In the Garden:
Pacific Northwest
June, 2007
Regional Report

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This well-weathered collection of leaves can be used as mulch over bare soil or added to the compost bin.

Improving Clay Soils

One of the biggest challenges a gardener faces is dealing with clay soils. What can you do to improve them? The answer is not simple, but there is still plenty you can do to improve heavy, poorly draining clay soils.

Soil is comprised of minerals, organic matter, air, and water. The mineral portion is made up of sand, silt, and clay particles. Sand particles are big enough to be seen with the naked eye; silt particles require some magnification to be seen; viewing clay particles requires an electron microscope. Soils with a large proportion of clay tend to be heavy and poorly drained because the clay particles are so small and packed together that water has a hard time finding a pathway through.

Determining Your Soil's Texture
One way of getting a rough idea of your soil's texture is the squeeze test. Two or three days after a good rain or watering, take a handful of soil and rub it between your thumb and your index finger. Sandy soil feels gritty; silty soil feels silky, like moist cornstarch; clay soil feels slippery. Now gently squeeze the soil into a ball and release it. If it crumbles, it has a fairly well-balanced texture. If it holds its shape, it has quite a bit of clay. If you can roll it into a cigar shape, it has even more clay. There isn't much you can do about your soil's texture, but you can improve its structure.

Soil Structure
Soil structure refers to the way the particles in soil assemble together as aggregates. Clay soils tend to aggregate into sticky, horizontal plates. When these plates dry, they shrink and crack. Water drains very slowly, and plant roots tend to follow the cracks, so they miss all the nutrients in between. For the healthiest garden plants we want to create soil with a good structure. We want a soil that is crumbly, that doesn't crust over or make clods, that absorbs water readily and holds some water yet drains well.

So, how can we improve clay soil? By the addition of organic matter. Organic matter improves soil structure by increasing the size of the soil aggregates, making them less likely to shrink and crack.

Improving Soil Structure
You can add compost to a garden by digging or tilling it in or using it as mulch. Incorporating large amounts of compost into a vegetable garden can be fairly easy, just dig it in after you clear a bed in the spring or fall. In perennial or ornamental beds, it's easier to apply compost (or other organic materials) as mulch, right on top of the ground. Just be sure to keep the mulch away from the plant stems to avoid diseases.

Preserving Soil Structure
One point to remember when working clay soils: it is very important that the soil not be too wet or too dry. Working very wet or very dry soil can further damage the soil's structure, leading to formation of big, hard clods.

Here's how to check for soil moisture: pick up a handful of soil and give it a squeeze. When you open your fingers, if the soil crumbles, it's too dry; water it and wait a day or two to work it. If the soil forms a solid ball, it's too wet; wait a day or two. If it holds together loosely, your timing is perfect and you can dig!


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