how much compost to add to clay soil

18 years ago

We've just plowed a 100' x 100' garden in SW Missouri (Ozarks); had soil testing done and have added the recommended lime and fertilizer. The soil is very clay-ey and sticky and we'd like to add organic matter. We have a source for organic compost, but I don't know how much to add per square foot. The soil test report says Organic Matter 7.1% but I don't know if they mean that's what it contains, or that's how much to add; nor what that translates into in terms of pounds per square foot. I have searched the forum but didn't find this kind of information. Any advice would be appreciated. - Kathrynne

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Oil_Robb

11 years ago

No till? Every farmer from the beging of time tills ...do you think we have ten years to wait for worms to do what we can do in 1 hour? unless its around shrubs and trees I say till er in and loosen and feed that soil, I till all kinds of OG on a regular bases and with my drip lines I grow bumper crops. In my 60' by 60' garden I till 12" of organic matter in every year and every other year I add 3" of shredded wood from the municipal pile. I started with rock hard clay (cement) and 3 years have great soil.

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Comments (17)

jean001
18 years ago
Minimum amount to be of any vaue is to mix in 2 inches.

Because this is your first time, I'd suggest 4 inches, but do it 2 inches at a time -- add 2 inches and mix in, repeat.

In successive years, again mix in 2 inches.
ericwi
18 years ago
If you put down about 6 inches of compost on a small section of your garden, and tilled it in to a depth of about 10 inches, you might end up with workable soil in that spot. Getting the compost thoroughly mixed in with the existing clay will take some effort, and that is why I have suggested that you begin with a small section of your garden. When you have a section ready for planting, you can estimate how much labor and compost would be required to complete the entire garden.
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I gardened in SF clay for decades so I know what you're talking about. You cannot work California clay in January. That is the rainiest month, and right now you've gotten huge amounts of rain even for January. Wet clay should not and cannot be worked. I never planted anything in the ground (pots only) in December or January - the clay is too wet and too cold. A planter box should have purchased soil mix in it. Not clay. The comment about using plastic is to protect soil about to be planted in from getting excessively wet. Not for the whole yard. I would instruct my clients to tarp the area they wanted me to plant fruit trees in so it would be workable. Once clay is wet, you have to wait for it to drain a few days before you can work it. (Hence the tarping beforehand.) Squeeze a bit in your hand. If it squeezes through your fingers and remains in one piece with ribbons sticking out where it came through your fingers, it's too wet. It should crumble at least a little bit before you try to work it. When it's dry enough, compost is the best thing. Gypsum does not work in California clay. You will not transform it overnight, or even in a year or two. It will always be clay, and that's not a bad thing - I miss clay. Here I'm gardening in dirty gravel, with no water retention or nutrient retention or any measurable organic matter. Most trees and shrubs will grow in clay just fine, with annual additions of a little compost. Lettuce and other vegies are a different story, they need a looser soil. That's where mounding up amended soil comes in. Dig up the clay, add some compost and maybe some purchased topsoil, and mix it all together and heap it up above grade. Don't just lay stuff on top of the clay and then plant in it, as water will sit on top of the clay and the plants' roots will then be sitting in water. Mix it up so it grades from amended soil to the native clay. Compost helps any soil, clay or sand. You just can't work clay soil after all that rain you've gotten. You'll have to wait for it to dry a bit.
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Until last year, I used to garden for my mother-in-law when I visited (2-3 times a year) and you could go to a pick-up place in one of the county parks (?) and take all the compost you wanted for free. I think you were supposed to be a resident, but no one was there to check. While I wasn't a resident, she was, so I felt ok about it. Over the years, I did it many times. I never had a truck, just my m-i-l's very fancy car, but I just saved the bags from when I bought other amendments or potting soil for her, and reused those and other assorted containers. Anyhow, I am not suggesting that you go to a different county to scam free compost, but that you look at other sources if you aren't happy with the compost from WF. BTW, compost from yard waste is perfectly fine to use. It heats to a much, much higher temp than home-made compost and is very, very unlikely to contain any weed seeds or pathogens. Aphids won't survive even home composting. Heck, they won't survive a blast of water from the hose. You can compost them yourself quite safely. "Some kind of mulch" is a whole 'nother story. Whatever that is, could, indeed, have brought in oak root fungus or some other disease. This would be especially true if the mulch was un-composted oak bark chips or something like that. But oak root fungus is so widespread in your region, your friend could have gotten in any number of ways. It often is dormant for many years before it starts showing symtoms, so it would be impossible to say it was "brought" in by any one item. If you can't get compost in your county for free, go to the nearest nursery or Home Depot and buy it. I would bet you nearly anything it would be cheaper than WF.
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Kimmsr
18 years ago
The soil test report says what is in the soil, so theoretically you have a humus level of around 7 percent. If you think it needs more, add more. According to most all soil scientists the optimum level of OM in soil is 5 percent, but I have not found that to be worth much and most of the soils (clay, sand, loam) I have worked with need much more.
