Some people use a tool that burrows a hole in the ground and allows you to funnel dry granular fertilizer down the hole, and same theory as the deep root injection, you poke holes and pore the fert in a circle around under the canopy of the tree. Maybe more expensive than spray. Or some people apply a top coat of granular fert under the bark mulch in the tree ring and cover it with the bark mulch and as it gets watered in it slowly seeps down and feeds the tree. Are you licensed to treat trees or just turf? In my state they require you to be licensed in tree and shrub if your treating more than just grass. Whoever you buy fert from ought to be able to help you pick out a great product with micro nutrients and iron for color. Lesco makes a great tree and shrub blend. Cost is a whole lot more than liquid. And at this time of year you'd better act quick to get results this year, the trees already did most of there uptaking in the fall for this spring and summer. Where as liquid would react alot quicker. Cost for granular in my area range from 13.00 bucks a bag to better stuff for about twenty bucks a bag. 40 lbs. bags. How many trees per bag depends on product. Check with your dealer. Hope that helps a little. I didn't see any one else replying , so i thought this might be a start. Good luck, sounds like a great money maker. Nothing easier that pouring or sprinkling fert, right??