strange soil profile

I had a retaining wall built behind the house as we are on such a steep slope and when the digger was in levelling the site I discovered a strange soil profile. It consists of less than 1 inch of topsoil, about a foot of solid clay that I always took to be our subsoil but underneath this there is a couple of feet of beautiful dark loamy soil. I think this may have been from when the current house was built about 7 years ago and they maybe dumped the subsoil on top of this area. My question is about how best to deal with this. Obviously the texture of the bottom layer is much more suitable for plants, however, it has been pretty deep down for some time. The top layers, although horrible sticky clay that cause drainage issues will contain all of the appropriate bacteria and funghi for topsoil. What do I do with it? Although I generally wish to adopt a no dig method I'm not sure if it is right in this area.

If I mix it up will I cause anaerobic conditions by incorporating the stuff that is growing on the surface into the deeper layers and destroy many of the beneficial organisms in the soil. If I leave it as it is will I continue to have the drainage issues or could these be solved with particular green manures (bearing in mind I'm in zone 6 I think). Growing conditions are pretty tough here (cold winters, lots of rain and a very short and fairly cold growing season).

Can anyone suggest what they would do?

since the clay layer is relatively thin, you should be able to perforate it to improve drainage. just dig a hole right through and refill with mulch. maybe experiment with different sizes and spacing.

using daikons is also a popular way to improve soil structure. the clay might slow them down, but daikons frequently grow 18" deep or more, so they might make it all the way through. when they're mature, break off the top to eat and leave the rest to rot and provide new channels for drainage and roots. plenty of other deep-rooted plants could also be worth a shot.

Are the Daikons really tough? The roots of the rough grasses that are growing in the area are not penetrating the clay at all.

Katy Whitby-last wrote:Are the Daikons really tough? The roots of the rough grasses that are growing in the area are not penetrating the clay at all.

they're fairly tough, but they've got their limits. easily available and affordable seed, though, so your risk will be minimal. several generations of daikon might be required, with each growing a bit deeper than the last.

mechanically penetrating that clay layer (with a spade) would almost certainly work to improve drainage immediately, but it would also require a fair amount of labor.

adding organic material on top would improve things gradually, but I'm not sure how long it would take to improve your whole mass of clay. probably quite some time.

in your situation, I would try out several approaches to find out what works. I don't think you want to mechanically mix the layers, though.

I agree with Tel about not mixing the layers. In these cases I'm a fan of deep mulches, to get more biological activity above, and percolating down into, the clay. And,yes, plants with deep roots like comfrey (though that will fairly permanent), daikon, and some of the fibrous-rooted cover crops like mustard or rapeseed to get OM into the clay and open it up.

Toby Hemenway wrote:I agree with Tel about not mixing the layers. In these cases I'm a fan of deep mulches, to get more biological activity above, and percolating down into, the clay. And,yes, plants with deep roots like comfrey (though that will fairly permanent), daikon, and some of the fibrous-rooted cover crops like mustard or rapeseed to get OM into the clay and open it up.

So would I be able to sheet mulching using cardboard which is what I had intended for killing off the grass in the area. Would the daikon and others be able to penetrate the cardboard or would I have to wait until it had decomposed?