There are a lot of different formulas for tree fertilizer, and, ideally, soil tests should be taken to determine what (if any) nutrients the soil is lacking before any fertilizer is added to the soil. It is possible to buy the individual ingredients that comprise most tree fertilizers and then make them yourself, but what would be the point in doing that? It would be easier and cheaper to buy a pre-mixed commercial tree fertilizer. Those gardeners wishing to make a homemade tree fertilizer need look no further than their own compost pile.
Choose a sunny location away from your neighbor's property. Ideally, a compost pile should be made on top of a concrete pad, but if no concrete is available make your pile on compacted earth if possible. Pile mulched leaves, grass cuttings, broken eggshells (calcium), and wood ash (phosphorus; clean barbecue ash is ideal) into a pile approximately 5 feet by 5 feet by 5 feet high.
Dampen the pile with water. The pile should be damp but not soggy.
Place a black tarp over the pile and weigh down the edges so the tarp holds in the heat produced by the decomposing pile.
Remove the tarp after three days and turn the pile with a shovel or pitchfork, placing as much material that was on the outside of the pile into the center of the pile as possible. Remember not to compact the pile--the more air that reaches the center of the pile the better. The pile should be heating up by this point--this is an excellent sign. Add more water if necessary to keep the pile damp. Replace the tarp.
Remove the tarp and turn the pile again after three more days. The pile should be very hot at this point. Dampen if necessary. Replace the tarp.
Remove the tarp and turn the pile once each week from this point on. When the pile has been reduced by approximately 50 percent and it is no longer hot in the center, your compost is ready to spread under your trees.
Spread your tree fertilizer on the ground, from a point 2 feet away from the trunk of the tree all the way out to the drip line of the tree. Do not pile fertilizer around the trunk of the tree as this could encourage pest problems or disease.