I am planning to plant a dozen of various fruittrees on a slope. There is a plenty of open space right below my house on the ridge side facing East and South.
The problem is that there were a lot of truckloads of clay or clay-type soil brought in and dumped on top of the natural forest-type soil, in order to level and raise the surface in this area. I made holes for the trees yesterday and after the rain they were all filled with standing water. Only in the hole on the edge, the water drained, since it was dug in the natural soil.
There is a layer of clay from 3' to 6' thick that I am planting these trees into. Should I penetrate all the way to the soil, that would allow water drainage, or the roots will be able to do it for me?
I have 70 fruit trees in very dense clay on a sloping hillside and they are doing fine. Mature trees will have an almost mirror image of their roots below ground, so your trees will get below the fill and into the natural soil pretty quickly. I wouldn't worry about it too much. Just keep an eye on them while they are young, make sure the water does not pond around the trunks, make little run-off trenches so you can see the water flowing on down the hill. Once the trees are 6-8 feet tall they will be into the native soil.
Don't overwater when the trees are young. The clay topsoil will hold water longer, which is a good thing, and your trees will be less stressed. Stick your finger down into the clay, or use the shovel to see if the clay is wet 6" down, and if it is, hold off a little before watering. Clay is also full of minerals that add nutrition and flavor to your fruit. If you mulch the top, use leaf and straw mulch, mowed weeds, manure, the worms will come up and aerate the clay nicely.
I would be more concerned about mice and rats chewing the trunks of young trees, girdling, so don't give them mulch to hide in near the trunk. Keep mulch a good forearm distance away so they will not want to be in the open. This will also allow you to see any ponding of water. I wrapped the trunks loosely with the netting that bulbs come in, and that kept the chewing to a minimum.
It's harder to get waterlogged on a slope. When the hole is filled, that water will puddle on top instead. If the clay has been there a few years, you can get a good idea about the fertility by looking at what is already growing - if it is a sad state of growth, trees might have trouble too until you get some biomass and nutrients going.
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