Home and Garden

Amending Clay Soil in the Southern Plains


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While the soils in the southern plains vary widely, many gardeners face the challenge of growing plants on heavy clay soil. This may be due to the natural composition of the local soil or the removal of the top layer of soil during construction. It is unfortunate that healthy topsoil is often displaced to build new houses, leaving houses on compacted sub-floors. I encountered this problem in my own courtyard during a recent planting project.

The good news is that clay soils are generally very fertile and can be modified. I like to do two steps: adding organic matter to improve soil structure before planting, followed by routine maintenance to protect and improve soil health.

Here a piece of modified clay soil sits next to unchanged clay soil. The former looks rich, smooth, and workable, while the latter is full of dense and crumbly clumps of clay. Photo: Kim Toscano

Add organic matter

The first step in improving the clay soil is to add organics, which are simply material derived from plants and animals. This includes compost, manure, straw, leaves, clippings, kitchen waste and much more. Organic matter improves the structure of clay soils and allows water and air to move more freely. It also feeds beneficial microorganisms that live in the soil and provides nutrients to the plants.

When I work on new garden beds or build degraded soil, the first thing I do is add large amounts of compost. Compost is organic material that has already decomposed into a nutrient-rich, soil-like material. My favorite tool for this job is a wide fork. It takes a little manpower to use, but it breaks through sound with surprising efficiency. In my yard project, my husband and I used a wide fork to turn the soil and put in a few inches of compost. To get the compost in as deep as possible, we used the double digging method.

with a wide forkA wide fork is a tough tool for breaking through tough soil. Photo: Kim Toscano

Avoid compaction

Once you’ve changed your soil, it’s important to avoid compaction. Clay soil compacts easily and compresses the pore spaces that are required for the movement of air and water. One way to preserve the soil structure is to avoid working on wet soils. You can tell if the clay soil is dry enough to work by gently squeezing a handful into your hand, then gently shaking your hand. If the soil sticks together in a ball, it’s too wet. If it crumbles easily, it is dry enough to work.

In vegetable gardens, I designate areas for walking in order to prevent the planting beds from compacting. You can also limit the use of heavy equipment such as rotary tillers.

Double digging compostDouble-dug compost makes it easier for plant roots to penetrate deeper into the ground without having to struggle with dense clumps of clay. Photo: Kim Toscano

Yearly maintenance

With our hot summers and the long growing season in the southern plains, organic matter in the soil degrades very quickly. Replenish the soil with a fresh layer of compost every spring. It is also helpful to use organic mulch such as wood chips, straw, cotton burrs, or cocoa bean husks. In addition to the benefits of mulching in maintaining soil moisture, they eventually break down and release nutrients to the soil.

Fall is the time when I like to add whole, non-composted materials like leaves, grass clippings, corn stalks, and other cuttings to garden beds. In the autumn and winter months, these materials get directly into the garden and provide the plants with valuable nutrients in the following growing season.

Garden with organic mulchNot only does organic mulch keep weeds out and regulate soil temperature and humidity, it also breaks down in the soil over time. Photo: Kim Toscano

Organic matter and soil nutrients

It is important to note that different types of compost and organic matter vary in the amount and type of nutrients available for plant use. For example, manure can be high in phosphorus and potassium, nutrients that can easily build up in excessive amounts in the soil. For this reason, I sparingly add compost with manure to more established garden beds.

The microorganisms that break down organic matter consume nitrogen and other nutrients in their work. Nitrogen can be limited in beds that have carbon-rich materials such as wood chips or straw incorporated into them. It’s a good idea to do a soil test in the spring after the decomposers have used their magic to see what your soil may or may not need.

Cover crops in winterIn the southern plains, you can grow cover crops in winter, many of which are edible or have some ornamental value. Photo: Kim Toscano

Use cover crops

Another way to break up clay soil is to plant cover crops with deep tap roots. Daikon radish is one such cover crop that is commonly used to improve the movement of water and air through the soil. Many vegetable gardeners like to use cover crops to protect the soil from erosion, add organics, and provide nitrogen.

Healthy soil is the key to healthy plants. If you take the time to improve the garden beds before planting, you will get great benefits from healthier, more resilient plants. And remember that organic matter degrades over time. So plan to renew your garden changes every year.

– Kim Toscano is a horticulturist, entomologist, garden designer, writer and graphic artist. She previously hosted Oklahoma Gardening, a weekly PBS television broadcast by the Oklahoma Cooperative Extension Service.

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Robert Dunfee