Apple (Malus domestica L.) is one of the most important horticultural crops in Iran, topping the list of exported fruits on the basis of tonnage. Potassium is one of the essential nutrients with significant roles in apple trees. The competitive effects between potassium and other ions, especially calcium, are very important for this plant. Researchers attribute specific ratios of N/K and K/Ca for the onset of apple fruits quality degradations. Therefore, special care must be given to supplying apples with the needed potassium in terms of amounts and the kinds of fertilizers.
An experiment was carried out during the year 2000 in Ziaran of Ghazvin to test different rates of two sources of potassium, namely, potassium sulfate and potassium chloride, combined with micronutrients.The design was a completely randomized block with ten treatments and eight replications. The treatments included the two sources of potassium, each applied at three different rates:(1)recommended rate of Soil and Water Research Institute, (2):50% higher,(3): twice the recommended rate. All treatments , except treament (1) and another teament that received only NPK, received micronutrients. Leaf and fruit chemical analysis and the apple quality aspects showed no differences between the two potassium sources. Increasing the rates of application of either of potassium sources lowered the calcium content of the fruits.
The potassium fertilizers combined with micronutrients caused significant increases in vegetative growth, nitrogen, potassium, magnesium, iron and manganese contents, as well as the size and weight of each fruit. The fruits TSS, and sugar and acidity levels of the juice also increased significantly with potassium treatments. However, the juice content of apples from the control plots were, at least, 8% higher than those from the other plots i.e. the juice contents decreased with potassium treatments. Potassium chloride treatments increased the levels of chloride in the leaves and fruits, but no more than the accepted levels. The results showed no differences due to different rates of potassium and from the rates point of view the fertilizer recommendation levels were the best treatments.