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I live in a USDA hardiness zone 5 temperate area. Our soil is definitely heavy clay. I have bags and bags of dirt that I can use to amend it, but I'm stuck as to how.

See, the clay is very heavy, and the temperature is quite hot (25-30° Celsius during the day time). After several days of dry heat, it becomes brick-like. I tried digging up existing soil (which kills the grass too), but it's back-breaking. I even got a pickaxe, and I can barely dent it.

There must be a better way to amend my clay soil and use up all my dirt. I also have a very fenced-in back yard, so I don't have any way to get a tiller back there (without going through the house). Manure is out of the option right now.

12

I have a similar problem in my garden. The clay soil is at least six feet deep and can only be broken up with a pickaxe or power machinery. Why?

  • as the glaciers melted in Ontario about 14,000 to 10,000 years ago they dropped the silt (clay) in huge moraines and drifts. Post ice age plants started building soil and took thousands of years to do so.
  • then, as is the standard practice for house builders in Ontario, the builder removed the top soil undoing 10,000 years of biological action

There are real challenges doing anything with the clay subsoil:

  • grass roots cannot penetrate the surface. The sod sits on top like a blanket and dies back in the hot summer when it cannot get water
  • if you dig a hole in the subsoil water just sits in it and does not drain. Most trees and shrubs will not do well so planting high or "proud" can be done
  • if you decide to raise the grade of your garden by adding lots of soil on top you might change the drainage of the area causing water to go into your neighbours yard or their house.

Here is what you can do:

  • establish what areas you want to have flowers, vegetables and trees in and raise the grade in that area by removing grass and adding topsoil. This is a raised bed. Depending on grading you may need to add french drains using my favourite outdoor landscaping material: four inch drainage pipe with sleeve. Available at most building supply centres this provides a cheap and effective solution to working with clay sub soils.
  • for areas where you cannot change the grade top dress the area every spring and fall with one to two cm of top soil. Repeat every year and your patience will be rewarded.
  • white and red clover have deep roots and make their own nitrogen. They will grow in clay and help break it up but are almost impossible to remove once planted. Will grow with grass and attracts bees.
  • instead of adding soil think about planting species which tolerate clay soils:

    • Amsonia Blue Star (Zones 5-9) - great plant
    • Asclepias tuberosa Butterfly Weed (Zones 4-9)
    • Aster (various) Aster (Zones 4-8) widely available
    • Coreopsis Tickseed (Zones 4-8) widely available and tough
    • Echinacea purpurea Coneflower (Zones 3-9)
    • Eryngium yuccifolium Sea Holly (Zones 5-10) picky foliage and seeds
    • Helianthus angustifolius Swamp Sunflower (Zones 6-9)
    • Helianthus x laetiflorus False Sunflower (Zones 5-9)( a bit weedy!)
    • Heliopsis helianthoides Ox Eye (Zones 4-9)
    • Hemerocallis Daylily (Zones 3-10)
    • Liatris pycnostachya Kansas Gayfeather (Zones 4-9)
    • Liatris spicata Blazing Star, Gayfeather (Zones 4-9)
    • Monarda fistulosa Wild Bee Balm (Zones 3-9) (subject to powdery mildew in my area)
    • Ratibida pinnata Drooping Coneflower (Zones 3-10)
    • Rudbeckia hirta Black-eyed Susan, Gloriosa Daisy (Zones 3-7) vary reliable
    • Sedum 'Autumn Joy' (Zones 3-10) great plant for fall blooming
    • Silphium integrifolium Prairie Dock (Zones 4-7)
    • Silphium laciniatum Compass Plant (Zones 5-9)
    • Silphium perfoliatum Cup Plant (Zones 5-9)
    • Solidago Goldenrod (zones 5-9) get the cultivars as they are better behaved
    • Vernonia noveboracensis Ironweed (Zones 5-9)
    • Yucca filamentosa Adam's Needle (Zones 5-10) (does not stand up well to having snow piled on it)

Edit: You can top dress twice a year, in spring and fall, whether you are working with grass or vegetables. @bstpierre will probably have some thoughts on this but I have not seen top dressing done more often as it is difficult to do so with actively growing plants.

