Hard Clay Soil

Calimoss(z10 CA)September 28, 2005

Hi All!

I live on the hot/dry side of San Diego and have an over abundance of clay soil. We had our yard landscaped last summer but I would like to add some fill-in plants in some places. The problem is the soil in my empty spots is rock hard! (We've had our fair share of 90degree days this summer).

When we had the yard landscaped they removed a great deal of the existing soil and added amendment and compost for the plantings. I did the same for a 5x3 area this weekend and it was truly back-breaking work!

Any tips on a less agonizing way to go about this?


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username_5(banned for no reason)

Use a pitchfork and stab it into the soil and rock back and forth. Don't break your back trying to lift it or turn it as this is unneceesary, just break it up a bit. Add generous amounts of compost and repeat. Take it slow and easy the idea is to work some of the compost into the soil, but don't try and get it all into the soil, this is not that important. Be content with whatever compost falls into the openings the fork creates as you rock it back and forth.

Leave a couple inches of compost on top of the soil. Repeat annually. The idea is that over time worms will tunnel through your soil to feed on the compost and the compost falls into the tunnel. Think of earthworms as tiny, slow motion roto tillers. You will always have the clay soil (which is actually a blessing in disguise in your climate as it holds water much better than the sand soil others in your state deal with), but the more compost that gets incorporated, the more workable it becomes. Also, after your initial compost treatment place 3" or so of organic mulch (wood chips, grass, chopped (with a lawnmower) leaves or whatever you have available. Allowing the clay soil to sit bare allows the rain to cause it to form a hard layer on top and the baking sun doesn't help either. The mulch and compost will result in greatly improved soil in about 1-6 months although the area is plantable immediately. Repeat this annually and your soil will improve every year.

    Bookmark   September 28, 2005 at 7:49PM
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vetivert8(NI-NZ zone 9a)

The trouble with clay is that you need to choose your time to work it. It's like Goldilocks's porrige!

Allow a couple of weeks to deal with each patch.
A week before water the area. If you're under water restriction - recycle the rinse water from your washing machine. It's usually quite safe.

Then cover the damped patch to be worked with black plastic or a rubberised sheet so the water is held in place without evaporating.
Take it off the night before you want to work the patch.

Next day have your compost and gypsum and mulch at hand. For mulch, use something that will hold water and rot down quickly. Chopped leaves, seaweed, grass clippings.

For the digging I'd use a big spading fork with heavy tines. And I'd work on no more than four square feet at a time because this is HARD work.

Expect the dirt to come up in big chunks. Smack them with the fork so they break up into smaller clods. Aim to get down at least a foot - and loosen the layer below if you can.

Add your amendments and mix them through. Water enough to damp the compost well. You might also want to add a soil wetting agent. Cover with the mulch and let it sit for another week.

Keep the area just damp and keep adding mulch. 'Never let a bare patch show'. And never let the mulch pack down to a thatch or a lot of your watering will just flow off.

You'll have an initial planting site prepared at the end of the process. If you could plant something like lupins for over the winter they could be dug in for spring, and they have strong enough roots to help break down the clods.

Be aware that the clay below your young soil may form a pan trapping the water. If you put in plants such as shrubs that have delving roots it is possible they will hit the pan and start coiling as if they were root bound. When you come to plant either choose shallow rooted plants or carefully lift out your 'new' soil, loose the clay below, then mix some of the new soil with it. Adding sharp grit (not sand) about 5-7mm will also help to open the soil and get water tracking through.

The advice given in the first posting is very useful and will help you build a soil to replace what you've lost.

Clay can be good for growing provided you can get air and water into it, and add lots of humus. It often holds plenty of water over dry times but the spaces for roots to forage are so close a plant can droop from thirst long before the soil is really dry.

    Bookmark   September 28, 2005 at 10:41PM
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don_brown(Zone 6A NS)

From experience, my advice has two main strategies....roto-tilling and amending. It is a lot of work, and you should restrict yourself to limited areas to work with at one time. But, that said, get a roto-tiller and till the area thoroughly until the clay is well broken up and there are no big lumps......just do not go down any more than a foot or so....you don't want to turn it into a mining expedition. Add LOTS of compost, dead leaves, grass clippings, peat moss and sand to the soil, and till it in thoroughly. (I use about three parts organic material and one part sand.) You may have to do this several times over a number of years, but eventually, it is going to become good workable topsoil.