Heathen1
18 years ago
Also, as someone with years of experience with clay soil. One application just won't do it. BUT You can help enormously by mulching with about 6 inches of hay. I did that one summer, and then in the fall, turned it into the soil. The soil pogressed better that year than the previous 5 years of dumping compost in it. I can't rave enough about it.
Kimmsr
18 years ago
Heather, it would not have made much difference which type of organic matter you used, just that you used some and enough. Hay, Straw, shredded leaves, pine straw all materials that would have done the same thing, get enough organic matter into the soil. It always takes a lot more than most people think it will. Keith Baldwin in his article in Taunton's Press old Kitchen Garden on amending clay soil said you needed at least 8 inches of organic matter to start.
peggy_g
18 years ago
IMHO there is no such thing as too much organic matter. Do follow the advice on the lime; having the pH in the desired range (6.0-7.5) allows nutrients to dissolve in water so they are availabe for uptake by the plants. Soil structure is also affected. The most common liming material is ground limestone. The calcitic limestone (calcium carbonate) is the type most often used. The other type, dolomitic lime, is only used if your soil also has a magnesium deficiency. Good luck changing the sticky soil.
dchall_san_antonio
18 years ago
I think you will get about all the microbial benefit compost can offer from 1/4 inch. If you need a mulch in addition, you can use cheaper mulches on top. 1/4 inch is 1 cubic yard per 1,000 square feet.
Heathen1
18 years ago
You're welcome. :o)
crumbo
18 years ago
I think ksimmer is correct when discussing the level (%) of organic matter in the soil. However, what you are doing by adding more organic material would be making temporary changes in soil structure. A good amendment for you would be a grain or nut hull, something with a little more silicon-- don't know if you have any rice producers in Missouri. This would be similiar in the short run to adding sand, and therefore you could add a foot or so.
mossyard
13 years ago
Should I mix compost into the whole bed or just where I plant things? Don't want to encourage weeds to grow but also don't want to mess up soil drainage by creating underground clay pots. :) Advice?
Also, when you plastic and bark how do you ever mix things into the soil, mulch, etc.? That may be an embarrassingly silly question, but seems like people recommend both plastic/bark and mulching/mixing, but they seem mutually exclusive...
mossyard
13 years ago
Should I mix compost into the whole bed or just where I plant things? Don't want to encourage weeds to grow but also don't want to mess up soil drainage by creating underground clay pots. :) Advice?
Also, when you plastic and bark how do you ever mix things into the soil, mulch, etc.? That may be an embarrassingly silly question, but seems like people recommend both plastic/bark and mulching/mixing, but they seem mutually exclusive...
humbll
11 years ago
Once you begin adding organic matter to the soil, it will be worked in by worms and other critters. Keep adding organic material to your soil, year after year, and avoid using chemical fertilizers and tillers because these destroy the microbial and fungal life in the soil which is what breaks down the organic material. It also drives away the worms. Use only organic aerated compost teas to supplement the plants needs as the season goes along. Adding good organic materials to your garden, including mulches such as wood chips and straw, composts, etc. will all eventually be broken down into smaller and smaller particles and brought down into the soil through the action of worms, and will provide a wealth of nutrients for your plants to thrive upon.
Kimmsr
11 years ago
There is some evidence that plastic on soil creates more problems than it cures because the plastic could lower air infiltration into the soil. I have found soil under plastic to be rather stinky indicating anaerobic conditions exist. Adequate amounts of mulch do as good a job at "weed" suppression as anything else but do not cause problems with air infiltration. The Soil Food Web needs air along with adequate amounts of moisture to work.
jolj
11 years ago
I add to the whole bed, no matter what I am adding soil, compost, water or mulch.
You know you have to much compost when you sink to your knees in the bed & have to call for help. :-D
C Man
3 years ago
100 foot by 100 foot is a good sized garden. If you are starting a fresh plot with little to no organic matter i would recommend bring in 15-20 yards of compost to mix into the top 10-12 inches. For a plot that size i also recommend renting a tractor or hiring a service to come in and rip the native clay to break it up initially, then disc it (breaks soil into finer chunks), then have your compost delivered and till it in.

If you intend to do garden this large plot year after year you should also invest in a good rototiller to help the process along. Some people are anti-till, but that is pure hogwash. Without a tiller you have no hope of enriching your soil in a garden of that size in YOUR LIFETIME. Sure earthworms and microbes will do a lot of the work but you need to mechanically mix in signficant organic matter which is not practical by just dumping it on top and shovel mixing it in without breaking your back or taking FOREVER.

You will have to repeat the importing of compost for several years in a row to build up the soil. Clay is not bad assuming it has sufficent organic matter mixed in. Clay is good for holding moisture and has signficant nutrients in it. Don't be dismayed. I did what you are hinting at 10 years ago in a garden i started from scratch.
floral_uk z.8/9 SW UK
3 years ago
Thread is from 14 years ago ......

how much compost to add to clay soil

Oil_Robb

Comments (17)

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