Summary The effects of cover crops and NPK fertilizers (in factorial combination) on the growth, crop yield and leaf-nutrient composition of young Cox’s Orange Pippin and Worcester Pearmain apple trees have been studied over a period of six years. The covers used were : (a) wild white clover, S. 184,-(b) perennial rye-grass (Lolium perenne, S.23), (c) timothy grass (Phleum pratense, S.50), and (d) natural sward as it developed (mainly Poa annua and Agrostis stolonifera). The major effects were due to cover crops. Nitrogenous fertilizer had significant effects in combination with timothy and rye-grass, but phosphatic and potassic fertilizers had no significant effects on tree growth or crop yield. Clover and natural sward covers had the least deleterious effect on tree growth and gave the highest yield of fruit. Timothy and rye-grass severely restricted tree growth and crop yield. This was due largely to competition for nitrogen, which was clearly reflected in a reduced nitrogen concentration in the leaves of the trees. Optimum growth, crop yield and fruit colour in Cox were associated with a nitrogen concentration of 2 . 0-2 . 2% in dry matter of leaf samples taken in August from the mid-third region of extension shoots. There is evidence from the composition of the leaves of the trees to suggest that clover made available additional nitrogen and phosphate. There was a positive relationship between the total nitrogen concentration of the tracheal (xylem) sap of shoots at the green-cluster stage and the leaf-nitrogen status in the previous August. It is suggested that the best method of establishing a permanent cover in orchards, with the least adverse effect on tree growth and nitrogen status, is to sow wild white clover and to allow the slow establishment of indigenous grasses by frequent mowing.