When to Apply Manure
Apply 1 to 2 inches of well rotted-manure in March while the tree is still dormant and again in July. You should wait until the year after planting to start fertilizing. If you live in a warmer region with mild winters and little danger of frost---zones 9 or 10 and up---you can apply manure three times a year: in March, July and September. In cold regions, applying manure in the fall encourages tender growth that can be damaged by freezing temperatures.
How to Apply Manure
Remove any mulch your have under your apple trees. Make sure the mulch is placed outside the drip line before you add the manure. Apply the manure under the canopy and spread to the drip line. Make sure you leave 6 to 7 inches between the well-rotted manure and the trunk of your tree as the manure could burn the tree trunk. The manure must be in contact with the soil. The apple tree's roots only go to the drip line of the tree---spreading the manure outside of the drip line will make it unavailable to your tree. If the well-rotted manure is more than 2 inches deep, it could possibly smother or damage roots close to the surface of the soil.
It doesn't matter what kind of manure you use (horse, chicken, cow, rabbit or goat) but it does need to be well-rotted. Well-rotted manure looks like soil and has no smell. Never use fresh manure as it contains ammonia, which can burn roots as it decomposes. Well-rotted manure is considered an organic fertilizer and adds nitrogen, potassium and other nutrients to soils. It also adds organic matter that helps loosen the soil, increases the oxygen content and attracts earthworms.
Advantages and Disadvantages of Manure
Well-rotted manure does not contain artificial additives, is hard to over-apply and does not release noxious chemicals as it is decomposes. It is not, however, a balanced fertilizer. You may need to add other additives like bone or blood meal to meet your apple tree's nutritive requirements. Have the soil tested under your apple trees every other year to verify they have the nutrients they need. Manure almost always contains weed seeds, with every application you may be adding thousands of potential weeds. Using several inches of mulch over the manure will help keep potential weeds in check. Never use manure from humans, dogs or cats. Even composted, these manures may contain diseases that can be transmitted to the soil and then to humans.