    Bookmark   September 29, 2005 at 7:51AM
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username_5(banned for no reason)

As a one time roto tiller myself I can honestly say that I no longer have any use for roto-tilling. The big disadvantage of roto tilling in clay is it makes the soil worse at the bottom of it's reach. The tines rub the clay smooth and make hardpan. One ends up with 6-8" of loose soil and a nearly impenetrable hardpan under it. Hardpan can be broken up over time via biological activity in response to the addition of compost and other organic material, but why create it in the first place?

I suppose if one wants to use a tiller to break up the soil for the first time rather than use a pitchfork that could make sense, but I wouldn't suggest buying a tiller for that, just rent it and after that keep adding organic material and organic mulch and the soil will remain decent.

    Bookmark   September 29, 2005 at 10:26AM
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Calimoss(z10 CA)

Thank you so much for the much needed advice. It never occurred to me that if left uncovered these areas were going to turn into clay rock!

    Bookmark   September 29, 2005 at 10:31AM
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rhizo_1 (North AL) zone 7

All great advice, though I would not bother with the sand. Organic matter of ALL kinds is the cure for that clay soil. I think that you will be really pleased with the results, and in less time than you think!

If you can find a source of triple-ground wood chips, it would be striking gold! That stuff makes the best soil amendment I've ever used.

    Bookmark   September 29, 2005 at 1:51PM
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bejay9_10(zone 9/10)

All of the above posts are great suggestions.

However, I might add one or 2 thoughts - if you don't mind.

As I am an older lady, my physical contributions are of necessity, limited. For those of us who like to garden - now, as opposed to next year or so, I would recommend building a raised bed.

These can be done in many ways, such as using some good soil and compost mounded up so that plant roots are elevated over the clay soil, providing better drainage.

I like to raise vegetables year round, so I have 9 cedar boxes, measuring approximately 3-4 ft. across x 5 or 6 ft. long. These are filled with my own home-made compost and soil mixtures. They are then instantly "workable."

In time, this material sinks down into the clay soil below, and if a continuous supply of manures, etc., are added, the resulting soil microbes, and angle worms will break up the soil below.

I plant my boxes year round - and now have celery, parsley, chives, cabbages, carrots, spinach all planted, plus still reaping summers harvest - tomatoes, peppers, eggplants from the boxes as well.

Just my 2 c's.


    Bookmark   October 5, 2005 at 11:01AM
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The key, mentioned several times above, in being able to dig (or rototill) hardened clay is the moisture level. If it is dried out, it is difficult or impossible to rototill or to dig with a spading fork. Too wet and it is mud.

In my area, we have hard clay above caliche and you can't get enough compost into the soil to make it loose. The only way is to replace it all to a depth of at least a foot with commercial topsoil. You can add things like ironite or gypsum to the soil to loosen it, but here that is just not enough. Our clay is very rich in nutrients, just hard as cement when dry.

You have several options: 1) water, then wait a few days for moisture to soften the soil, add some compost, and use a spading fork to spade it in, 2) work a spading fork into the soil enough to break it loose without turning it over, water, then wait a few days for moisture to soften the soil, add compost, and turn the soil over with a spading fork, 3) water several days before your first hard freeze, let the freeze break the soil, then spade in compost (since you live in zone 10, don't wait on this one). Breaking the soil loose before watering is the best option, because it will let the water break it loose more.

Good luck!

    Bookmark   October 5, 2005 at 6:44PM
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bobkat13(z7 Richmond VA)

Your other option, and the one that I chose, is to go on over to the Soil, Compost and Mulch forum and search for Lasagna. I've used this method with great sucess this year over my subsoil left by the builder (red clay that turned to thick muck when wet and cracked when dry).

