Remediating heavy clay soil

Discussion in 'Planting, growing, nurturing Plants' started by tresenglish, May 22, 2015.

  1. tresenglish

    tresenglish Junior Member

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    I have a site where I want to create a water harvesting/graywater irrigated urban food forest. Unfortunately, the site has shallow (26") heavy clay soil on top of bedrock (at least 6' of a calcium carbonate rock called caliche). I also want to concentrate 10s of thousands of gallons of roof runoff into about 10 basins. Three of those basins will receive graywater and so shouldn't have an overflow. (Though I may do that anyway for large rains.)

    How can I increase lateral percolation away from these basins?

    Since water can't go down, it has to go sideways at sufficient rate to prevent prolonged water logging. Because of cost, I don't want to just dig it up and replace or amend the soil. I would have to excavate and amend over 200 cubic meters. And in any case, the site is an occupied residential area and I don't want to dig up everything.

    I am considering a heavy crop of legumes and grasses (or other plants) with deep/large roots.

    Will this increase percolation out the sides of the basin, if I don't also add sand to physically break up the soil?
    What plants should I use that are adapted to alkaline soils and -8C to 42C temperatures?

    Since the problem is to get water out of the sides of the basins,
    Do I need to build some sort of French Drain around the edges of the basins?
    If so, can you suggest any designs or design issues?
     
  2. mullerjannie

    mullerjannie Junior Member

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    Lots of questions in there but let's give it a go.

    Your subject: "remediating clay". Build your soil from the top down instead of trying to dig etc. Clay can be broken by little workers such as worms or weeds such as Yarrow or deep rooted plants like dock and Comfrey or trees. The clay will be a short stop away from being rock hard without water or muck with too much water. But luckily it sounds like you have a water issue with plenty in supply.

    If you have LOTS of water. Plants species that needs lots of water, think of water melons for instance and bananas would probably love it. Alternatively go for evaporation, it is "scary" how many liters is evaporated through big trees, big acacia's in Africa does thousands of liters per hour. My guess is that you are not in Africa so find a local specie.

    Without pipes etc. Hugelkultuur also does a good job of acting like a sponge to move water about without pipes, this percolation does a great job moving water laterally and with some trail and error you might even be able to move it back up the hill if you want a material such as a tree fern trunk with sufficient capillarity to move water up the hill ( but it depends on the gradient). It would make for an interesting post though.

    If you build up organic matter it should help soak up water. Also if you get too much runoff build swales. There are 3 S's , slow, soak and something else! But it is also a good start to slow down water flow, which helps build organic matter (or moisten) the ground for critters aerating the clay.

    Drains, well if you drain the water where will it go?

    In terms of the alkalinity, if you have constant runoff it's probably exacerbating the condition, by building the soil with organic matter it should stabilize to a point where it won't be as extreme meaning you can plant at leisure.

    If you are subject to heavy rains then you can try to build something for "silt" catchment. A small inlet pond \ basin before the larger basin. You can then harvest this for "sand'. I suspect over a period of time most of your sand will end up at the bottom of those basins. If you can put in a system such as an inlet which you can harvest for sand you could move this back into the clay (with a bit of steady manual labor). However it would be worth while since the basins silt would be rich in nutrients.

    Turn your biggest problem into the biggest solution.

    Fun project!
     
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  3. Paul Dixon

    Paul Dixon New Member

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    serious improvement of soil moisture penetration in clay soil is by planting is a long long term project.
     
  4. Bryant RedHawk

    Bryant RedHawk Junior Member

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    First off, sand + heavy clay = concrete, not what you are looking for I am sure.
    Your idea of using plants to break up this heavy clay and add humus is sound.

    Plants to really consider in the mix include, comfrey, daikon radish, mangle, burdock, rape, oats, barley, wheat, cereal rye, any of the brassica family, field peas, etc..
    The way to use these is to let them grow then chop and let it lay where it drops, let the plants that will, get their second growth start and then add new seed for those that didn't make the comeback. This will let all humus building materials you put in do their job of rotting. This will most likely be an ongoing process for at least two full years, maybe more. Once you get the foundation going, you can add other items to help it along, compost teas will help bacteria get a foot hold and provide the nutrients they need to help build soil, finding mushrooms and using them, in a slurry, to inoculate the area will also help, both the soil and the plants will become healthier and more productive as everything moves towards balance.
     
  5. DC Brown

    DC Brown Junior Member

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    I agree with Bryant. I've worked out an expensive way to 'fix' clay but it's just not practical as soon as you move from a few sq metres.

    I've found all beets/beet greens, dandelions, comfrey, dock, plantain etc will grow on compacted clay, and more importantly, open it up for other plants. Always adding nitrogen fixers, clover seems to do best then hairy vetch. When cover cropping oats work great in clay.

    I've found the best mineral mix to be gypsum (1/2) and oystershell (1/2) with basaltic rock dust and char (treated with nitrogen eg fish hydrolysate).

    Angular sand is ok, porous is not. It's to do with the clay platelets getting purchase in porous substrates and forming bonds across them - aforementioned concrete.

    To get food growing relatively fast I run a chicken tractor over first, then make raised beds to allow drainage for the clay, swale paths between them to allow water retention, then add minerals, compost, plants and mulch. Most things will grow well immediately following this treatment.

    Then there's the concept of just making a lasagne bed over the clay and letting time/plants/no till heal the soil. Works great!
     
  6. DC Brown

    DC Brown Junior Member

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    PS - berries, berries, berries. Berries love clay.
     
  7. songbird

    songbird Senior Member

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    the strawberries here seem to do well enough in clay, but they really do much better in soils with more organic material, so putting a nice layer of mulch on top of clay and then putting strawberries in that is a very good combination. the clay underneath keeps the moisture pretty well which the strawberries like and by the time the strawberry patch is ready for remediation (in a few years) the mulch has helped feed the worms, the worms have opened up drainage channels and moved more organic materials down deeper and the whole area is improved. the mulch also really helps during any dry spells to keep the ground from cracking and even if it does crack then the mulch can work its way down the cracks so it's not really that bad.
     
  8. Flatland

    Flatland Member

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    I'ld second the gypsum. At my old place I had very heavy clay used gypsum and horse poo and ended up with lovely soil. It was the gypsum that really opened up the soil. Dig it in if you can and have a strong back otherwise just sprinkle it on top, and maybe just scratch the surface a bit. Use heaps lots more than what the bag says.
     
  9. Bryant RedHawk

    Bryant RedHawk Junior Member

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    I agree, gypsum is great, and if you find a new house construction site, you can often get the leftover drywall scraps for free, just make sure they are pure and not treated with anything. I've gotten many of these scraps, ground them down to almost powder and used them in lots of gardens that needed the gypsum.
     

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