One of the biggest factors in the productivity of a home apple orchard is fertilization, and fertilizing at the right time is as important as giving the right fertilizer in the right quantity. In fact, quantity depends directly on the age of the tree. The amount of fertilizer in pounds that each tree needs 0.5 times its age. So the questions of when and how much are inextricably linked.
The first time you will need to fertilize your apple trees is one month after planting. Since apple trees need plenty of nitrogen, a 10-6-4 fertilizer is a good choice. Note that if you use a 5-10-10 fertilizer you will need to use twice as much by weight as you'll need of the 10-6-4 and if you use a 20-5-10 fertilizer, you'll need half as much by weight. As you can see the relevant nutrient is nitrogen. Sprinkle it on the ground under the tree, making sure to keep all fertilizer at least 6 inches from the trunk of the young tree to avoid fertilizer burn. You will want to fertilizer newly planted trees twice more their first season in May and in June.
In years two through four, you will need to fertilize four times each year in March, April, May, and June. From the fourth year onward, apple trees need to be fertilized at most twice. Fertilize first when the buds begin to open. If there is less than 12 inches of new growth between this time and May, apply fertilizer again. If there is 12 inches or more of new growth, you will not need to fertilize again. In fact, this means that the tree has plenty of nitrogen and giving it more fertilizer at this time may actually lead to a decreased crop size or even no crop at all.
A more accurate way to ascertain whether your trees need a second feeding is to test the soil in your orchard. This should be done every two or three years.