Q: I think I may have clay soil. How can I tell, and how can I make it more suitable for planting?
A: El Paso County’s heavy clay soil can become good soil. Adding organic material and relieving compaction make a world of difference to your plants.

Here’s an easy (“quick and dirty,” so to speak) method to determine what type of soil you have. Wet a handful of soil. Compress it in your palm. If it stays together and is a bit slippery, you have clay. If it sifts through your hand without taking any shape, you have sand. If, by a miracle in El Paso County, you have a handful of sweet smelling, dark, crumbly soil, you have loam. If it turns out you do have clay, read on!

Choosing a soil amendment [CSU Fact Sheet 7.235]
Gardeners in many areas of the country can take good soil for granted, but gardeners here in Colorado have to work for it. Heavy, sticky, clay soil needs organic matter, and lots of it, to become good garden soil. The top eight to ten inches of soil, where most of a plant’s roots live, determines the success of your gardening efforts. Plant roots need both air and water, just as people do. Clay soil is too compact to allow roots to “breathe.” Adding organic soil amendments to the soil lightens soil texture, discourages compaction, adds nutrients, improves drainage and aeration, moderates soil temperature, and provides pore space, which is essential to plant growth. Sandy soils can also benefit from added organic matter, which improves the sand’s moisture retention and adds needed nutrients.

Established beds will need regular additions of soil amendments, since organic matter breaks down quickly. If your gardening style calls for moving plants in and out of a bed regularly, then soil amendments can be added whenever you move a plant. Established beds will benefit from yearly topdressing with compost or an organic mulch.

Do not work in clay soil when it is wet. (That’s how pottery is made.) Mowing wet lawns or working in wet flower and vegetable beds compacts the soil and spreads fungal diseases.

Soil Amendments vs. Mulch
Amendments are worked into the soil. Mulch is placed on top of the soil. Soil amendments and mulches can be either organic (such as ground bark) or inorganic (such as gravel). After organic mulch decomposes, it can be worked into the soil as an amendment. Decomposed plant material from just about any source can become an organic soil amendment. It is decomposed enough when you can no longer tell what it used to be. Well-decomposed organic matter is black and crumbly, and smells fresh and loamy. Examples of possible plant material include last fall’s leaves, spent mulch, kitchen vegetable scraps, or the summer’s bygone annuals and vegetable plants.

Undecomposed organic matter is best thought of as a “work in progress” for gardeners interested in improving their soils. If you can still tell what it was, it’s not decomposed enough. Undecomposed organic matter (such as wood chips or shredded leaves) is fine as mulch, but should not be worked into the soil during the growing season. The microorganisms that decompose it will compete with your plants for the nitrogen in the soil.

As always, there’s an exception to the rule: under-decomposed organic material can be tilled into the soil in the fall, in preparation for spring planting. The winter will allow time for the under-decomposed organic material to break down. Freezing and thawing cycles also help.

Coming soon! Dealing with Clay Soil, Part 2
Contributed by Nadine Salmons, Certified Colorado Gardener