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In the answer @bstpierre gave to this question on ammending soil he mentioned mixing layers of soil during double digging is a bad idea. I was wondering why is this?

I imagine mixing the layers at the initialization of a vegetable patch (converting from lawn to garden) would be a good thing. For example layering in the following way:

  • compostable material + compost in the deepset layer
  • turf and top layer of dirt
  • mixed dirt
  • more compost
  • more dirt

might lead to a very rich, deep soil. Then, this procedure would only be repeated if the garden seems like it is depleted of nutrients.

At any rate, this is what I did last fall to prepare a new patch for the coming season. I dug down about 2.5' hoping that the turf and compostable stuff deep down would compost. I have a clay soil. I guess I can report back in a few months to say how it went, but it would be good to know what people have already experienced.

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Om Patange
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1 Answers1

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Once you get below the root zone, the only thing you're affecting is drainage. How deep the root zone is varies from plant to plant; see e.g. "Root Development of Vegetable Crops". Even when taproots go very deep, you probably want to focus on where most of the root hairs are, for most of the time the plant is growing, which is of course less deep.

Subsoil is usually far inferior to topsoil, so if in mixing you're putting topsoil below the root zone, you're wasting it.

Some folks think you shouldn't till at all. I'm not there, because I use winter rye that needs turning in, because I add compost every few years, and because I have invading tree roots that I have to keep an eye on.

In composting, there are four key ingredients: carbon, nitrogen, water, and oxygen. When you bury plant material where no air can get to it, you won't get much composting.

Ed Staub
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