Adequate potassium supply to apple trees is essential in order to achieve regular yields of high quality fruit. Potassium is particularly important in ensuring sufficient acidity in apples and for providing the characteristic taste to more acidic cultivars such as Cox.
In the past, as with nitrogen, there was a tendency to over-supply trees with potassium fertiliser. It is universally accepted that excessive potassium in the fruit at harvest increases their susceptibility to bitter pit during storage. In Cox, high potassium in leaves and fruits was associated with increased Gloeosporium rot, core flush, lenticel blotch pit and late storage corking.
Although high potassium fruit may be less prone to some forms of breakdown, the adverse effects of excessive potassium generally out-weigh any possible beneficial effects. It is particularly important to omit fruit with excessive potassium from long-term storage. However, Cox apples that contain less than the recommended minimum amounts of K should also be omitted from long-term storage due to the possible development of low temperature or senescent breakdown.
Growers need to ensure that leaves and fruits from all orchards are analysed each year in order to determine the requirement for potassium fertiliser. For Cox an upper limit for potassium of 1.6% (dry weight) is advised.
Fruit potassium levels in any particular orchard will vary from year to year and it is important to develop a ‘trend-line’ by regular analysis. Crop load has a major impact on the potassium concentration in the fruit with heavy cropping generally reducing the concentration. Flower or fruit thinning increased fruit potassium levels in Cox and increased their susceptibility to senescent breakdown and core flush during CA storage.
Where cases of severe potassium deficiency occur, foliar sprays of potassium sulphate (10-15 kg 1000 l-1) can be applied to supplement soil application of fertilisers. Three applications may be made at 14-day intervals starting at petal fall. Sprays containing potassium are not advised for any other purpose, as they are likely to promote the development of bitter pit.