How do I work with soil that has a high clay content?

Question

The clay content in my garden soil is rather high. Tilling it is a royal pain, especially if there is even the slightest hint of moisture in the soil. It just turns into big sticky clumps instead of nice fine dirt. Needless to say, this is less than ideal for planting.

Is there anything I can do to the soil to improve this situation? Any treatments or substances I should be tilling into it to make it easier to work with and more accommodating to my plants? Any special equipment that would make tilling easier?

Answer 1

I've recently moved into a house on heavy clay (Gloucestershire, UK) and have been working the soil for just over a year now. We decided to turn the central lawn area into several raised beds (ideal for clay-y soils with poor drainage) for a kitchen garden and leave the established shrubby borders. The first thing I did that was enormously helpful for cultivating the vegetable area was stripping the sod and then, using a garden fork to loosen the soil, turning the sod into a fork's depth of soil. I did this in the fall and left large clods of clay to be broken down by the winter weather and frosts. By the next spring, I found that the clods were breaking down nicely and the worms had been at work eating the grassy material that had been incorporated. I added some bought top soil and compost or manure to the raised beds which is slowly starting to help as it breaks down.

If you live in a frost-free zone, or aren't digging in your lawn, in general, the best way to start 'lightening' your clay is to add organic matter; well-rotted manure or rich, home-made compost are best for this. While peat will also help to lighten your soil, I suggest NOT using it since the destruction of peatlands has severe ecological implications. Other tips: never cultivate your clay soil after rain / when it's wet; avoid regularly walking on areas in which you intend to dig or plant as this contributes to compacting the soil.

Answer 2

Here is a good article about improving clay soil. I found this part interesting:

Compost plays a key role

Composts are integral to my clay soil management plan. Because of the humified nature of compost and its low concentrations of oxidizable carbon and available nitrogen, compost is relatively resistant to further decomposition, and additions of compost to the soil over time can increase the soil's organic carbon and humic matter content. I add compost not so much to provide nutrients as to provide stabilized organic matter that will improve the physical properties of the soil.

Answer 3

I'm in much the same boat I usually ruin half the garden. I've tried just not tilling it and that certainly works, in subsequent years you may be able to just use compost you turn into the soil with a pitch fork and double digging.

If you tilled it when you absolutely knew everything was dry it would be too late to plant cool season crops. So... Maybe plant tomatoes and cukes and zucchini on the side of the garden you want to till in the dry season and get that soil good and worked up. Then repeat on the opposite side next year. By the third year you may have a garden that requires much less tilling.

Answer 4

The best type of manure for lightening clayey soils is stable (not farmyard) manure. Make sure you stack it under cover for several month before using it.

Answer 5

I MISS my clay soils! I've just moved to another state, Oregon, smack in the middle of cinder cones and my new soil is pumice and sand. Easy to work but watering is going to be every day until I can get my 'slaves' (micro and macro organisms) to find my garden, eat, make babies, do lots of pooping. I could use a tiller but I've gotten use to doing the work myself as I'll never use a tiller in clay, even dry. I don't think clay ever gets dry unless baked into bricks, sigh.

Tilling clay is like rotating the concrete drum. How is concrete made? It is made with gypsum, lime, gravels, clay (portland cement), water and mixing. Clay is a bit different than sand and silt particles. Very small and FLAT particles of rock. These rock faces have charges that attract other clay particles and the flat sides get together and stay together. These charges are amplified by movement; tilling moist clay increases these charges. A couple of decades ago, people routinely added gypsum, river gravels, to their clay soils (spraying them with water to 'soften' the clods) and with tiller in hand made...their new concrete patio! Or should have. This would be a bummer for someone new to gardening. They probably never tried again.

But clay has great properties; holds moisture, nutrients don't wash away each time you water and a little goes a long way. I manually (arghhh) mixed compost as I double dug my beds...half-heartedly...remember the less you manipulate clay, the better. And I never add gravel or sand or any other mineral (unless my soil tests show a deficiency). Double digging is the most work I do in my garden, I do it ONE TIME the first year. Depending on the subsurface layers, I try to go down 18" to 2' and if there are layers of blue clay, shale, old lumber, roots, garbage, rocks I'll pull them out or punch holes through them for better drainage. The top 6" usually has all the organic material so be careful you don't turn all that over and bury it 18" below.

Plant roots, something like 95% of all the roots in the world are within the top 6" of soil. When adding mulch (hopefully decomposed mulch) to the top of your beds, after the plants are in the soil, be careful not to add more than 2-3". Too much will bury the plant's feeder roots and they won't get enough air and water. Careful with shallow rooted plants such as Daphne, Rhododendrons and Azaleas. Only use an 1" of mulch for these plants. And keep the mulch (and soil) away from the base of woody plants. Mulch keeps moisture trapped in the bark and microorganisms will girdle (kill the vascular system just beneath the bark) and will slowly but surely kill the plant.

