How To Improve Clay Soil | Best Amendments To Add

Dealing with clay soil can feel like a losing battle, but it doesn’t have to be that way. 

What’s the best way to amend clay soil? You can use biological amendments, such as compost, compost tea, and compost extract, to improve your clay soil. It might also be appropriate to mix gypsum or sand with your topsoil, though this approach won’t be as effective in the long run. 

The following will describe how and why to use different amendments and give you some general tips for improving clay soil. 

Before beginning, it’s best that you are certain that your soil is indeed clay. Head over to this article on clay soil identification, then proceed with the advice given below.

Best Amendments for Clay Soil 

You’ve probably heard about mixing calcium or sand with clay soil.

However, more recent discoveries have shown that restoring healthy soil is about promoting a thriving community of microorganisms below the ground.

That’s because beneficial bacteria, fungi, and predators will naturally create air and water passageways that your clay soil so desperately needs.

The best way to get this community of microorganisms into your yard is to use biological amendments. 

Best Amendments For Clay Soil

Compost

The best thing you can add to clay soil is excellent compost.

Unfortunately, not all compost is the same, so you need to find a trusted supplier or make your own (learn all about compost here).

But how can you tell whether compost is any good?

Smells

Compost that you apply to your soil should not have any bad smells. So if you get anything eggy or vomit-like coming off the compost, don’t go near it. 

These bad smells are not an inevitable part of gardening. They mean that your compost was made without enough oxygen, and pests and pathogens have taken hold. 

Temperature

You should also make sure your compost has cooled down to ambient temperatures before purchasing or using it.

If it’s still hot, the decomposition process hasn’t finished, so you can’t know whether the end result will be any good. 

Shelf Life 

You need to know how long the compost has been sitting around on the shelf.

As time goes by, compost will lose the microorganisms you hope to introduce to your soil. So if possible, you would want your compost to be no more than a few weeks old.

If you buy plastic sacks of compost from a large garden center, it can be tough to know whether there is any life left in the bag. 

Pesticides

If you’re making your compost, you have more control over what’s going on inside.

Make sure you choose green materials that haven’t been exposed to pesticides. Otherwise, you aren’t going to get an amendment filled with life.

Instead, you’ll just be putting lifeless decomposed matter into your yard, which could be toxic enough to kill the microbiology you already had in place.

In the best case, it would just be a waste of money and have no positive effect in your garden. 

Decomposition

And finally, you want to be sure that any manure in your compost is fully decomposed. If you can still make out visible chunks of manure, it’s not ready to be used in the garden. 

As manure passes through the digestive tract of animals, it can pick up a range of plant and human pathogens.

Only by complete decomposition with plenty of oxygen can you kick these unwanted visitors out of the party and ensure that your compost is not doing more harm than good. 

Testing 

It is possible to get your compost tested for biodiversity by sending it to the right kind of soil technician.

I say the right kind because some labs test for nutrients rather than microorganisms. You don’t have to do that, though.

If you at least follow the guidance above, there’s a good chance your money will be well spent. 

Compost Extract 

Another biological amendment you could use on your clay soil is compost extract. 

Compost extract is just water that’s full of beneficial microorganisms. This happens when you massage compost into water or by using an air pump and tank. 

It takes practice to make a good extract, and you won’t know how well you’ve done without a microscope.

So I recommend that you approach a compost company to purchase some. You need to buy a local extract because it will have the best microorganisms for your environment.

For instance, cold-resistant microorganisms from Alaska won’t do well in the California heat. 

It is possible to buy plastic jugs of extract online (like this 1-gallon jug on Amazon) or in garden centers.

Still, I recommend finding a dedicated compost company in your area, as these are much more likely to be successful.

Plus, you can ask the salespeople for help and advice specific to your region. 

Compost Tea

Compost tea is similar to a compost extract, but it has additional food resources so that the microorganisms have a better chance of reproducing when they hit the ground.

Just like with the extract, I recommend you get in touch with a local compost company and seek their advice.

Ask whether they have a microscope on site and if they check levels of biology in all their batches. 

If the answer is no, then I wouldn’t go there, but I admit, I am overzealous because understanding soil health is my career!

So if you are using compost tea or extract that seems to be having fantastic results, you don’t have to ditch it just because of my recommendations. 

Traditional Amendments For Clay Soil

A gardener using a small shovel to add a white powder to garden soil.

All the amendments mentioned above will help get a good set of microorganisms (bacteria, fungi, protozoa, beneficial nematodes) back in your soil.

Their natural glues will make aggregates (clumps) in the clay soil, so water, air, and roots can move freely.

They will also release nutrients from the soil particles so that your plants don’t get stressed.

There are some more traditional amendments that you should be aware of. These aren’t categorically bad; they just deal with symptoms rather than root causes.

In the long run, this can mean you’re spending a lot of money without seeing lasting change. 

Gypsum (Calcium Sulfate)

Gypsum, aka calcium sulfate, is frequently spread on clay soil.

This can help break up the compaction, creating some of the same air, water, and root space that microorganisms can. In the short run, this can be helpful. 

However, the calcium will leach out with the first rainfall.

This means that local streams and rivers get an unexpected overload of calcium, which can change the pH and wreak havoc for local wildlife.

Having said that, gypsum can also reduce the runoff of other agricultural chemicals, so more research needs to be done to really understand the impact it has on the natural world. 

Losing that calcium also means losing your money.

So gypsum can be a solution, but it’s impractical and expensive in the long run, and it can have unforeseen impacts on wildlife. 

