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Understanding and Improving Clay Soil

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Clay soil is prevalent many parts of the United States, and it can be a real pain if you happen to decide that you want to plant a flower or vegetable garden. While many trees and shrubs grow well in clay, the roots of the majority of annuals, perennials, and vegetables just aren't strong enough to make their way through. And if spring flower bulbs are your dream, forget it. Bulbs tend to rot over the winter in clay soils. With a bit of background about clay, and a strategy for improving your soil structure, you'll be able to grow flowers and vegetables to your heart's content.

What is Clay Soil?

Clay soil is defined as soil that is composed of mostly clay particles. Soil that consists of over 50% clay particles is referred to as “heavy clay.” To determine whether you have clay soil or not, you can do a simple soil test. Most likely, you probably already know if you have clay soil. If your 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.

Positives of Clay Soil

Even clay soil has some good qualities. Clay, because of its density, retains moisture well. It also tends to be more nutrient-rich than other soil types. The reason for this is that the particles that make up clay soil are negatively charged. They attract and pick up positively charged particles, such as calcium, potassium, and magnesium.

Negatives of Clay Soil

In addition to the drawbacks mentioned above, clay also has the following negative qualities:

  • Slow draining
  • Slow to warm in the spring
  • Compacts easily, making it difficult for plant roots to grow
  • Tendency to heave in winter
  • Tendency to be alkaline

Improving Clay Soil

Improving your clay soil will take a bit of work, but the good news is that the work you do will instantly improve the structure of your soil and make it easier to work with. Most of the work is done up front, with some annual chores to continue improving your soil.

It is best to improve an entire planting area all at once. I often see advice about just improving individual planting holes as you go along, but I don't recommend this practice. When you dig a planting hole in clay soil, then plop in a plant and nicely amend only the soil you use to backfill, your plant will be happy for a little while. But what you've essentially done is make the planting-hole equivalent of a flower pot. Eventually, the plant will start sending out roots, but when they reach the limits of the nicely amended soil you backfilled with, they will have a hard time expanding into the hard clay around them, and will start circling around in the planting hole instead. You'll end up with a perfectly root bound plant, and it won't grow as large or as healthy as it should.

Decide how much area you want for your new garden. If you are improving an existing bed, you'll just have to dig out any plants you want to keep, and then you can get started. If you are preparing a brand new bed, there are a few more steps to consider.

To improve your soil, you'll need to add six to eight inches of organic matter to the entire bed. You can add any organic matter you can get your hands on. Grass clippings (as long as they haven't been treated with chemicals), shredded leaves, rotted manure, and compost are all perfect choices. Spread your organic matter on top of the soil. Here's where the manual labor comes in. The organic matter needs to be mixed into the top six to twelve inches of soil. Digging it in and mixing it with a shovel is a great way to do this, as it moves a lot of earth without pulverizing the soil particles the way tilling can. However, if digging is just too hard on your back, using a tiller is a fine method.

When you're finished, your garden bed will be several inches higher than it was originally. It will settle some over the course of a season, but the soil structure will keep improving as microorganisms in the soil work to break down all of the organic matter you've added. The bed can be planted immediately, however. You'll be adding more organic matter on the top of the bed once or twice a year. This will continue the process of improving the soil's structure and offset any settling that happens.

Testing for Fertility and Adding Fertilizers

After a season or so, it's a good idea to collect a soil sample and have it tested 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 be perfect for growing healthy plants.

It's a bit of work. But you can be secure in the knowledge that you won't have to waste time struggling with clay soil again.

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