No gardener wants to find clay soil in the area they hoped to turn into a thriving, productive garden bed. Clay soil is notoriously difficult to work, having a putty-like consistency when wet, and turning into a brick when dry.
However, clay soil also has some characteristics that can help in the garden: it retains nutrients and water better than other types of soil.
The best part of clay soil is that it’s simple to improve with the right methods. If you have clay soil, you’ll be able to turn it into workable beds for vegetables, fruits, and herbs in just one season.
In this post, we will discuss what clay soil is, how to tell if you have it, and how it can impact your garden if left unaddressed.
We will also go over all the right strategies for improving clay soil, and even dispel some common myths about the process.
What is Clay Soil?
Clay soil is made up of at least 25% clay particles. Clay particles are far smaller than other soil particles, such as sand. By comparison, clay particles can be 1,000 times smaller than sand particles.
Additionally, clay particles are uniquely flat, stacking up tightly like a deck of cards, unlike particles like sand, which are round.
Because of the shape and size of clay particles, clay soils can become compacted easily. Picture a stack of bricks (representing clay soil) and a large tub filled with beach balls (representing sand or another large, round soil particle).
The beach balls have more space between them for water and air to flow, while the small, flat bricks create a barely penetrable barrier.
This finely textured soil has both challenges and benefits in the home garden. It’s much harder for air, water, fertilizers, and root systems to move through clay soils, especially if they become compacted.
For the same reasons, clay soils are better able to retain more water and nutrients, which is an advantage.
By improving clay soils with the strategies outlined below, you can enjoy the benefits of clay soil while greatly reducing the downside.
How Do I Know If I Have Clay Soil?
There are several ways to find out if you have clay soil.
First, you can always have a soil test conducted. Soil tests will give you a wealth of information beyond what type of soil you have, and are inexpensive.
Your soil report should include specific recommendations for improving your soil, too. Contact your local cooperative extension office to get started.
Observing your soil will clue you in to its type. When wet, does your soil become a sticky putty glued to the bottom of your boot? When dry, is it hard and cracked? If so, you have clay soil.
You can also perform a couple hands-on tests. First, grab a small handful of soil. It should be wet, so add water if necessary.
Form the soil into a ball, then squeeze or roll into a ribbon. If the ribbon reaches two inches in length without breaking, you likely have clay soil.
How Does Clay Soil Impact the Garden?
The structure of clay soil helps it retain water and nutrients better than other types of soil, but this same structure also causes the following issues for plants:
Hard-to-Work Soil: Clay soil vacillates between a putty consistency when wet and a hard, brick-like texture when dry. Neither of these are good gardening conditions.
Stunted root growth: While trees and shrubs generally don’t have an issue growing in clay soil, plants with smaller root systems such as vegetables and herbs struggle to penetrate this dense soil.
Often, plants grown in clay soil are unable to extend their root system beyond the hole they were planted in, causing them to be root bound just as if they were stuck in a too-small container.
Lack of drainage: Clay soil can retain too much water, leading to root rot and insufficient oxygen.
Lack of Soil Life: Clay is a hostile environment for the worms and microorganisms that are essential for a thriving garden.
Intensified Poor Soil Conditions: If there is a nutrient or mineral imbalance in your soil, it will be amplified in clay soil.
Practical Ways to Improve Clay Soil for Your Garden
Fortunately, clay soil is easy to improve by implementing strategies that increase air, water, and nutrient flow.
You’re essentially breaking up that brick wall of compacted clay particles and creating more space and porousness in your soil structure.
All of the strategies below are relatively simple, but do require consistent time and effort each season. Using some of these strategies in combination will bring the best results.
Some are practices that are beneficial for any soil in any garden, so regardless of your soil type, you might want to incorporate them into your garden toolkit.
1: Aerate Clay Soil for Better Plant Growth
Aeration creates air pockets in the soil, which improve drainage and lessen impaction. Aeration should be done twice a year, in the fall after garden cleanup and in the spring before planting.
To aerate compacted clay soil, you can use a handheld tool such as a broadfork or digging fork. To aerate a large area easily, buy or rent a tow-behind aerator that attaches to a ride-on mower. Avoid tools such as spiked aerator sandals; these are best for maintaining soils that are already in good condition.
Work backwards when aerating. Otherwise, you’ll end up re-compacting the soil as you walk or ride over it.
2: Amend Your Clay Soil with Organic Matter
The best amendments for clay soil are organic matter such as leaf mold, bark, manure, and compost.
Amendments should be added right after aeration, as the aeration holes create an easy entrance point for them to be worked into the soil.
Beyond improving soil structure on its own, organic matter attracts microorganisms and worms, which further loosen the soil as they move through it. Worms also leave castings, increasing the amount of organic matter available.
Compost is an ideal amendment because unlike manure, you really can’t overdo it. In addition to improving the soil structure, compost contains mychorrhizal fungi which create a compound called glomalin.
Glomalin binds clay particles together while covering this larger particle in a wax coating, which creates more space for air and water to flow.
Manure is rich in nutrients but too much can damage growing plants. The proper amount of manure per square foot varies depending on the type and whether it is composted, so make sure to follow instructions carefully.
Leaf mold is simply composted deciduous tree leaves. Leaf mold loosens the soil, adds organic matter, and is rich in nutrients that plants need to thrive. It also retains moisture well.
Many gardeners have an abundance of leaves on the property already. At the end of the season, shredded or whole leaves can be worked into the soil in the fall, or composted and used the following year.
Finely shredded bark can be worked into clay soil to loosen it and provide organic matter, or added as a layer of mulch which will break down over time.
