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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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    You dug down 3.5 feet ???
    – Ed Staub
    Mar 12, 2012 at 17:41
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    @EdStaub 2.5', sorry, but yes, I did. I wanted to get to the clay and mix it around so that I would have deep and rich soil. It took a couple of weekends with a shovel.
    – Om Patange
    Mar 12, 2012 at 19:44

1 Answer 1

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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.

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  • +1 -- "Subsoil is usually far inferior"; it's just diluting the good topsoil with poorer subsoil. Though I will say that I've seen drawings of root systems for certain plants, e.g. beets, that would go down 4' if they could. If you had topsoil that went down 2 or 3 feet deep, certain plants might really thrive.
    – bstpierre
    Mar 13, 2012 at 14:20
  • @bstpierre - Thanks - what else goes deeper than a foot?
    – Ed Staub
    Mar 13, 2012 at 14:38
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    See, for example, p14-15 in "How to Grow More Vegetables (7th Ed.)". The bulk of the roots are in the 1' zone, but the six they show go down further: sweet corn to 3-4', lettuce to 2', tomato to 3-4', carrot to 5-8', cauliflower to 2' (but mostly at 6-12"), and beets are crazy wide and deep, spanning 8' and 9-10' deep. You can see similar charts by doing a google image search for "vegetable root system". My beds are built up and have rich soil to about 2' deep; I get great carrots.
    – bstpierre
    Mar 13, 2012 at 16:40
  • My soil is 8" deep, so that is how far my roots go. My beets roots don't go that far, maybe because I cultivate, but I get huge beets.
    – J. Musser
    Mar 14, 2012 at 13:23
  • @jmusser - Right. What's at 8" - hardpan? My expectation is that if there's nothing useful to grow down into, the roots will grow out sideways instead. This may cost the plant something, but I don't know. It probably increases the optimum distance between plants, FWIW. Based on what was in the (online) book I referenced above, I suspect that most of us generally underestimate root depth and spread - I know I did, till bstpierre's comments.
    – Ed Staub
    Mar 14, 2012 at 14:19

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