EFFECTS OF LONG-TERM FERTILIZER PHOSPHORUS APPLICATION ON SOIL AND CROP PHOSPHORUS AND CADMIUM CONTENTS

The effects of four rates of fertilizer phosphorus (P) application (0, 9.8, 19.6 and 39.2 kg P/ha per year) on soil and crop P and cadmium ( Cd) contents were measured in a field trial begun in 1968 and cropped each year with barley in south west England. In 1996, available and to tal soil P and Cd were measured in seven soil layers (0-20, 20-25, 25- 30, 30-35, 35-40, 40-45 and 45-50 cm). Offtake of P in the crop was me asured, or could be estimated, throughout the trial period. There was a linear relationship between P balance (total applied - total offtake ) and P application rate with a balance of zero at a rate equivalent t o 17 kg P/ha per year. The rate of P required for the economically opt imum grain output was equivalent to 30 kg P/ha per year. No evidence w as found for available P enrichment of soil layers below 25 cm. There was no evidence of Cd enrichment of either soil or crop after 29 years of P applications.