Colour of the skin is a very appealing apple quality to consumers. Therefore, growers generally seek to improve fruit colouration. Crop nutrition can help to achieve the desired appearance.
Nitrogen excess can reduce red skin colour
Over-use or late applications of nitrogen reduces the red colouration of red apples. This can be advantageous in green apples, as it will improve greenness and minimize red colours.
Phosphorus has an effect on skin colour
Foliar application of a fertiliser containing phosphorus, calcium and magnesium, has been shown to enhance red peel colour, and an increase in the concentration of flavonoids in ‘Fuji’ apples.
Trials in South Africa show that phosphorus has a positive effect in fruit colouration. It does so by increasing the production of an enzyme involved in the production of anthocyanin, one of the main compounds in fruit that determines its red colour. Phosphorus source and timing are important. Foliar phosphate is very effective after flowering and at early ripening where there is a need to improve fruit colouration.
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 increases anthocyanin content
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.
Zinc and Manganese
Zinc helps improve apple colour, while manganese helps improve background, green colour in apples.
Soil applied magnesium improves the colour of red fruit largely through the increased production of carbohydrates, which are the building block for red pigments.