Clay Soil: Identification, Improvement Tips & What To Grow

Clay soil has a pretty bad reputation. We tend to think of it as waterlogged, compacted, and difficult to manage.

However, it is a misunderstood soil type that can be utilized with a few simple tips.

What is clay soil? Clay soil simply has more clay particles than other soils. It tends to be rich in nutrients like calcium, potassium, and iron and tends to hold moisture and provide support for plant roots. However, it also gets compacted, drains poorly, and limits the function of microorganisms.

This article will help you understand how to grow healthy plants in clay soil without wasting your money on expensive and ineffective inputs. I hope you find it helpful!

Tips for Growing Plants in Clay Soil

I’m going to start by giving you some quick tips for growing plants in clay soil.

Feel free to bookmark this page and come back to it again and again as you work to improve your soil. 

1. Stop Tilling 

A cultivator might seem like the best way to deal with pesky compaction that clay is famous for.

However, it only fluffs up the very top layer of the soil while making compaction even worse a few inches down.

In turn, this compaction will lead to waterlogging, diseases, and an exodus of your worms. 

2. Cut Back on Chemicals 

Many gardeners use herbicides and pesticides to control weeds and pests, but there is a catch.

For every bad thing you kill, you will also be killing many beneficial microorganisms. 

Those helpful little critters, like bacteria, fungi, and protozoa, will eat away at your compaction layers if you give them a chance.

They also use natural glues to stick to soil particles, which open up passageways in clay soil so water can drain through properly and oxygen can reach your plant roots. 

3. Choose The Right Plants 

Work with what you’ve got. Choose flowers like pulmonaria, bergenia, or roses for clay soil. Vegetables like potatoes, peas, and cabbage would also work well.

Having said that, you can grow nearly any kind of plant in clay soil if you manage it carefully.

It’s possible to get clay particles to flocculate, or clump, and act like a balanced loam. (More on that later).  

4. Get Composting 

A layer of fantastic compost will significantly improve your clay soil.

A good compost will have beneficial microorganisms, like bacteria and fungi, which will make their way down into the clay and start putting things right.

It’s incredible what nature can achieve when we stop getting in the way, but we have to be the guardians that get that biology back into the dead soil! 

5. Think Long Term 

Sometimes it takes more time to avoid the work than to just get on with it. If you are struggling with your clay soil, take time to read this article to the end.

Understand the roots of your problems, and address them now. In the long run, you’ll be grateful that you put in the effort. 

Clay Soil: Definition, Characteristics & Types 

Now that I’ve given you some quick tips, I think we should dive a little deeper into what clay soil actually is.

This information will help you take care of your garden appropriately so that you can transform waterlogged clay into healthy, balanced soil. 

What Exactly Is Clay Soil? 

There are three types of soil particles: sand, silt, and clay. 

Sand particles are the biggest, which you can see with the naked eye. To see an individual clay particle, you need to put your soil under a microscope. 

All soil has a combination of sand, silt, and clay. So clay soil is mainly made up of clay particles.

To clarify, clay isn’t necessarily a unique substance with a different chemical makeup than sand or silt. It just comes down to how much the particles have broken down. 

They all started as parent material. This means the particles were once rocks or mountains.

Through weathering, chemical reactions, and the tireless work of bacteria and fungi, parent material breaks down into microscopic soil particles we call clay. 

The “ideal” soil that most people want is called a loam. Loam describes an equal balance of sand, silt, and clay.

Each soil type has its own characteristics, but loam is typically preferred because it has the benefits of all three kinds.  

Clay Soil Characteristics

Clay soil has some positive and some negative characteristics. 

The good thing about clay is that it’s rich in nutrients that plants need to thrive.

It also holds water, so you don’t waste money or time watering a sandy garden that just lets all the moisture run straight away.

Finally, clay provides a proper support system for roots compared to sand. 

But clay can be a bit too intense! 

It’s made up of tiny particles, which tend to sit on top of each other like dinner plates.

It doesn’t take much to cause clay soil to compact, which means that water and air can’t move through the soil.

This creates waterlogging and anaerobic (low oxygen) conditions, which are a breeding ground for pests and diseases.

Backed-up water can also drown beneficial organisms like worms, leading to your soil becoming lifeless. 

It can be challenging for roots to move through compacted clay soil, so plants can’t access as many nutrients or water droplets. This can make your garden less resilient. 

Types of Clay Soil 

There are a few different ways to classify clay soil. 

