Uptake of 32P labelled phosphate by clover and ryegrass growing in mixed swards with different nitrogen treatments

SUMMARY Lolium perenne cv. S.23 and Trifolium repens cv. Olwen were sown together in 1975, fertilised then and in 1976, and finally given nitrogen doses of either 50, 100, 200 or 400 kg/ha (as N) combined with 0.64 times as much potassium (as K2O) in 1977. As nitrogen increased, grass yield increased, but clover decreased. Grass roots absorbed more 32P than clover roots, and nitrogen increased this difference. Grass roots bore more mycorrhiza than clover roots. The difference in 32P uptake between grass and clover was less in June and July than in August. Clover roots took up most phosphate from the upper layers of soil, while grass absorbed 32P rather uniformly down to 25 cm. It was concluded that optimum fertiliser placement for clover growth was a surface dressing in the early season.