How to Amend Clay Soil: 6 Ways to Improve Soil

David Kuchta, Ph.D. has 10 years of experience in gardening and has read widely in environmental history and the energy transition. An environmental activist since the 1970s, he is also a historian, author, gardener, and educator.

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Garden soil, or loam soil, ideally consists of a mix of clay, sand, and silt. Too much clay will cause water to pool, drowning plants, worms, and beneficial micro-organisms. Too little clay can make the water run right through your soil, not allowing your plants to absorb nutrients.

Clay is composed of the smallest of soil particles, primarily silicates (silicon and oxygen). Clay easily bonds with calcium, iron, magnesium, and potassium—elements that are essential for plant growth. But because clay traps water in its small pores, those water-borne nutrients are unavailable for root uptake.

The goal of amending clay soil is to increase the size of the particles that make it up. This will allow water to drain properly, bring oxygen into the soil, and make nutrients available to your plants.

Here's how to amend clay soil to make it suitable for growing most kinds of plants.

Conduct a Soil Test

Squeeze a moist handful of garden soil in your fist. If the clump immediately falls apart, your soil is too sandy. If it feels heavy and sticky and forms a ball that holds its shape, your soil has too much clay. Silty soil will feel slimy when wet and become powdery when dry. Good garden soil will hold its shape but crumble if you start poking at it.

When in doubt, consult with your state cooperative extension service for a more scientific test.

Grow Plants

Clay soil is relatively life-free, so to aerate your soil, add life to it. Organic matter is the leaven of soil, creating air pockets like yeast does in bread. Plants that are good at breaking up clay include those with deep tap roots, like daikon radishes, and those with fibrous root systems like rye or clover. When the plants die, their organic matter creates a natural compost that allows water to drain through. The bacteria and worms that break down the organic matter and fertilize your soil.

Add Compost

If you don't want to wait a year for plants to grow and die, add compost. Compost is organic matter already broken down by bacteria and worms (through "vermicomposting"), making their nutrients ready for root uptake. Compost or composted cow manure (which has little scent) is commercially available and garden centers. It can be added on top of the soil as a mulch. In places where the ground freezes, the natural process of freezing and thawing will work the compost into the soil. In places where it doesn't freeze, rain will leach the nutrients into your soil.

Add a Mulch

If compost isn't available or is too costly, add a mulch of other types of organic matter, such as leaf litter, pine needles, lawn clippings, straw or hay from local farms, or untreated wood chips. Add two to eight inches on top of your soil, and fork or dig it in, trying not to turn over the soil (which disrupts its structure).

Add Worms

Adding worms will accelerate the decomposition of organic matter in the soil. Worms tunnel through your soil and naturally aerate it, while the excretions they leave behind is their own way of composting your soil.

Add Air

When all else fails, drive a garden fork into your soil. Rock it back and forth without turning over the soil completely. Otherwise, you risk breaking up the structure of the soil, leaving it more susceptible to being either compacted or eroded.

The best way to amend clay soil is to add organic matter, such as plants with good root systems or compost. Organic matter helps aerate the soil, which will make it more suitable for growing.

Sand may improve the soil texture, but it doesn't improve the structure of your soil, which is what makes nutrients and water available to roots. In fact, the process of adding sand is likely to disrupt any existing structure, without adding any new life to the soil.

Don't expect overnight success. Start with a raised bed while you work on amending the clayey areas of your garden. Converting clay soil to healthy soil is a gradual process that can take multiple planting seasons. Loam isn't built in a day.

There are many ways to maintain the health of your soil, including no-till gardening, crop rotation, covering your soil with a mulch or cover crop, and more. You can also practice organic gardening and avoid using pesticides.