3: Using Worms and Castings to Improve Clay Soil
Rich in nutrients and microorganisms, worm castings are another great addition for improving soil structure.
Until you’ve made some progress on your soil, though, don’t add worms directly. Because it’s hard for worms to move through clay soil, they will eventually relocate to more favorable areas of your garden.
Once you start to enrich your soil with organic matter, though, introducing worms to your garden is a great way to aerate your soil and add nutrients.
4: Till in Amendments the Right Way
If tilling in amendments, take care not to worsen the soil structure in the process. Working the soil when wet, or tilling too deep too fast, may create long-lasting clumps that make the soil even harder to work with.
Clay soil should not be too wet when tilling. The soil is at the right moisture level if you can form a ball with your hands that falls apart easily when squeezed or poked. If the ball sticks together, the soil is too wet.
Begin with your tiller at its most shallow setting. Make a full pass over your beds at this setting, then increase the depth by two inches. Continue to do this until you’ve reached your desired depth.
5: Other Soil Amendments: Use With Caution
Both peat moss and gypsum can be used to improve clay soil, but are best used in very specific circumstances. Otherwise, they might do more harm than good.
Peat moss is not ideal, as it can create a bog-like consistency when combined with clay. Peat also retains moisture and nutrients so well that it can create soil toxicities. Peat is only recommended if you get regular soil tests.
Gypsum, or calcium sulfate, is a naturally occuring substance that is often recommended as an amendment to improve clay soil, but it’s generally unnecessary (and potentially harmful) in home gardens.
Gypsum is primarily used at the commercial level to prepare soil for tilling. It’s effects at breaking up and softening clay soil are short lived; after a few months, clay soil will return to its original state. Since gypsum doesn’t improve soil over time, use an amendment that does, like compost.
Additionally, gypsum can cause soil problems. It adds large amounts of calcium to the soil while breaking down salt deposits.
Unless your garden soil is already low in calcium and high in salt, gypsum can throw off your mineral balance, adversely affecting your plants.
However, if you live in a coastal or arid region, with over salinated soil that will benefit from added calcium, then gypsum might be an appropriate short term strategy for making your clay soil workable. Still, you’ll need to incorporate other methods for long term improvement.
6: Grow Clay-Busting Plants
Want to aerate your clay soil and introduce organic matter, all at the same time?
If so, clay-busting plants are the way to go.
These are plants that have sizable root systems that can break through clay soil. At the end of the season, instead of harvesting the plants or pulling the root systems, simply chop and drop the plants.
Or, if you’ve planted a root vegetable, just leave it in place. The roots will decompose underground, leaving air pockets and adding organic matter simultaneously.
Some clay-busting annual plants to try:
Daikon radish: This root vegetable can penetrate up to two feet into the soil. You can harvest some for eating, and allow the rest to continue to grow and flower. Before winter, just cut back the tops and leave the radishes in the ground to decompose.
Mustard: Mustard is a great choice because it has a huge, fibrous root system that can grow through compacted clay soils. Just chop and drop at the end of the season.
Sunflowers: Sunflowers also have strong root systems that can grow through clay. Plus, they have the added benefit of attracting beneficial pollinators to your garden.
7: Plant Cover Crops
Cover crops, or green manures, can be grown on clay soil and tilled under before they go to seed. This adds nitrogen, loosens the soil, and works in organic matter without adding weed seeds.
Additionally, some cover crops have deep taproots which penetrate up to three feet, breaking up impaction while bringing nutrients up to the topsoil.
Cover crops can be planted in spring for fall tilling, or in early fall for spring tilling. They also function as a “living mulch” when planted along with other crops.
Cover crops with particularly deep taproots are alfalfa, fava beans, and bell beans. Other commonly used cover crops to improve clay soil are clover, winter wheat, and buckwheat.
8: Build Contour Beds
Contouring your garden, or adding high and low elevation points, can help improve clay soil. This doesn’t have to involve heavy equipment, but can be as simple as incorporating terraces and raised beds or mounds into your garden landscape.
Contouring will help improve growing conditions in clay soil. The high points will dry out more easily, making for great growing areas, while the low points will naturally trap organic material that can break down and improve the soil.
How to Apply Organic Matter to Clay Soil
Regardless of the type of organic matter you choose, a good rule of thumb is to add 6-8 inches of organic matter to your garden beds and work it 6-10 inches deep into the soil. After this your beds can be planted for the first time.
To prevent your soil from returning to its previous clay state, reapply 1-3 inches of organic matter each year, in the fall or spring.
The most economical way to purchase bulk compost or organic matter, if you don’t make your own, is to have it delivered in bulk by the cubic yard.
One cubic yard of organic material will cover approximately 100 sq feet of ground in a layer 3” deep.
Why Adding Sand to Clay Soil Could Be Doing More Harm Than Good
While it might seem tempting to add sand to clay soil, the larger sand particles won’t improve the structure of clay soil unless sand is added in significant amounts (at least 3 parts sand to one part clay).
Instead, the smaller, flat clay particles will fill in the space between the bigger, round sand particles, creating a concrete-like soil that’s even harder to work. For this reason, avoid using sand entirely.
Improving clay soil may seem like a daunting task at first, but it’s actually straightforward and simple.
Aerating and amending your garden beds each season will transform your clay soil into the foundation of a beautiful and productive garden. Incorporating the other methods outlined above will only expedite this process.
Cameron Jenkins moved from the city to a small farm where he lives with his wife and daughters. The farm is divided between the garden, pastures, hayfields, the start of an orchard, and 13 times as many pets as people. Their farm vision is to grow produce and raise animals in unison with nature. When Cameron is not farming (or writing about it) he spends his time playing with his children, reading, cooking, and napping with his pet pig.