    Bookmark   October 7, 2005 at 9:13PM
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bobkat13(z7 Richmond VA)

And to be more precise about my answer: it takes the high school boy that builds my lasagna beds about an hour to build about a 60-70 square foot bed that can be planted in immediately. Not a raised bed. It goes like this: pick a spot you want the bed. Lay down cardboard, or thick sections of newspaper. Overlap the sections. Cover the bottom paper layer with mulch - about 2" thick. If you have horse manure - that's the next layer. If no horse manure, you can scatter alfalfa pellets (even alfalfa cubes, if your nearest feed store doesn't have the smaller pellets). Feel like wetting the stuff so far? Go ahead. Next throw on a layer of rotten hay or straw. Then mulch. Then - oh, say a whole bunch of used coffee grounds that you get for free from Starbucks. Cover with a final 2" of mulch. Pick the spot you want your plants to go. Scooch all the stuff aside that you just layered up and make a hole bigger than the plant. Pour in some store bought dirt (such as Scott's Garden Soil or Black Velvet). Put your plant in, fill in with more dirt, water.
The plants seem to like it, you can scrounge up a lot of the ingredients for free (use fallen leaves or pine needles in place of mulch, or use grass clippings in place of manure or alfafa). I've planted all kinds of perennials, annuals, and bushes (including roses) in this within the last 3 months. Some of the rose bushes have tripled in size and same thing with other stuff. Nice blooms, too.

    Bookmark   October 10, 2005 at 8:34PM
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All of these methods are great, but what if my bed is already planted? We planted lots of perennials, but didnt realize the clay basin until we putin a flowering cherry...any ideas on how to loosen things up without rototilling? It keeps coming up for me to jam a large digging bar in the soil several times to attempt to break holes into the soil, then filling those holes with some gypsum or something...any thoughts?

    Bookmark   October 17, 2005 at 5:59AM
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username_5(banned for no reason)

If your bed is already planted then you put the organic matter on top of the soil and cover with mulch. The clay below will be improved over time. You can also amend everytime you dig a hole for a new plant. If your soil is very tightly packed and drainage is an immediate issue then a pitchfork or other tool can be used to poke holes into the soil around plants and then compost or other organic matter can be raked in.

Gypsum is more for sodic soils rather than all clay soils. Here is a link that explains when gypsum is useful

    Bookmark   October 17, 2005 at 9:21AM
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Even though you have already put in plants, you still have a lot of options. Adding compost to the top works great, and it is easy, but works over time. If you need to speed up amending your soil, here are a few suggestions:
1) Buy some earthworms (if you don't have them already), dig a few holes where you can, put in compost and a few worms, and keep the area moist (but not wet enough to drown the worms). You can dig deeper and closer to established plants than you might think as long as you don't dig too much or pry on plant roots. Dig holes and add compost in different spots next year.
2) Break the soil with a spading fork as you add compost or mulch, just don't pry perennial plant roots loose.
3) Contact your local county extension agency for help. They are the professionals, they know your area soils better than anyone, and some of their services are even free. Hard clay soil in my area has a pH that is way too alkeline.
4) Lower the pH and loosen the soil by adding sulfur or other commercial products in addition to good compost.
5) IMHO the best compost available is made from cotton gin waste. It will lower the pH more gently than commercial products. When I was a kid, once a year we would use an auger to dig a 3ft deep hole about 3ft away from fruit trees in our orchard. We would throw cotton burrs, any plant waste material we could find (leaves, grass clippings, etc.), and soil into the hole, then add a few worms close to the top. The trees, the worms, and the watermelons we grew in the orchard all loved it.

Good luck

    Bookmark   October 18, 2005 at 10:43PM
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SIMPLE task!
Just use you electric drill with a large drill bit...... 3/8-1/2 bit and longer the better. I use a 10" long bit. You can drill through anything. just slowly move the drill back and forth as it sinks into the soil. Good luck! Woody24

    Bookmark   June 10, 2013 at 11:49PM
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After much work and amendment expense breaking down 6ft X 30ft of hardpan, I figure it would be better to have your local soil supplier dump a truck load of topsoil in your yard. Live and learn

    Bookmark   April 13, 2014 at 4:41PM
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