Healthy soil is the foundation for vibrant plant growth, so it can be frustrating when the soil just doesn’t cooperate. Cracked soil prevents the roots from getting the proper ratio of water and oxygen, so it’s important to understand why your soil is cracking in order to fix it.
Soil cracks when it has a large amount of clay and it is exposed to hot, dry conditions. Clay sticks together when the soil is wet, but as it dries out, the clay shrinks and causes cracks that are nearly impossible for roots to penetrate.
Clay soil is frustrating, but there are steps you can take to improve the growing conditions of your lawn and landscape.
Why Does Clay Soil Crack?
Clay soil has the ability to hold a lot of water, which is why it drains so poorly in wet climates. As clay absorbs water, it expands, and the tiny particles stick to each other and prevent proper drainage.
When clay is exposed to hot, dry weather, the water evaporates and the clay shrinks. As it shrinks, it condenses and cracks.
Tilling clay soil only makes it more likely to crack. As you till, the clay turns to dust, and then quickly binds together to form even stronger blocks.
Bare clay soil sticks together more than planted clay soil. Thick, strong roots can penetrate clay blocks and help with aeration, and dead roots and plant material will help add organic material, which improves porosity.
Clay soil can also crack during the winter as water within the soil freezes and expands. When the weather warms up and the ground thaws, the clay particles will settle and bind to each other, and then pull away from each other as the weather gets hot and the water evaporates.
How to Improve Clay Soil
Clay soil retains more nutrients than any other soil texture, but the nutrients are unavailable if the soil is so dense that roots can’t grow.
You can’t change clay soil into a different texture. Texture is the classification of how small the soil particles are in a given area as a result of weather patterns, parent material, and erosion.
You can improve clay soil by increasing porosity but the only way to improve porosity is by adding organic matter on a consistent basis.
The most common type of organic matter to incorporate into your soil is compost, but there are other options depending on your soil’s specific properties:
- Composted manure. This is manure that has been aged at least 1 year. Composted manure is high in nitrogen, so adding too much may burn plants.
This can be a good amendment for soils in wet climates where nitrogen may leach out or run off but you will need another, bulkier amendment to increase porosity.
- Wood chips. Brown, carbon-rich materials like wood chips are great for increasing porosity. However, adding raw plant materials directly into the soil can pull nitrogen away to help with the decomposition process, which lowers available nitrogen for established plants.
If you have a bare patch of clay, you can add bulky, brown material like wood chips, and after a few years, the nitrogen content should return to normal and you can begin planting.
- Worm castings. This is a wonderful, potent, fertilizer, but it’s not a good option for improving porosity. Most clay soils are high in nutrients, but they’re too dense for plants to reach them.
Worm castings are the perfect amendment for established plants to encourage root growth, but they will not improve porosity.
- Grass clippings. Tilling green, nitrogen-rich material into your clay soil may improve porosity faster than adding brown, carbon-rich material like wood chips.
However, raw green materials will significantly lower nitrogen content until they have broken down, and it won’t increase porosity as much as brown materials.
- Cover crops. You can plant a large area of clay soil with mustard, clover, or other cover crops and then till them under to help improve organic matter content and break up the top layer of soil.
Cover crops will grow a dense root system that helps to prevent large blocks of clay, and when you mow or till them under, the top growth adds organic material that helps to improve porosity.
The goal of adding organic matter is to increase porosity, but some homeowners try to achieve the same end result by mixing sand into their clay soil.
If you add sand to clay and add water, you get concrete. Sand will not improve clay soil.
If you have a bare patch of clay soil, you can till in large amounts of organic matter and let it break down. Do not till wet or dry clay; wait until the soil is damp but not soggy. Only till the soil as much as absolutely necessary:
- Working the soil when it is too wet will remove oxygen and create a thick paste that is worse than when you started.
- Working the soil when it is too dry will result in a fine dust that stick together with the first rain.
- Working the soil too long, regardless of the soil moisture, will result in removing oxygen and creating a more dense soil structure.
If you have an established lawn or landscape in your clay soil, you will have to add organic material without disturbing the root systems of the grasses and plants. You can do this through annual aeration and topdressing:
- Rent a core aerator in late spring to early summer. These remove cores of soil, which opens small spaces to rake in compost. Tine aerators will not work for topdressing because they create very small holes.
- Set the aerator for the densest spacing your lawn can handle. Aerate your lawn.
- Calculate how much compost you need for topdressing. Multiply the length of your yard by the width to determine square footage, and then multiply that by .041 to determine cubic feet. Divide cubic feet by 27 to find how many cubic yards of compost you will need.
- Order bulk compost from a garden supply center, and rake it over your lawn to fill in the holes left by the aerator.
Repeat this process each spring to slowly incorporate organic matter into your clay soil to prevent cracking.
How to Manage Clay Soil
Even if you continue to add organic matter and aerate your clay soil, it will still have clay particles that can cause problems if they are not managed properly.
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Clay is prone to compaction, and even if you add compost on a consistent basis, clay can begin to stick together and become blocky if there is a lot of heavy foot or vehicle traffic.
Maintain clay soil by doing the following:
- Irrigate with small volumes of water over long periods of time. Instead of applying 500 gallons in a half hour, try to apply 500 gallons over 2 hours. A slower rate of application can help the soil absorb moisture instead of creating runoff. As clay absorbs moisture, the roots have access to water and can continue to penetrate into the clay. Applying water too quickly, which results in runoff, can keep water from ever reaching the root zone.
- Avoid foot traffic or vehicle traffic when the soil is wet. When clay soil is saturated, it is easy to compact, which pushes the oxygen out of the soil. This results in large blocks of bound clay, which cracks and splits as it dries.
- Mow consistently. Frequent mowing (which depends on your turf) stimulates root growth and encourages deeper, more fibrous root systems. This helps to prevent splitting and cracking while promoting aeration and adding more organic matter into the topsoil.
- Aerate and top dress each spring. Continue to aerate and topdress to keep a balanced amount of oxygen and moisture in the soil.
If you stop adding organic matter, clay soil will eventually revert back to a blocky, cracked texture. Organic matter will decay over time into microscopic compounds that wash away or are absorbed by plant roots.
Consistent application is the only way to improve and maintain clay soil.
Clay soil is one of the most productive soil classifications because it holds on to nutrients so well. If you can incorporate organic matter into the soil and prevent heavy traffic, you will notice a more spongey, airy soil texture that resists splitting and cracking.