My new garden soil has very little organic material, probably 1" of the top soil otherwise it is 3' of uniform pumice and sand, blown from cinder cones 50 miles away. I haven't dug any deeper. In fact, I am only double digging down 1 foot so I don't lose any organic matter. Sure doesn't fluff up like clay did. If this soil were clay, I'd have 3' high beds which would reduce to 1' within a month or two. The beds won't be compacted from being walked on and should last forever, IF you continually top dress with decomposed organic mulch. I also love green cover crops for the winter. Keeps the weeds down, the roots keep the soil friable and when the plants turned over and chopped up in the spring before going to seed, these cover crops add a lot of food for your soil.

I was able to find the local power company in need of places to dump their chips and had 40 yards or more dumped on our property. I won't use any of it in my soil until it is decomposed. Maybe for the walkways in my new greenhouse. It will take years to break down. Our climate is cold and the decomposers are sluggish, I guess. I'll keep 5 or 10 yards to decompose, the rest for my horse's paddocks and the rest we've covered our driveway with 4" to keep the dust down. Works great.

And no, I won't use my horse manure. I spray my horses poo with pyrethrins to reduce flies. I want insects, worms, bacteria to thrive in my soil and my horse poo won't be decomposing properly and the pesticide I certainly don't want in my soil. I till my paddocks and plant pasture seed in the fall. Grass seems to do just fine in yucky soils as long as they get fertilized and watered. One of my neighbors has decomposed horse manure (3 years old) but I need to find out what she gives her horses and what her pesticide practices are before I use any of it in my beds.

I won't double dig again unless I enlarge my beds or move again! The most important thing I do besides watering is watching for nutrient deficiencies and keeping insect/rabbit/deer/fungus/virus/bacterial/mice/rats damage to a minimum. Feed the good guys and the bad get fed too. But a healthy ecosystem albeit made by me, takes both. The good eat the bad. If no bad guys what will the good guys eat? I always top dress my beds with decomposed organic mulch. This keeps the good guys good, grin. The good guys are your 'slaves' doing most of the work for you as they did in a natural ecosystem. Checks and balances. Humans love to control everything. As a gardener, a lazy one to boot, knowledge is power. A good manager delegates!

The first year is the most difficult until you get the soil pH stabilized and full of organic matter. I'll be sending in soil samples to our Cooperative Extension Service for testing once per year to help me figure out a fertilizer program. The products for plant nutrients or fertilizers are quite high tech...grin. They are mostly organic, more slowly taken up than the big blast of inorganic fertilizers and they now come infused with bacteria and mychorrizae! You can make compost 'teas' out of some of these products and water life back into the soil.

In my 'sandy' soil I am going to need to be vigilant with nutrients as they are leached out with every watering. In clay soils you will need to be more careful about how much and what kind of nutrients to add. Clay holds onto nutrients, some more than others. Too much of one nutrient can cause deficiencies in others. Too much fertilizer can kill plants, especially if the fertilizer is inorganic. Once as a new nursery employee I was asked to fertilize. Unfortunately some of the plants had already been fertilized the month before and someone had forgotten. The next day, toast. Big shrubs, expensive trees gone. I tried removing the soil, washing of plant roots and replanting but still casualties occurred. I felt awful. But even worse than that was when I led crews for landscape maintenance.

I had a few landscape maintenance clients that thought if a little works, more is better! Maybe they were talked into it by the chemical company..."Hey how about a 'deep root feeding'?" Remember what I said about most plant roots? Deep roots are for stabilizers, not for uptake of nutrients. The chemical company SAW the signs we put up saying what and when was fertilized. My client's entire landscape was killed, lawn, trees, shrubs, hanging baskets! Guess who got sued...not me! Or my company. Fertilizer is NOT PLANT FOOD. Plants make their own food. Nutrients in the soil are usually part of an elaborate ecosystem. We have to add nutrients because we've 'mined' the original ecosystem...taken trees, indigenous plants, topsoil away leaving a sterile site to build a home or a parking lot.

Can't plant trees to replace a forest. Sometimes I wonder just how educated our 'leaders' are!

So us gardeners have to understand ecosystems, soils, geology, water, chemistry, climate, weather just to grow a tomato plant. Otherwise, you are just lucky. A 'green thumb' is earned through study and experience. No one is born with one...

Answer 6

I'm afraid it's lots and lots of organic matter. And keep adding it. There was another question on this just recently also with some good information. There is good information in these answers. You've the most nutritious soil, but the most hard work. Potatoes and some legumes will break up the soil. Time your tilling to a couple of days after rainfall, it will be less sticky. I live in the south Midlands in the UK on heavy clay, so I have felt your pain. I've also added grit and grit sand, but this is expensive. I haven't had many failures in my garden, so persevere it will be worth it. If you can bear to do it, double dig, this will prevent or get rid of an iron pan. Scaffold planks used to walk on, will preserve the soils structure, and prevent, compaction. If your plot is big enough, is there the capacity to use a rotovator? Then leave until broken down by frost. The worms will come you've no need to add them, especially if they are in the manure that's rotting down. Like Shanna, I don't recommend peat and for the same reasons. I put anything on my clay, from an old bag of compost that's been hanging around too long. Grow bags usually used for tomatoes, anything to lighten the soil. There is a very good reason why there is a pottery near me the good local clay soil .......... Hard work or not a toss up between clay or sandy soils - clay all the way.