The mushroom compost would be fine. You can also cover the root area - as widely as possible, even beyond the drip line is best - with any organic (in the sense of natural, i.e.; shredded bark, wood, leaves, compost, etc., not in the meaning of chemical free, and not gravel, although in the right place, that's also attractive, and fulfills the other requirements) mulch, putting it no deeper than 4", and putting no mulch in the 2-4" next to the trunks of the plants. Mulch will eventually break down - one reason to use it, others being limiting weeds, keeping root areas cooler, and keeping moisture in the soil - so you will need to add an inch or so per year, if you do begin. Mulch also breaks the force of water, so is more likely to stay put, and not wash away like your soil did. If you are planting on a slope, avoid pine bark - it tends to float and wash away. You can use a "natural" fertilizer, such as cottonseed meal or alfalfa meal, compost or one of the Organic fertilizers (of whatever brand you chose; Espoma is a good one, 'though not all the types are purely Organic) lightly before spreading the mulch, as they are mild and not likely to burn. If you plant to use a chemical fertilizer, then I would spread an inch or two of whatever you use as a mulch down first, as the chemicals are much more water soluble, and might burn the exposed feeder roots. I trust this has answered your question, and not confused you further!

I mulch leaves right back into the soil... it's free fertilizer, I only wish I had more trees on the property. Pine needles on the other hand I can't mulch up, the tractor will just keep blowing them out. I have a bagger for my tractor that I use to clean these up with. The only problem is that there is always so many that I'll have to empty the bags a good 20 times. This is actually the worst I've ever seen it. I'll gather some up and spread them around the base of some of the plants for added winter protection and mulch. I'm not going to add as much as I was going to because I just found out they blow around easily. Well at least they do today with winds around 30MPH. It's actually a lot worse right now, the entire picture above is tan, hardly any grass poking through. The winds are increasing, blowing them further

This is why I took pains-at the very outset-to stress the point that it is a matter of degree. In no way would a light coat of pea gravel-or any other course aggregate-cause trouble. And as Dave in NoVa rightly asks, how many older trees have no shade of their own root zone? Not many, I'd warrant. The simple answer is, this should not be a problem. I guess if we want to split hairs endlessly, then yes, we can endlessly dream up ways in which it would cause great harm. But that would all be BS.

We have actually done this. The developer of the subdivision planted sycamores on both sides of the street about 10 feet from the curb in the 60's. Ours succumbed to the blight they get and we had it taken down and the stump ground. Of course all the roots were all over the front yard. We have clay soil also. I moved the planting spot in from the street and over from the lot line about 10 feet each way. I bought a willow oak from Home Depot in October of 2001. It was not a big one at all, maybe 10 to 15 feet. Today it is about 55 feet. Be sure to dig the planting hole twice as wide as the root ball, but do not dig it deeper. If you dig it deep and fill in with soft dirt, any water will run past the tree roots and pool at the bottom of the hole. You want the flare to be slightly above ground level. I used those tree rings made out of recycled tires for the first few years, so we didn't have to mow around it. The only problem we had with roots was, as they decayed, mushrooms sprouted where they were. Those are long gone now though.