Garden soil is much more than simple dirt and when it comes to clay soil, it can be complicated. Learning how to amend clay soil can help you become a better gardener. It's not as difficult as you may think; improving clay soil involves mixing organic materials, such as bark, sawdust, peat moss, composting materials, or manure, directly into the soil.
Examined closely, garden soil, including clay, is a complex mixture of mineral particles, organic material, moisture, living organisms, and chemical nutrients. Among the most critical factors in a soil's ability to grow plants is its texture, dictated by the size of the mineral particles found in it.
The ideal soil for gardening is loam or loamy soil. It has a broad mixture of sand, clay, intermediate mineral particles, and a heavy dose of organic material. Depending on the size of these soil particles, the texture can range from very porous (sandy) to extremely dense and resistant to water movement (clay).
Clay soil is prevalent in many parts of the United States, and it can be very problematic if you are trying to grow a flower or vegetable garden. While some trees and shrubs grow well in clay, most annuals, perennials, and vegetables don't have roots strong enough to force their way through dense clay. If spring flower bulbs are your dream, forget it—most bulbs tend to rot over the winter in clay soils.
Clay soils can be improved, however. With some background information and a well-designed strategy, you'll be able to grow flowers and vegetables to your heart's content.
What Is Clay Soil?
Clay soil is soil that is comprised of very fine mineral particles and not much organic material. The resulting soil is quite sticky since there is not much space between the mineral particles, and it does not drain well at all.
If you have noticed that water tends to puddle on the ground rather than soak in, it is likely your soil is clay. Soil that consists of over 50 percent clay particles is referred to as “heavy clay.” To determine whether you have clay soil or not, you can do a simple soil test. But chances are you probably already know if you have clay soil. If the soil sticks to shoes and garden tools like glue, forms big clods that aren't easy to separate, and crusts over and cracks in dry weather, you have clay.
Advantages of Clay Soil
Even clay soil has some good qualities. Clay, because of its density, retains moisture well. It also tends to be more nutrient-rich than other soil types. The reason for this is that the particles that make up clay soil are negatively charged, which means they attract and hold positively charged particles, such as calcium, potassium, and magnesium.
Disadvantages of Clay Soil
In addition to the drawbacks mentioned above, clay also has the following negative qualities:
- Slow draining
- Slow to warm in the spring
- Compacts easily, making it difficult for plant roots to grow
- Tendency to heave in winter
- Tendency to be alkaline in pH
Improving Clay Soil
Improving your clay soil will take a bit of work, but the good news is that the work you do will instantly improve the structure of your soil and make it easier to work with. Most of the work is done once, although some annual chores are necessary to prolong the soil improvement.
It is best to improve an entire planting area all at once, rather than to attempt improving the soil in individual planting holes as you need them. If you dig a planting hole in clay soil, then drop in a plant and nicely amend only the soil you're using to backfill, your plant will be happy for a little while. But you have done nothing more than create an in-ground flower pot. Eventually, the plant will start sending out roots that will be stopped in their tracks when they reach the clay walls of the planting hole. You'll end up with a root-bound plant that won't grow as large or as healthy as it should.
Start by defining the growing area for your garden bed. If you are improving an existing bed, you can dig out any plants you want to keep and set them aside in pots until your soil improvement is completed. If you are preparing a brand new bed, you'll need to go through the basics of starting a new garden bed.
To improve your soil, you'll need to add 6 to 8 inches of organic matter to the entire bed. You can add any type of organic matter. Grass clippings (as long as they haven't been treated with chemicals), shredded leaves, rotted manure, and compost are all perfect choices. Spread the organic matter on top of the soil. Here's where the manual labor comes in. The organic matter needs to be mixed into the top 6 to 12 inches of soil. Digging it in and mixing it with a shovel is a great way to do this, as it moves a lot of earth without pulverizing the soil particles the way tilling can. However, if digging is just too hard on your back, using a tiller is a fine method.
When you're finished, your garden bed will be several inches higher than it was originally, but this is not a problem. Your garden bed will settle throughout the season as the organic material breaks down. The soil structure will continue to improve as microorganisms in the soil work to break down all of the organic matter you've added.
The bed can be planted immediately. Plan to add more organic matter in the form of compost once or twice a year. This will continue the process of improving the soil's structure and offset any settling that happens.
What Not to Do
Surprisingly often, people imagine that the proper way to improve dense, clay soil is to add the opposite kind of mineral material—sand. After all, loamy soils, viewed as ideal garden soil, are a mixture of sand and clay. Unfortunately, when sand is added directly to clay, the result is something that more accurately resembles concrete. The reason loamy soils are great for plants is that they have a large ratio of organic material in them as a foundation for the sand and clay. Without lots of organic material, clay plus sand equals an awful garden. Clay soils are best improved with the addition of compost and other organic materials only.
After a season or so, it's a good idea to collect a soil sample and have it tested to see if you have any nutrient deficiencies or pH issues. The report you get back will offer suggestions for how to improve the garden further. Add any organic fertilizers or soil amendments outlined in your report, and your bed will continue to be perfect for growing healthy plants for years to come.
Organic material will need to be continually added to prevent your garden from returning to its heavy clay state. Fortunately, this becomes a self-fulfilling process as garden plant material breaks down and works its way into the soil. Rather than cleaning your garden down to the soil line each fall, allow leaves and other plant material to decay naturally and become part of the ongoing ecosystem of good garden soil. If your garden is occasionally mulched with more compost, there will be little additional work you need to do.
Soil Conditioning. Clemson Cooperative Extension Home and Garden Information Center.
There's no break for people who garden in clay. Oregon State University