You can look at the percentage of clay particles compared to sand and silt, or you can consider the way it behaves.

You could also divide clay by the origin of the parent material. 

Parent material

For example, kaolin clay is soft white clay used to make porcelain.

It is mainly made from the mineral called kaolinite, though some other minerals can sneak in there in small amounts. 

Behavior

Montmorillonite is a common type of clay soil in the U.S.

It’s classified by its ability to absorb moisture and swell and its capacity to hold nutrients. Several different minerals can be present. 

Illite is another type of clay, which is nonswelling.

It is structurally different from montmorillonite, but you could only see that under the microscope. With the naked eye, you might just notice a different color. 

Percentages 

Finally, you might just be looking at the percentage of clay particles compared to sand and silt. This is normally the most helpful thing to consider for us growers. 

Clay soil with 10% clay particles shows less exaggerated characteristics than clay soil with 40% clay particles. 

Problems like waterlogging and compaction can be more severe in 40% clay soils, but the nutrient content will also be higher. 

How To Identify Clay Soil 

A close up of several layers of red clay soil.

Soil that has a high percentage of clay can be identified with the naked eye.

You might notice it sucking and popping as you walk through it, or you could feel the velvety texture in your hand. When dry, you will see that clay soil tends to crack. 

That’s not very precise, though. So here is a simple but effective test you can do to your soil. 

  1. Take an empty water bottle. 
  2. Fill it 25% full with soil from your garden.
  3. Fill it to 75% full with water (you need some air space in the bottle).
  4. Screw the lid on firmly.
  5. Shake vigorously until the soil and water are mixed.
  6. Place the bottle on the table and wait for a minimum of 30 minutes (maybe longer).

When you come back to your bottle, you should see three distinct layers of soil particles.

Sand will be at the bottom because it’s heavier and it settled fastest. Next, you’ll have the silt. The top layer will be your clay. 

You can measure the different layers with a ruler to determine the percentage of clay you have in your soil. If you struggle to work out percentages, this little calculator will do it for you. 

If you’ve got 33% of every soil type, congratulations! You have a loam, and many gardeners around the world will be jealous.

If you have a clay soil, that’s okay too. There are some benefits and drawbacks of this soil type, but it’s certainly not a disaster. 

Advantages and Disadvantages of Clay Soil

Where Are Clay Soils Found? 

You’ll often find clay at the bottom of lakes or the sea bed.

So any area that used to be covered in water may be high in clay, such as a valley that was carved out by an ancient river or glacier. 

This is because clay soils tend to develop in moisture when a film of water surrounds the clay particle. 

If you want to find out more about the soil type in your specific area, you can type something like: “Soil Type Arkansas.”

Depending on your state, you’ll find a wealth of information online, particularly from the websites of local universities. 

What Grows in Clay Soil? 

Before I go on, I just want to remind you that it is possible to get clay soil to act like a loam.

It takes some understanding of biology, but you can achieve it in just a couple of growing seasons. In that case, you can grow nearly anything in clay soil. 

However, if the right microorganisms are not supporting your clay soil, it will limit what you can grow in there. 

Here are some plants that you can grow in clay-dominant soil without investing too much time and effort into improving conditions.

How Do You Know If There Is Too Much Clay in Soil? 

You know there’s too much clay in the soil if it forms deep cracks when it dries out.

You might also have frequent waterlogging problems, and you might notice that the earth is heavy and sticky and it doesn’t have many worms in it. 

To be fair, a lot of these issues come down to management. 

If you stop using pesticides and start using no-dig gardening practices, some of the problems associated with “too much clay” will disappear by themselves.

Perhaps a loam soil will take more mistreatment before growing becomes impossible, but that doesn’t mean anything is categorically “wrong” with clay soil. 

We just have to be a bit more thoughtful and do everything we can to keep it in top shape. 

How To Improve Clay Soil: Amendments 

You don’t need to bring a truckload of sand into your yard and dump it on top of your clay soil.

That’s not going to fix anything in the long term, and it’s going to cost you a lot of money. 

That’s not to say that all amendments are pointless. Here are some things that I would recommend adding to your clay soil.

Compost 

Good compost will be teeming with beneficial microorganisms.

After you apply it to your garden, these microorganisms will slowly make their way down into the soil.

They will eat away at the compaction layer as they move around, and they will also use their natural glues to stick tiny clay particles into bigger lumps called aggregates. 

