A.This depends on soil fertility and soil type. On peaty soils it is best to apply P and K during the growing season because peat has a poor capacity to retain fertilizer. Potash shouldn’t be applied to sandy soils during late Autumn/Winter as it is readily leached from sandy soils.
On non sandy mineral soils where soil fertility is poor (Index 1) it is best to apply a fairly large application of P and K in the Autumn in order to promote tillering and root development. As soil fertility improves (index 2 or better) the timing of P and K becomes less critical but there are time and labour advantages to applying P and K with nitrogen during the growing season. Risk of losses should be less with smaller more frequent applications of P and K during the growing season compared to a large single application out of season.