Apple trees need a lot of sunlight with excellent soil fertility to be large fruit producers. Making sure your apple tree has ample morning light to dry dew from the plants reduces disease. If the environment is properly selected, there is little you need to do to fertilise your apple tree.
When planting a new apple tree, do not place fertiliser directly in the hole the tree is going in. Do not fertilise for the first weeks after planting either to allow the roots time to develop in the soil.
Apple trees under four years old need to be fertilised with nitrogen twice each year in the spring and summer. Mature trees may only need one fertilisation in the spring if the crop is good.
If your soil pH is above 7.8, nitrogen is the only fertiliser your apple tree needs. If the pH is between six and 7.5, you will need to add phosphorous and potassium to the fertiliser with a ratio of 3:1:2 (nitrogen:phosphorous:potassium).
Create a ring around the tree trunk that is 3 feet wide, but not having any contact with the trunk of the tree. This will maximise fertilisation without harming the trunk.
Use growth as an indicator of proper fertilisation; your tree should have 12-18 inches of growth annually when bearing fruit.