If you dig a hole in clay and back fill with soil you will create a bowl of poorly draining soil that will not work for vegetables or fruit. A raised bed will allow drainage. The depth you require depends on what you are growing. Apple trees tolerate clay quite well so planting them high so the top half or third of the root ball is above the clay and surrounding them with soil will work. Carrots need at least six inches of soil probably twelve inches for best results. Leaf lettuce and similar veggies can get by on a bed of soil that is only six inches of soil. The taller the bed the better, less bending for your back, warms up sooner in the spring.

  • Property development is often a synonym for strip mining. Our lot had it done and we're down to a nasty clay layer that takes a lot of work to make it fertile and keep it friable. Last year, we bought the lot next door that thankfully due to the econolypse and a long period of local financial malaise, had never been touched. The soil there is 2-3 feet deep. Beautiful garden area. – Fiasco Labs Jun 16 '13 at 15:00
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    Two questions: 1) won't soil areas just create "outdoor bowls" where the roots can't penetrate into clay? 2) What's the maximum frequency I can top-dress? Also, I'm more into veggies than flowery stuff. – ashes999 Jun 16 '13 at 19:19
8

I have the same situation. For garden beds, I've had the best luck with lasagna gardening. I leave the grass in place, and cover it with several layers of cardboard and/or newspaper. I think I am well known at the grocery store as the lady who asks to raid the bin where they dump the broken-down boxes. If I have enough compostable material, I'll add that over the top of the cardboard, cover it in soil, and then cover it in a thick layer of mulch. For some of my gardens, I've run out of compostables, so I just smother the grass with the cardboard and cover the cardboard with mulch.

In either case, by the time the cardboard breaks down, the grass had died and the worms have moved in to start composting it. I do have a little grass that survives to pull around the edges, but it generally isn't too bad. And the condition of the soil is way better than it was. I like to use leaf mold as mulch for these beds, but often end up with wood chip mulch, as I do not have enough trees yet to make my own mulch. If I could afford all leaf mold, I think that would be best.

Good luck!

  • 1
    I've done no-till gardening for two years now in my yard, and I can attest that by adding leaves, grass clippings, and so on, the soil is starting to get very rich. I had maybe an inch or two of top soil, now it's about 6-12 inches deep in spots. – thomasw_lrd Jun 17 '13 at 17:04
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I agree with the lasagne gardening/sheet composting replies above in that adding organic matter is the very best thing you can do for clay soil. We have heavy clay soil here and we compost leaves every fall in black garbage bags (mix a bit of dirt in and make sure there are holes for air circulation and that it stays moist but not wet) and till those bags of leaves into the garden in the spring when the soil is workable. During the summer, we pile on several inches of straw in the pathways and around the plants, which composts during the warm wet months and also gets tilled in, in the spring with the semi-composted leaves. After just a couple or three years of doing this, our heavy clay soil was much, much improved.

One more thing - don't bother with sand if someone should suggest it. You would have to dump a ton of it into your plot to do any good at all, and it doesn't always stay near the top where you put it.

5

In your comment to kevinsky's answer you mentiond you're more into veggies so you might want to consider growing things like diakon radishes and red potatoes in the areas you want to loosen.

These plants will push into and break up the clay. I've heard of Asian gardeners planting fields of diakon crop and not havesting the roots, just letting them rot then tilling. You could probably even pull out the plants once they're done growing and then start the layering to get a head start.

I'm sure there are other veg that will work as well.

3

Sticky clay requires the addition of calcium (and magnesium to a lesser degree) to cause flocculation of clay particles and contribute to the crumb-like texture which is the hallmark of good soil with good drainage.

Compost, while a good (eventual) source of humus, is not a significant source of minerals. Quoting from: http://www.soilminerals.com/compost_manure_humus.htm

Let's look at another factor in using compost or organic matter as a mineral source: How much would we need to use to add significant amounts of needed minerals to the soil? This gets a little difficult to quantify, but going back to research done by Davidson and LeClerc in the 1930s, we find that the amount of Potassium found in ash from commercial vegetables was around 7%, the amount of Calcium averaged about 2% (they also measured 95%+ moisture content and 20% ash from dry matter, which leaves only 1% total ash, but let's be generous and stick with that 1 1/4 lbs we came up with above).

