Improve Clay Soil for New Or Existing Lawns

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Sydney has over 15 years of experience in lawn maintenance, landscape design, and organic gardening. She has an A.A.S. in Landscape Design/Organic Produce Production from the Nebraska College of Technical Agriculture.

What Is Clay Soil?

Part of what determines soil quality is its texture. Soils can be classified as sand, silt, or clay, with a number of variations. The goal, however, is loam, which is an even mixture of all three. Technically, the texture is measured according to particle size. However, there’s a “poor man’s test” that is adequate for any homeowner hoping to maintain a nice lawn.

Simply dig up some topsoil, and wet it until damp, but not sopping. You should be able to squeeze it without water dripping out. Once you squeeze it, open your hand flat, and just examine the sample. If it crumbles, you have sandy soil. If it holds its shape, but falls apart when you poke it, you have a silt soil. If it looks like it belongs on a potter’s wheel, congratulations, you have clay.

All soil will be somewhat of a combination, but if you’re trying to decide between silt and clay, treat it like clay. The remedies for clay soil are good practice for anything except sandy soil.

Existing Lawns

It is entirely possible to improve clay soil that already has an established lawn, but it will take more time and effort. Also, it may result in needing to reseed your lawn anyway, since a lawn growing in clay may not do well in a soil with more organic matter.

Organic Matter

Organic matter is the magic elixir for all soil problems. It adds stability to sand and creates porosity in clay. It adds nutrition in depleted soils and brings in microbes and earthworms to break down dead plant matter.

In clay specifically, organic matter will bring earthworms who will tunnel through the soil and help with porosity and allowing oxygen to plant roots. It also helps with drainage and will increase the average particle size, which will slowly turn soil into a loamy consistency. A proper balance of oxygen and water will keep fungus and rot from growing, and help beneficial microbes and nematodes be more active in the soil.

The easiest way to add organic matter into your soil is to use a mulching blade on your lawnmower. As clippings decompose, they create a layer of compost on top of the soil. You can also mow over leaves in the fall instead of raking them, as this can also add organic matter into the soil.

Thatch

Due to the nature of clay, the organic matter from clippings and leaves will accumulate on top of the soil. This is called thatch, and although it is helpful in adding nutrition, it also keeps in moisture. In loamy and sandy soils, this would be a good thing. But, clay has difficulty draining, and holding in water can create problems with fungus and rot. In dry clay, thatch can also prevent water from penetrating. Either way, a heavy thatch layer on top of clay will create some problems.

Instead of removing thatch, consider aerating your lawn in the spring to provide holes where organic matter can accumulate. Earthworms will tunnel through these holes, and pull organic matter with them. Progress will be slow, but after a few years of aerating and allowing organic matter to accumulate, your soil should start to improve.

There are three ways to aerate your lawn:

Spike aeration. The tool used for spike aeration will have long tines that poke holes in your lawn. This is the least efficient way to aerate, and can actually cause problems with compaction, especially in clay soils.
Core aeration. This process will cut and remove small cores of soil from your lawn. It is better than spike aeration but is only effective for minor soil problems. Clay soils will require more work to produce a healthy lawn.
Liquid aeration. This is a fairly new process that uses wetting agents to help break down surface tension to allow water to penetrate the soil. Results show that it penetrates deeper than manual aeration, but you lose the benefit of having pockets of organic matter integrated into the soil.

Drilling

Instead of aerating, which pokes small holes in your lawn, you could also drill large holes and use them as miniature compost wells. Then, fill them with organic waste from the kitchen and garden. Vegetable peels, coffee grounds, etc.

As the organic waste breaks down, this will create larger pockets of organic material in your lawn. The downside is that you will have more noticeable holes in your lawn for a few weeks until the grass begins to grow over them. The upside is that this is a much faster way to improve the soil.

Another benefit to drilling over aerating is that a drill will help with porosity on the sides of the holes. Aerators, on the other hand, punch holes in the soil. In clay, this can result in smooth-sided holes that do not allow air to pass through. While organic matter will still collect, and eventually be worked in, the process is much slower than if you used a drill.

Bare Soil

If you have bare soil, and are wanting to establish a lawn, you have one huge advantage, and that is turf choice. You can choose a grass that will thrive in clay and will hold up well under stress.

Tall Fescue. Best for cooler climates. The roots are deep penetrating, so they can help keep soil from getting too compacted.
Buffalograss. This is a warm-season grass. Like tall fescue, buffalograss has deep roots, which helps keep soil from getting compacted.
Bermuda Grass. Bermuda doesn’t tolerate cold temperatures. So this is a good choice for lawns in the southern part of the country. It spreads very quickly, and is very drought resistant.

However, before planting anything, you should take steps to amend your soil.

Tilling

Tilling a clay soil can cause more problems than it solves. As clay is broken up, it will dry out and harden and create a lumpy textured soil with no water retention. If you continue to till past this point, the end result will be a fine dust that retains too much water. If it rains, and then the soil dries out, it will create a sheet of dried pottery over the area. Both results are worse than before you started.

