Apple trees require nitrogen, phosphorus, potassium and trace minerals. For home growers, fertilisers should have a higher nitrogen ratio. Common 20:10:10 fertiliser is suitable. A rule of thumb for small orchards is 450 g of fertiliser per year of tree age, to a maximum of 2.7 kg per tree. A fertiliser ratio of 10:5:5 requires double the amount of fertiliser to achieve the same concentration of minerals as a 20:10:10 fertiliser.
Nitrogen is critical for both tree growth and fruit development so apply it once a year. Nitrogen is the most important part of an apple fertilisation program. Phosphorus uptake is from deep soil. It should be added deeply at planting. Surface applications rarely move to deep soil root zones. Potassium is necessary for good fruit size, colour and flavour. It promotes winter hardiness and general good tree health. Know your trees. Different species of apples may have different requirements. Dwarf, semi-dwarf and regular trees also have different needs.
Choose organic or non-organic fertiliser. Either can be applied to the soil or as foliar spray to the leaves. Carefully follow application instructions for either type. Compost may be used. Both organic and non-organic fertilisers are available in dry forms. There are several liquid organic fertilisers, including fish emulsion and seaweed concentrates.
How to apply
Apply dry fertiliser to soil by scattering it under the tree out to the canopy dripline. Spray liquid fertiliser on the soil in the same area. Keep both organic and non-organic fertiliser at least 15 cm away from the trunk of the tree. Apple trees grow best with little competition from sod. Increase the amount of fertiliser up to 25 per cent if sod is present. Liquid fertilisers can be sprayed onto the tree leaves. Foliar application makes nitrogen immediately available. Spray when drying conditions are good to avoid fungi and disease.
When to apply
Whichever form or type of fertiliser you choose, the earliest application should be in March at bud formation. A second application should be done in May after fruit set. If the tree is young or if no fruit has set for another reason, the second application may be omitted. July is the latest time to fertilise. After that, weak growth that is susceptible to winter damage or disease may result.
Trees that have been heavily pruned will require less nitrogen that year as they grow new wood. Since trees store some nitrogen, a lighter fertiliser may be used the first summer after heavy pruning, and a regular nitrogen programme can resume the following year. Monitor branch tip growth -- the average is 10 to 15 cm per year. If growth is less, consider a slight increase in nitrogen the following spring. In general, fertiliser with 10 per cent nitrogen will translate into 0.1 per cent nitrogen content in the leaf test. This is an optimum level for growth and fruit production. Higher nitrogen fertiliser (21:0:0) is appropriate for newly planted trees until they are three years old. At four years, switch to the ratio for mature trees.