1 1/4 lbs= 566 grams

566 grams x 2%= 11.3 grams Calcium per 100 lbs compost

566 grams x 7%= 39.6 grams Potassium per 100 lbs compost

Even a sandy loam requires at least 2,000 lbs of Calcium per acre for best growth. What if we measured the minerals and found that we needed to add 1,000 lbs of Calcium? How much compost would that take, at 11 grams per 100 lbs? I'll spare you the arithmetic: It would take about 4,000,000 lbs: Four million pounds of that 75% moisture content compost per acre to add 1,000 lbs of Calcium. Wait, it gets worse: While we were adding that 1,000 lbs of Calcium we were also adding almost 4,000 lbs of Potassium, far too much. Well balanced soils need about 1/7th as much Potassium as Calcium, so this soil would call for about 280 lbs of Potassium per acre; we would be adding over 3,700 lbs too much, assuming that we were crazy enough to try adding four million pounds of compost anyway.

Putting that in terms a backyard gardener could relate to, one would need 90,000 lbs of compost per 1,000 square feet of garden just to bring the Calcium level up to par.

Concerning how to add organic matter to soil, I would favor grass clippings to anything else.

1) Its easy to find in large quantities, like leaves. 2) Grass is green, therefore having chlorophyll, which is a compound containing 4 nitrogens around a magnesium. 3) Grass has proteins which also contain nitrogen.

Dr. William Albrecht explained why nitrogen is important in organic matter: http://www.soilandhealth.org/01aglibrary/010120albrecht.usdayrbk/lsom.html

Thus when decay has proceeded to the point where the carbon-nitrogen ratio is significantly decreased, a residue of a more stable nature is produced. Thereafter the carbon-nitrogen ratio is narrower and remains more constant. This corresponds more nearly to the condition that holds in the case of the organic matter in virgin soils. Its further decay, which is slow because of the relatively low level of carbon, liberates nitrogen in place of storing or preserving it. Because of its high carbon content, the decomposition of fresh organic matter requires additional soluble nitrogen to be used as building material by the micro-organisms, which obtain it from the soil, often exhausting the supply to a degree that is damaging to a growing crop. The amount of increase in organic material corresponds, in the main, to the amount of nitrogen available. The extra carbon in the fresh material is lost from the soil. Thus when soils are given straws, fodders, and similar crop residues of low nitrogen content, only small increases in soil organic matter can result--in the main, only as large as the added nitrogen will permit. Many tons of common farm residues and wastes per acre are needed to produce a single additional ton of organic matter in the soil.

The restoration of soil organic matter, then, is a problem of increasing the nitrogen level or of using nitrogen as a means of holding the carbon and other materials. This is the basic principle behind the use of legumes as green manures. In building up the organic content of the soil itself, it will often be desirable to use legumes and grasses rather than to add organic matter, such as straw and compost, directly. If legumes and grasses are to be successfully grown on many of the soils of the humid regions of this country it will be necessary, first, to properly fertilize and lime the soil. Legumes use nitrogen from the air instead of the soil, and thus serve to increase the amount in the soil when their own remains are added to it. Commercial nitrogen used as treatment on straw for the production of artificial manure in compost piles, or when plowing under straw in the field after the combine, may be considered in the same category. Small amounts of added nitrogen may in this way make possible the use of large amounts of carbonaceous matter in restoring the soil.

So the remedy for hard clay would be lime and grass. You could apply the lime, then a thick layer of grass clippings, then apply your bagged soil over the grass clippings to trap the nitrogen in the grass from escaping as ammonia into the atmosphere, then apply more lime (unless bagged soil is already high in calcium) and nitrogen fertilizer. The result would be fairly healthy soil, with no digging. This is how I renovated my lawn. For the garden, I alternate layers of grass and soil.

I would cite more references, but for reasons I can't imagine, I am not allowed.

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