Even if you planted it in the topsoil, if you have clay under it, it can cup the water and not absorb. The quickest solution for your yearly garden would be deep raised beds. It's expensive, but it's quick. Lasagna method gardening also works. Next is to till in organic matter into the clay. Back breaking, fairly quick, fairly cheap process. You should be obsessing over compost. Call your local phone company and ask for their tree chippings to be delivered to your area, it's a free source of organics. David the good's book "compost anything." Is a cheap and short guide to doing just that. The easiest, slowest and cheapest option is to let plants do the work for you. Plant alfalfa or fava beans, the deep taproot will break up the soil over time.
As far as The trees go, Clay is wonderful for trees, but you need organic matter on the surface of the soil or it'll bake into a brick-hard impermeable parking lot.
*Dig your hole 3 times the size of your pot. Don't dig too deep, as it will cause your tree to settle and sit too low if you do. You want the sides of your hole to be irregular and not perfectly round and smooth, or the roots will hit that smooth wall and just circle around in a loop. Don't amend your soil with anything but crumble it up loosely so that the roots can easily move through it once you back fill.
When you pull your trees from the pot, gently dig down along the trunk until you find the first lateral root. Often, nursery staff will plant the left-over bare-root stock that they get in the spring. They'll bury the trees too deeply, and so part of the trunk is actually below the soil line. You want to plant your trees so that the trunk is above ground and the roots are below. As you sit your tree in the hole, the first root should be an inch or two above the level of the soil outside the hole. That way, your tree is planted slightly higher than the surrounding soil. People think, "I'll plant it a bit low, and then all the water will seep into the hole and it will keep it well hydrated." Wrong -- trees that sit in low wet spots tend to rot or suffer from fungal diseases. Plant it right at soil level or slightly higher.
If there are roots that have run around the outside of the pot, gently untangle them. You don't want them to slowly strangle your tree. If you are unable to untangle them and pull them outward, then snip them off with a sharp set of shears. As you place the tree in the hole, take a few minutes to gently direct these roots outward.
Slowly backfill the soil, again, using the soil you removed from the hole. If you were to add a bit of WELL AGED compost at this time, it wouldn't be terrible, but it's not necessary. Do not add anything that will burn the sensitive roots -- this means no manure or fertilizer. The ground should gently slope away from the trunk (a drop of an inch or two) until it reaches the outside of your moat -- a 4 inch circle of soil that will hold the water in when you give your tree a drink. Usually, you will need to find some additional soil, as there is never enough from the hole and the pot to sufficiently build a little round berm around your hole.
Ultimately, you'll want fungal dominated soil to form a symbiotic relationship with your tree roots. If you have a mature fruit tree in the area where
mushrooms pop up out of the ground after a hard rain, go get a couple of scoops of that soil and put it next to the root ball of your tree. Fungi are fun guys. Your tree will be happy to bond with the fungal network.
Gently pack the soil around the tree. Don't stomp around heavily on the soil you've just backfilled. You'll be crushing those little roots and will be needlessly pushing all the air from your soil. As you water the tree, you'll see it compress a bit. You may need to add another shovel-full of soil here and there to get it back to level. Again, you don't want a low spot or depression where water will sit over the root ball.
THEN -- the key for clay soil, mulch mulch mulch mulch mulch. Wood chips.
Coffee grounds. More wood chips. Compost -- so that you get a couple of zillion bacteria and other microorganisms into that freshly loosened soil. Aged wood chips -- so that you get a nice fungal community going around the tree. And if you have some on hand, more wood chips. 4 to 6 inches of wood chips. Pull them out away from the trunk of the tree, but starting a few inches from the trunk, lay down a thick layer of biomass (chips, leaves, old sticks, compost). The biomass will keep the soil cool and moist, will encourage fungal growth, will be a haven for worms, and will keep that clay from compacting. The worms will move the organic matter down into the soil profile in the years to come. With clay, you really can't have too much mulch.
Stake your tree as needed.
Water deeply and as your tree gets stronger, less frequently. Thus, you'll train the roots to go deeper in search of moisture.
If you had to trim the roots, you'll need to trim a corresponding amount of branches above ground.
*I copy/pasted this part from another forum because it was easier than explaining this repeatedly to different people.
