Midsummer Potassium Sprays. During the past several years, there has been increasing use of foliar applications of potassium based on undocumented claims that such sprays can enhance red color of apples. These sprays have been applied regardless of actual potassium status of the fruit trees. Low soil potassium is widespread in humid regions, including western Washington; however, potassium deficiency is uncommon in central Washington orchards. Recent evidence suggests that depletion of soil potassium may be enhanced in orchards that have been farmed for very long periods of time, are on very sandy soils, especially where evaporative cooling is used, or where high-frequency fertigation through trickle (drip) irrigation systems is practiced. Although there is justification to apply potassium fertilizer where potassium levels in trees and grass cover crops are truly low, we have received an increasing number of reports of potassium sprays inducing severe bitter pit in apple and cork spot in pears. These disorders are associated with high potassium-to-calcium ratios in fruit.
Caution: indiscriminant use of potassium sprays can increase the incidence of bitter pit in apples and cork spot in pears.