Interactions between calcium (Ca), potassium (K) and magnesium (Mg) in apple tree nutrition were investigated in a pot experiment with ‘Cox's Orange’. Trees low in Ca supply were strongly dependent on Mg supply. Without Mg there was a very early and total leaf-fall. This led to a considerable yield response of Mg supply. During Ca deficiency, application of K enlarged the deficiency, resulting in a reduced K effect. Application of K reduced the Ca concentration in the leaves and Mg application depressed it further. This was not the case in the fruits, where K supply alone decreased the Ca concentration, while Mg supply, together with K, to some extent increased it. Spraying three times with MgSO 4·7H 2O, 1% or 2%, confirmed that fruit Ca is reduced by spray-applied Mg.