Paliyath et al. (2002) studied the effect of soil and foliar phosphorus supplementation on the post-harvest quality of apples (Malus domestica Borkh. cv. ‘McIntosh’ and cv. ‘Red Delicious’) and found that phosphorus fertilization increased the percentage of red skin on both varieties at harvest.
They have also found that fruit from sprayed sides of the trees subjected to foliar treatments with phosphorus and magnesium or phosphorus and calcium, from blossom until a week before commercial harvest, had increased red colour compared to those from the non-sprayed side.
Potassium also increases anthocyanin content in apples, which improves the fruit colouration. However, supplies need to be in balance with other cations, particularly calcium, as preferential uptake of potassium over calcium can lead to quality problems such as bitter pit.