This will open up air passages, so bigger organisms like worms and insects will crawl into the soil.

Over time, your dead, compacted soil will become a healthy ecosystem where plants can thrive. 

Compost Tea/ Compost Extract 

After you’ve added your initial compost, you might want to periodically top up the microorganisms in the soil with a soil drench.

You can make this by massaging a good compost into water, so the microorganisms get removed from the organic matter and become part of the solution. 

You can then sprinkle this extract onto the ground so you don’t have to keep building a huge mountain of compost. 

A compost tea is like an extract, but it also contains food for the microorganisms to multiply in the solution.

This would be better to spray on your actual plants rather than the soil. 

Cover Crops 

Compaction is a major problem with clay soils, so you should do everything you can to minimize its impact. For example, you should never leave the ground bare. 

Instead, you can plant cover crops to protect the soil from rainfall.

If the rain hits the bare clay soil directly, it will form a new compaction layer, and all the hard work you did to restore microbiology will be set back. 

How To Fix Waterlogged Clay Soil 

It’s better to prevent waterlogging than fix it. Once the water has backed up to the surface of the soil, your microorganisms and worms will have drowned.

Conditions will have become anaerobic, so harmful bacteria and fungi will be growing instead of beneficial ones. 

However, if the problem has already started, you can drain the water and then start better management in the future. 

You can use a spade to run a slit through the soil, which water can run through and escape.

Once most of the water has drained, you’ll want to add some new compost or topsoil to make up for any soil that the rain washed away. 

You should also consider fixing any compaction layers that stop the soil from draining and try getting more microorganisms into the ground to increase passageways for water and air. 

How To Break Up Clay Soil Without Tilling 

As a trainee soil scientist, I’ve seen the damage of tilling first hand. Tilling will destroy the beneficial bacteria and fungi in your soil.

It also causes carbon dioxide to blow off into the environment, and it tears apart the structure that your plants need to grow well.

Did I mention it causes rock-hard compaction layers beneath the surface?

So, I’m not a fan of tilling or using a cultivator. 

That said, sometimes you do need to till one last time. You would apply your fantastic compost or compost tea that’s full of good microorganisms, then dig it into the ground.

The idea is that you are getting the biology right down to the compaction layer, so the bacteria and fungi can break apart the soil once and for all. 

Another way to break up clay soil without tilling is to spread gypsum over the land.

I’m hesitant to recommend this because spreading minerals on the ground in large quantities is associated with water pollution.

Having said that, some early studies suggest that gypsum can reduce phosphorus runoff on agricultural land, so it could be beneficial for water quality in some cases. 

I’m holding off on my verdict until more peer-reviewed studies are released because unintended consequences for our actions are often far more complicated than we initially suppose. 

Can Worms Break Up Clay Soil?

Worms are great for improving soil structure, but it’s difficult for them to get into clay soil if it’s compacted or waterlogged.

When their burrows become filled with water, you’ll notice worms come up to the surface. That’s why you often see them on the road following heavy rainfall. 

The best approach is to put a thick layer of good compost on top of your clay soil.

Worms can live in this top layer while slowly eating away at the ground below and improving its structure.

If they are forced to evacuate clay soil due to too much water, they will die very quickly unless they have some leaf litter to hide in. 

Final Thoughts 

Clay soil isn’t the easiest soil to work with, but it does have many benefits.

It’s much better at holding moisture and providing structure than sandy soil, and it’s got tons of nutrients.

Like anything in life, we just have to make the most out of what’s in front of us. 

Try to avoid using chemical inputs or disturbing soil structure with too much digging.

Use compost and compost extracts to get microbiology back into the soil, and take steps to reduce compaction like planting cover crops and backing off with the tilling. 

You might be surprised at what happens when nature is allowed to take its course!

Recommended Reading 

  • Soil Biology Primer, Dr. Elaine Ingham
  • Teaming With Fungi, Jeff Lowenfels
  • Teaming With Microbes, Jeff Lowenfels
  • Mycorrhizal Planet: How Symbiotic Fungi Work with Roots to Support Plant Health and Build Soil Fertility, Micheal Phillips
  • Growing a Revolution: Bringing Our Soil Back to Life, David Montgomery
  • No Dig Gardening, Charles Dowding
  • Gaia’s Garden: A Guide to Home-Scale Permaculture, Toby Hemenway
  • Soil Science for Gardeners, Robert Pavlis