 1.8 kg soil/pot) were app lied to the original phosphorus treatments. The value of the curvature coef ficient of the Mitscherlich equation fitted to the relationship between yie ld of dried shoots and the amount of fresh-phosphorus applied increased wit h increasing amount of phosphorus applied 20 years previously That is, the fresh-phosphorus treatments became more effective for producing dried wheat shoots as the amount of previously applied phosphorus increased.