Sand or Gravel

Some people add sand or gravel to their clay soil to help give it some structure. This doesn’t have the same consequences as adding gypsum, but it’s not practical on a large scale. 

In a small yard or raised bed, mixing sand with clay soil can be effective.

However, in a large vegetable garden or agricultural setting, you’re never going to be able to import enough sand to make a long-term difference. 

General Guidelines for Improving Clay Soil

Now that we’ve looked at the appropriate use of amendments, I’ll give you some more management tips for improving clay soil. 

1. Promote Healthy Soil Food Web

The biological amendments that I suggested are going to get some much-needed life back into your soil, but they will soon die if you spray pesticides or use inappropriate biological controls. 

For example, if you spread a bacteria designed to destroy a harmful fungi, it will also eat up the beneficial fungi you just worked on getting into your soil.

Of course, pesticides will kill pests and pathogens, but they will also destroy the organisms that reduce compaction and build structure in your soil. 

So use any pesticides (organic or chemical) in moderation. This means using them at a small scale and only when they absolutely can not be avoided. 

2. Avoid Tilling and Digging 

Tilling and digging will crush the microorganisms in your soil.

It can also make compaction worse, so your clay soil becomes even more waterlogged, and oxygen and plant roots can’t make it down past the surface. 

Advancements in soil science clearly show that no-dig gardening is best for your soil, particularly clay soil, which must be managed very carefully to prevent degradation. 

3. Use Cover Crops

Cover crops or “green manure” will help reduce compaction caused by the rain. Any raindrops hitting bare soil are going to be contributing to compaction and waterlogging.

Bare soil also means that your microorganisms aren’t getting fed below the ground because they rely on plants to provide sugars in return for nutrients. 

The reason plants feed bacteria and fungi with sugars is because they both provide helpful functions that help the plant thrive.

For example, bacteria pull nutrients out of the soil particles, and fungi can form armor around the roots of plants so diseases can’t find a way into the plant. 

The symbiotic relationship can’t be maintained if there aren’t any plants photosynthesizing. In turn, your garden won’t be as resilient. 

4. Consider Mulching 

Mulching is important on so many levels. When it comes to clay soil, it gives worms a place to hide out if their burrows get too waterlogged.

If you don’t provide mulch on the surface, worms will get flooded out to the surface and quickly die from exposure. 

Without worms, clay soil will soon lose oxygen because the burrowing passageways help fresh air make its way down into the ground. 

You can use wood chips, straw, or leaf matter for mulching. It will also reduce weed pressure and help moisture stay in the ground in the hotter months. 

Length of Time Needed To Improve Clay Soil

You can improve clay soil in a single growing season, but you might need several years to get your soil to its absolute best.

Still, you will notice a considerable difference in a single year if you use quality amendments and commit to the management practices I described above. 

If your clay soil remains waterlogged and compacted year after year, it means you have not restored biology to your land.

You need to think about whether pesticides from neighbors could be causing you problems or if you have overlooked some other aspect of soil management. 

Don’t worry if you do slip up at first. It would take many lifetimes to understand how to work in harmony with our garden.

Unfortunately, we only have one, so we can only do our best!

How To Improve Clay Soil in Existing Lawns

If you’ve already got your lawn in place, you probably don’t want to go ripping anything up.  

In this case, you would be best off using a compost extract or tea as a soil drench, spraying it over your yard with a watering can or industrial sprayer, depending on the size of land you are dealing with. 

You would also need to stop using pesticides and chemical fertilizers. Feeding your clay soil nitrate-based fertilizers is like feeding your kids a diet of candy.

If you want your soil to be healthy and balanced, it needs the wide variety of nutrients that bacteria and fungi extract from soil particles. 

Best Fertilizer for Clay Soil 

I understand why you’d want to use fertilizers for clay soil. You might notice your lawn becomes more lush and green after spraying, so it seems like a great idea. 

What you can’t see is what’s happening below the ground. Because your grass has access to nitrate “candy,” it doesn’t bother feeding bacteria and fungi.

Unfortunately, this means that the beneficial bacteria and fungi die or move out. 

So far, no problem. Except this means you are stuck in a dependency cycle, where your grass will need fertilizer for life.

As soon as you stop spraying it, your grass will start wilting because the soil is lifeless. 

Honestly, the best thing you can do to help out your clay soil is to use a compost tea or compost extract, teeming with good microorganisms.

Bacteria, fungi, protozoa, and beneficial nematodes will do more for your clay soil than fertilizer ever will.

They will build structure and reduce compaction, whereas fertilizer will only improve aesthetics while undermining your garden’s long-term health. 

How To Deal With Waterlogged Clay Soil 

If your clay soil is waterlogged, you want to drain some of the excess moisture away.

You can dig a shallow trench so water can run out, but bear in mind that this will bring some valuable topsoil with it. 

Once water has drained, add a thick layer of good compost to replace any lost topsoil. 

The compost should kick-start colonization by good biology, which will prevent the problems from repeating themselves in the future. 

Final Thoughts 

You can start improving your clay soil in a single year if you know what you’re doing. The key is promoting a healthy, thriving soil that is teeming with microorganisms.

You can’t achieve this if you are using pesticides or tilling because both of these things will set you back to step one. 

Helpful amendments include compost, compost tea, and compost extract. In some cases, you can use gypsum and sand. 

Be careful with gypsum because it soon leaches out of your soil, which comes at a financial and environmental cost.