Understanding Clay Soil and How to Improve It

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 strong enough roots. Clay particles are extremely dense and resistant to water movement, which is not conducive to root growth.

In clay soil, the bulbs of spring flowers simply rot over the winter, for example. 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.

Soil that consists of over 50 percent clay particles is referred to as “heavy clay.” 

Clay 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.

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.

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

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 may be several inches higher than expected, but it will settle throughout the season as the organic material breaks down.

Amending Clay Soil Over Time

Clay soil cannot be changed overnight. Depending on how much clay your soil contains, it takes years of continually adding organic matter to prevent your dirt from returning to heavy clay. Fortunately, plant material you leave each fall will break down and work its way into the soil and help to transform the 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's results will offer suggestions on how to improve the garden further, such as adding organic fertilizers or soil amendments to help you grow healthy plants for years to come.