Suppose I fitted my skid sprayer (up to 10 gallons per minute, 500 psi) with an air-spade-like attachment passed through an upside-down 5 gallon bucket with a hole in the top. I bet I could bore some holes. With the right sort of head maybe I could bore 4" diameter holes 18" deep all around a tree, backfilling these with compost. Mud everywhere, but I'd cover the whole mess with wood chips. Is something like this going to work? If it did, it seems like it'd be preferable to an auger, and maybe more effective at soil decompaction than a root feeder tool.