One of the best materials for fertilizing your trees is likely already a major component of your landscaping. Trees require nitrogen-rich fertilization in order to grow lush, healthy foliage. Grass clippings are high in nitrogen. While you could just pile the fresh cut grass around your trees, the benefits would be minimal, as the nitrogen won't leach far enough into the soil to benefit the roots. Instead, use the grass clippings to make nitrogen-rich compost to use for an organic tree fertilizer.
Place a 4-inch layer of grass clippings in the bottom of a compost bin. Spread it out evenly using a garden fork, breaking up any clumps as you do so.
Top with a 4-inch layer of brown organic waste. This includes dry leaves and leftover vegetable waste from the kitchen such as peelings.
Mix the two layers together with your garden fork. Top with an additional layer each of grass and brown waste, mix and repeat until the pile is 3 feet high.
Mix in one to two shovelfuls of garden soil or compost and mix in to get the microbes in the compost started.
Turn the compost with your shovel or fork at least once a week and water the pile as needed to keep it moist but not soggy.
Apply three to four shovelfuls of the nitrogen-rich compost around the base of the tree. Water in well so the nitrogen begins leaching into the soil toward the roots. Composting trees right before a rain storm works well. Apply compost in spring and at midsummer.