How many trees are we talking about? If it's just a few go and get some potting soil at your neighborhood garden center (might even find it on clearance right now). Dig the hole wider and deeper than you need and refill it with the potting soil. Also dip the roots in water gel before planting. If you are buying a potted plant just add some water gel to the bottom of the whole before planting. Or, and I'm not kidding, toss a couple of disposable diapers in the hole (shred the plastic covering). The urine absorbent in a diaper is water gel. Throw asoiled diaper in and you've added fertilizer too.

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