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The Dirt on Dirt Science of Gardening

What’s your soil type?


Can’t decide if you’re sandy or silty? Try this simple test.

If you’ve never heard of soil types, you may be surprised to learn that soils are typed not by their mineral content but by their texture—that is, the size of the soil grains. There are three basic soil types: sand, silt, and clay. Sand particles (.05–2 mm across) are gritty, silt particles (.002–.05 mm across) are floury, and clay particles (smaller than .002 mm) are sticky when wet.

The exact minerals involved don’t matter for classification—if you have a soil with grains that are all 1 mm across, it’s called “sand,” no matter whether the grains are quartz, mica, basalt, or anything else.

You can get some idea of your soil type by just grabbing a handful of it and squeezing. If it immediately crumbles and falls apart, it’s a sandy soil. If it crumbles slowly, it’s probably a mixed soil—sand, silt, and clay. If it sticks together in a wad, complete with imprints of your fingers, then it’s a clay soil.

If you really want to get down and dirty, you can figure out the exact proportion of sand, silt, and clay in your soil. Here’s how:


• A clear quart-size jar (such as a mayonnaise jar)
• One teaspoon water softener (such as Calgon)
• A stopwatch (or an ordinary watch)
• A marker or grease pencil

To do and notice

1. Fill the jar 2/3 full of water.

2. Stir in the teaspoon of water softener.

3. Fill the jar almost to the top with the soil to be tested.

4. Screw on the lid and shake the jar hard and fast for at least 20 seconds.

5. Set the jar down on a tabletop and start your stopwatch. After 20 seconds, you’ll see a dark layer forming at the bottom of the jar. Mark this line with your marker. This is the sand.

6. Wait two minutes more, then mark a second line where the next layer has formed. This layer is silt.

7. The third layer—the clay—can take anywhere from overnight to a week to settle out. When the water looks clear, you’ll know that all the clay has settled.

What’s going on?

You should see three distinct bands in the settled soil in your jar: sand at the bottom, silt in the middle, and clay on top. (Any coarse crud on the top of the layers or floating near the surface is organic matter.) The thickest layer in your jar determines your soil type. You can measure the thickness of these bands to calculate the percentage of sand, silt, and clay in your soil.

Ideal garden soil has some of each type—sand to help with drainage, silt to boost mineral richness, and clay to trap moisture and keep nutrients from washing away. Too much clay in soil means that there are scarcely any air gaps in soil, which can suffocate plant roots and beneficial microbes alike, and it also makes for poor drainage. Excessively sandy soils can drain too well, leaving plants vulnerable to drought without frequent watering.

The best soils contain about 10%–20% clay, and roughly equal proportions of silt and sand. A soil with these optimal proportions is called loam. A good loam also contains plenty of organic matter (as much as 5%–10% in rich soils) to supply nutrients.

Oh—you might be wondering why you added the water softener. Water softeners are deflocculants; they separate the clumps of soil into their small constituent particles, so that they can settle by size.


What about pH?
What is potting soil?
What about fertilizer?
What's your soil type?

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