What's the difference between the cost of soil for a berm on top of the impeded drainage layer and filling giant holes dug out of it? I'd think less soil would be used for the berm, plus you wouldn't have the expense of digging holes out of the existing material and disposing of the material so generated. You don't necessarily need a huge depth of soil for trees, especially ones grafted onto fully dwarfing rootstocks. You are likely to need some kind of support system for these as the comparatively feeble roots that dwarf the tops also aren't very good at holding them upright over time. Install drainage if needed as determined by percolation test ... All planting holes must be dug at least three times as wide as the root ball or wider but no deeper than the root ball to avoid settling. Where drainage is marginal, all plants should be planted shallow or in mounds or berms ... If the existing soil is very poor, remove and replace with good field soil or place at least six inches of good field soil on the surface. However, you should match soil types as backfilling with a good sandy loam in a heavy clay will serve as a collection point for water and the roots will suffocate ... --Whitcomb, Establishment and Maintenance of Landscape Plants (1987 (1991), Lacebark Inc., Stillwater) Here is a link that might be useful: Books Available

I always thought decomposed granite was crushed granite, what is left over after the larger bits have been screened off. Here in Phoenix the terms decomposed granite and 1/4 minus are used interchangeably. But selling decomposed granite with clay in it seems like a scam to me. 50 years ago in Upstate NY many people used crushed granite and decomposed granite for driveways. Concrete and asphalt driveways were very rare then in our area. Larger crushed rock was put down, generally not rolled because that cost money. You drove on that for a year which pushed the rock down into the soil and you raked loose stuff back over into the ruts. Over time that made a good base. Then you brought in some smaller rock and repeated. Then decomposed granite. The bigger rock helped keep the decomposed granite stay in place. The bigger rock pushed down into the soil reduced future rutting. But you still had to re-level and bring in more material every 3-5 years or so. Here in Phoenix I like decomposed granite a lot. I like a thin layer, 1/2" max. More than that and it's like walking on the beach.

The OP might do well to consider moving the material around to make mounds, berms and trenches. Doing so he/she could create places to bring in soil for beds for particular types of gardening. Although I haven't worked much with decomposed granite myself, I think that it will support many types of plants as it is. But those plants that need a different soil can be nurtured with imported clay soils. In Alaska last year, I helped make some garden beds built up on a rock and gravel fill. The location was a beach that had been built up with a boulder retaining wall and backfilled. The 'soil' was very 'young', full of stuff like decomposed granite. After making the raised beds (we used a chainsaw mill to make the boards from yellow cedar), we filled them with whatever soil we could scratch up. We screened the soil to remove the stones and used the stones between the beds for walking paths. Hard work. Some vegetables did ok in that raw soil, and cold wet climate; lettuce and radish for example. We used kelp, sawdust, and humus from the forest as mulch and soil amendments. Very different than the clay back here in Boise. I wonder what climate the OP lives in.

Starting a new garden can be frustrating! I feel your pain. I might recommend starting the seeds in trays or pots, then transplanting into the ground when they're big enough to handle. That might help you deal with what you've got this year. I wrote this for someone else, actually 2 other people, but hope you may find some value in it also... If you've got more time than energy, like I do, smothering and lasagna is the easiest way to start a new garden bed for free, or almost. Sooo much easier than digging up grass. Just spread newspaper (about 10 sheets thick) or cardboard, overlapping well, until the area you want to be a bed is covered. Then cover the paper with 4-6" of finely shredded mulch and wait for the grass to die, usually 4-6 weeks but could be longer for some grasses. I've done this many, many times. My latest one is really ugly but I'm just trying to make lemonade out of lemons with this one... drought (probably aided by grubs) killed the grass here so I decided that would be the new sunny front bed I was considering. I did dig out a little spot that had hardly any grass and put some Cannas and Gladiolus there, a tiny baby maple tree, some Hibiscus cuttings which still just look stupid 'cuz they're "dead" sticks in the ground, then kind of working around it with smothering, and bark chips, which aren't my preference but I had them available. They don't stay in place if it ever rains really hard. Anyway, with this, I'm not planning to leave the bark chips there, they're just making sure the newspaper is held firmly to the contours of the ground to block the air and light from reaching the grass, which is what is needed to kill it. Whenever I can find more shredded hardwood, I'll replace the bark chips. Anyway, the newspaper decomposes and does not need to be removed later, just dig through it to add plants in the ground. I've also smothered grass with stuff that was handy, but does have to be removed to use the bed, like sheets of metal, old egg crate mattress topper, the bags of mulch that will cover the spot, whatever's handy. I think it's easier to wait for the grass to die than dig it up, and I don't mind if it has to get more ugly in the process of getting more pretty. One other benefit of smothering with a leave-in-place substance like paper or cardboard is that the weed seeds that may be in the ground are unable to germinate as they might be if you just dug up the grass and/or tilled. The lasagna comes into play if you add amendment layers to your smothering. For example, you could put the paper/cardboard, then kitchen scraps, ready to use compost, leaves, yard trimmings, whatever organic material (OM) that is handy, then the mulch (or not, if the other stuff is a thick enough layer to hold the paper in place and block the light.) It's not necessary to have lasagna layers when smothering, but when planting later, there's a huge improvement if a lot of OM was placed there. AND, while...