In the Garden:
Southwestern Deserts
July, 2011
Regional Report

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Flowers perform best in soil texture with a mix of sand, silt, and clay particles, and lots of organic matter.

Soil Texture

The towering haboob dust storm that recently roiled across much of southern and central Arizona left behind a thick layer of dust that coated plants, hardscape, cars, and everything else in its path. As I swept what seemed like a bucket of residue off my sidewalk, its siltiness brought to mind the topic of soil texture. It may not be something gardeners think about too often, but soil texture does have an impact our gardening tasks.

Soil texture is defined as the percentages of sand, silt, and clay particles in the soil. Sand is the largest particle, clay is the smallest, and silt is in between. You can feel the difference. Rub a few pinches of soil with water between your fingers: sand is gritty, silt is smooth like flour, and clay is sticky.

Texture influences how much water and fertilizer we will have to apply to keep plants healthy. For example, sandy soil drains more rapidly than silt or clay, so it will need more frequent irrigation. Frequent irrigation, in turn, means that nutrients will be leached through the soil more quickly, requiring more fertilizer (depending on plant type, of course). On the positive side, sandy soil offers good aeration for roots.

Clay soil retains water longer, so generally you can irrigate (and fertilize) less frequently. On the negative side, clay particles are so tiny that they pack together like a tight jigsaw puzzle, creating poor drainage and poor aeration, which can contribute to root rot.

Silt particles have characteristics between sand and clay. Silt has better drainage than clay and retains nutrients better than sand. A mixture of all three, plus lots of organic matter, combines to create a healthy garden soil for annual flowers and vegetables.

How to Determine Soil Texture
Here's an easy method to determine basic texture that you can do as a fun little science project for yourself or with the kids. Sift a few handfuls of soil to remove rocks and debris. Put one cup of sifted soil in a straight-sided quart jar. Add one tablespoon each of laundry detergent and salt. These help disperse different types of soil particles. Add tap water almost to the top and secure the lid. Shake for five minutes. (Or let all the kids take a turn shaking up a storm.)

Set the jar where you can see it and it won't need to be moved. Over a couple days, soil particles will gradually settle out into layers. Sand particles are the largest and heaviest and will sink to the bottom first, followed by silt, then clay. The thickness of the layers will help you determine the relative percentages of each.


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