Gardening Gardening Basics

Understanding Clay Soil and How to Improve It

How to improve clay soil

The Spruce / Jacob Fox

Clay soil can be a challenge for gardeners. While some trees and shrubs grow well in clay soil, most annuals, perennials, and vegetables don't have roots strong enough to force their way through dense clay. In soil, it is the amounts of minerals—sand, silt, and clay—that determine its texture. Sand particles are especially large and porous whereas clay particles are just the opposite: tiny, hard in their dry state, slick and sticky when wet, extremely dense, and resistant to water movement, all properties are not conducive to root growth. In clay soil, the bulbs of spring flowers simply rot over the winter. 

However, it is possible to improve clay soil by adding organic matter, such as bark, sawdust, peat moss, composting materials, or manure to the soil. Read on about how to determine whether you have clay soil, which methods work to amend clay soil and which don’t, and how to turn clay soil into great garden soil over time.

What Is Clay Soil?

Clay is one of the three basic soil types. It is mostly made up of very fine mineral particles, measuring only 0.002 mm in diameter (for comparison, sand particles range between 0.05 and 2.0 mm in diameter) and not much organic matter. The resulting soil is quite sticky since there is not much space between the mineral particles, and it does not drain well at all.

If you have noticed that water tends to puddle on the ground rather than soak in, it is likely your soil is clay. Soil that consists of over 50 percent clay particles is referred to as “heavy clay.” 

Chances are you probably already know if you have clay soil. If the soil sticks to shoes and garden tools like glue, forms big clods that aren't easy to separate, and crusts over and cracks in dry weather, you have clay. You can also do the squeeze test. Take a handful of moist (but not wet) soil from your garden and squeeze it firmly. If you open your hand and the soil holds its shape even when poked, it is clay.

Gardening gloves holding a handful of clay soil

The Spruce / Jacob Fox

The Disadvantages of Clay Soil

The negative sides of clay soil include:

  • Slow draining
  • Slow to warm up in the spring
  • Compacts easily, making it difficult for plant roots to grow
  • Tendency to frost-heaving in the winter
  • Typically has an alkaline pH, which is not suitable for planting vegetables that need a pH between 6.5 and 7.0
slow-draining clay soil
The Spruce / Sandhya Moraes 

Improving Clay Soil

Improving your clay soil takes an effort but the good news is that it will instantly improve the structure of your soil and make it easier to work with. It is best to improve an entire planting area all at once, rather than to attempt improving the soil in individual planting holes as you need them. 

Start by defining the growing area for your garden bed. If you are improving an existing bed, you can dig out any plants you want to keep and set them aside in pots until your soil improvement
is completed. If you are preparing a brand-new bed, you'll need to go through the basics of starting a new garden bed.

Here are the steps to improve clay soil:

  • Add 6 to 8 inches of organic matter to the entire bed. Untreated grass clippings, shredded leaves, rotted manure, and compost are all perfect choices. 
  • Spread the organic matter on top of the soil, then work it into the top 6 to 12 inches of soil. Using a shovel is better than a tiller as it moves a lot of earth without pulverizing the soil. 
  • Plant the bed immediately if you like. 
  • The garden bed will be several inches higher than originally, but it will settle throughout the season as the organic material breaks down.
illustration of how to improve clay soil

The Spruce

Warning

Do not add sand to clay soil. Adding sand does not help to improve clay soil. In fact, it creates a bigger problem. Although loamy soils, viewed as ideal garden soil, are a mixture of sand and clay, adding sand directly to clay yields something that resembles concrete. The reason loamy soils are great for plants is that they also include a large ratio of organic matter as a foundation for the sand and clay. To improve clay soils, reach for compost and other organic matter only, not for sand.

Amending Clay Soil Over Time

Clay soil cannot be changed in a single gardening season. Depending on how much clay your soil contains, it takes years during which organic matter will need to be continually added to prevent your garden from returning to its heavy clay state. Fortunately, this becomes a self-fulfilling process as garden plant material breaks down and works its way into the soil. Rather than cleaning your garden down to the soil line each fall, allow leaves and other plant material to decay naturally and become part of the ongoing ecosystem of good garden soil. Continue to mulch your garden with more compost and also plant cover crops to add green manure to the soil

After one or two gardening seasons, it's a good idea to do a detailed soil test (available at your local Extension Office) to see if you have any nutrient deficiencies or pH issues. The report you get back will offer suggestions for how to improve the garden further. Add any organic fertilizers or soil amendments outlined in your report, and your bed will continue to be perfect for growing healthy plants for years to come.

 

FAQ
  • What is clay soil good for?

    Heavy clay soil has mostly drawbacks but its big advantage is that it retains moisture well. In sandy soil, the opposite happens—the soil does not hold water and rapidly leaches nutrients.

  • How do you fix clay soil?

    Improving clay soil is unfortunately not a quick fix. Depending on the clay content in the soil, it is ongoing. After the initial addition of organic matter, plan to add more organic matter in the form of compost once or twice a year. This will continue the process of improving the soil structure and offset any settling that happens.

  • Can I plant in clay soil?

    It is not recommended to plant in clay soil. Instead, amend the whole area before you plant. If you dig a planting hole in clay soil, then drop in a plant and nicely amend only the soil you're using to backfill, your plant will be happy for a little while. But you have done nothing more than create an in-ground flower pot. Eventually, the plant will start sending out roots that will be stopped in their tracks when they reach the clay walls of the planting hole. You'll end up with a root-bound plant that won't grow as large or as healthy as it should.