However, tilling can be very practical if you are adding organic matter to the soil. First, water the area. You should never till dry or wet soil. It should be damp and moist. If you’re creating dust, add more water. If you’re creating a huge mud puddle, come back tomorrow.

Compost

Next, find a source of organic material. The best by far is compost or composted manure. For clay, you will want to spread a layer 2” thick across the area and till it in. You will need to measure the area you plan to amend in order to buy the correct amount of compost.

How To Measure For Compost

Measure the length and width of the area you plan to amend.
Multiply the length and width by .17’ (2”).
The result will be what you need in cubic feet.
Divide cubic feet by 27 in order to find how many cubic yards you will need.

For example, the area I want to amend is 20’ x 36’. I plan to till in 2” of compost:

20’ x 36’ x .17’= 122.4 cubic feet

Compost bags will be measured by cubic feet, so this will give you an idea of how many to buy. However, you can also buy compost and composted manure by the cubic yard in a dump truck. To calculate how many cubic yards you need, simply divide your cubic feet by 27:

122.4/27= 4.53 cubic yards

Buying in bags is more expensive but can be easier to apply. As of this writing, 3 cubic feet of compost was $8.48 on Home Depot’s website. For this example project, you would need 48 bags, and it would cost you $347.68 before tax (assuming no bulk discount). The bags would weigh, collectively, over 3,000 pounds. This would probably mean multiple trips to get the compost home.

If you were to buy compost by the yard, it is generally cheaper. This can depend on delivery costs and who you purchase from. Home Depot sells compost by the yard, and 5 cubic yards of their compost is currently $235.95 with a $99 delivery fee.

Many garden centers will offer free delivery if you meet certain requirements. However, you will need to be able to allow a dump truck to dump compost on the area you’re amending. If that is not possible, you must find a way to transport this compost to the area you’re tilling. This would take about 50 trips in an average wheelbarrow.

Is all of this work and money worth it? Yes. You could forego the compost and just till, but you will spend much more in the future on water, fertilizer, aeration, and other treatments to maintain a healthy yard. Plus, you won’t be able to go back and till in compost unless you destroy your lawn and replant it. Better to set yourself up for success the first time.

Soil Amendments

Soil amendments can be added during the tilling process or as a topdressing to an established lawn. While soil conditions change depending on the region, clay soils are generally high in nutrients.

The problem with clay is the texture. It is so fine that it traps water and nutrients and prevents them from being leached into the substrate.

The goal with amendments, then, is to improve texture. Nutrients are generally widely available, so this is not as much of a concern.

Also, grasses that do well in clay are usually very tolerant to soil deficiencies. So amending purely for nutritional value may be a waste of money.

Of course, the only way to know for sure is to do a soil test.

Humic Acid

When organic matter decomposes, the end result is humus. And the integral part of humus is humic acid. This is the part of humus that actually adds vitamins, minerals, and trace elements.

Humic acid is not a fertilizer. While it does add nutrition into the soil, it does much more than that. It is a soil conditioner, and adds many elements back into the soil that cannot be found in a fertilizer.

If you have bare soil, compost is still better than humic acid as an additive, since compost will include the bulk material that helps with soil texture. However, if you have an established lawn, humic acid is easily applied as a topdressing and will help the overall health of the soil. It is a much better alternative than fertilizer.

Manure

Manure is a tricky soil additive. Fresh manure has too much nitrogen and will kill any plants you apply it to. It must sit for at least a year before it can be applied safely.

Different animals have different quality manures. Horse manure is generally the best for organic matter, and chicken manure is the best for nutrients. But, there may be cow, sheep, goat, and even rabbit manure available.

Manure quality is also related to diet. Grass-fed cattle will have high-quality manure, whereas feedlot cattle will have more disease and much less organic content, making their manure almost pointless. Horses and cows that eat a lot of roughage can pass a lot of weed seeds in their manure. Using that manure on your lawn can be disastrous.

To avoid the headache of finding the perfect manure amendment, just buy a composted manure mix. The composting process can kill weed seeds, and a mixture of manures will provide the most balanced nutrient profile.

Gypsum

There are many claims about gypsum that are simply not true. While generally heralded as an all-purpose amendment, the results of adding gypsum are actually quite negative in most circumstances.

The two exceptions are coastal soils with high salt content and heavy clay. Gypsum can improve soil texture and fertility in heavy clay, and can help remove salt from coastal and arid soils.

Sand

Sand should never be considered as a soil amendment. The three ingredients in concrete are water, sand, and cement. Cement is basically manufactured clay.

Adding sand to clay will not help with drainage. It will, however, create an all-natural, organic concrete slab.

Conclusion

It’s easy to give up on clay soil and let the weeds take over. However, unhealthy soil can have consequences beyond unruly lawns. Waterlogged soil can encourage rot in outbuildings, which can attract pests and breed mold. Poor drainage can result in standing water that breeds mosquitoes. Fertilizer treatments and weed treatments are basically pointless without first fixing the soil.

A healthy lawn can help keep many other costs down and will help to protect your home. Focus on the soil, and the rest will happen naturally.