For those of us who have large trees, big roots, this is great to get some water under the top say 6' of soil.  My 5 giant greet thujas sit next to a 10' concrete swimming pool, and that exacerbates root problems but the trees are great privacy.  Not only can you use fertilizer pellets in these, but you can also just run it with water dripping......slow dripping is best for all big trees.  Just be careful not to force the shaft into the ground, do it easily, because I did break one.  Not a professional treatment, but I paid $250 to do my 5 trees last year.  So I could break a few and still be a happy camper.