How to Quickly Determine Soil Structure

Knowing your soil structure helps you grow healthy plants.

An important part of raising healthy plants is understanding your soil structure. According to the USDA, soils are composed of three main elements--sand, silt and clay. The varying percentages of each material determine a soil's texture or structure. Knowing your soil structure helps you make decisions about supplying nutrients, water and organic matter. It also helps you choose appropriate plants for your garden--yarrow, poppies and irises do well in heavier clay soils, while thyme, grapes and butterfly bushes thrive in lighter sandy soil.

Things You'll Need

• Trowel
• Bucket
• Clear glass jar or plastic bottle with smooth sides and lid
• Funnel
• Dish soap
• Ruler with millimeter scale
• Calculator

Instructions

1. Dig a hole approximately 1 foot deep and 6 inches across. Put all the soil from the hole in your bucket. As you dig, pay attention to the soil conditions--check for distinct layers versus well-mixed soil. This gives you an idea of how well your soil retains water and distributes it to your plants. Mix the soil in your bucket thoroughly with the trowel. Remove any living things, such as worms or bugs.

2. Fill the container halfway with soil. If your jar or bottle has a narrow neck, use a funnel to make the job easier. Add water and a few drops of dish soap to fill the jar to the top. The dish soap acts as a surfactant, allowing the soil particles to separate easily and settle out. Put the lid on the jar and shake gently, turning it upside down a few times to fully mix the soil and water.

3. Let the jar sit for at least an hour. If you have a lot of clay or silt in your soil, it may take more time for everything to settle out. The water may appear colored from leached minerals or tannins--this is fine as long as you can see fairly well through the liquid. If the water remains cloudy, let your sample settle overnight.

4. Inspect the soil sample for distinct layers. These are (from bottom to top) the sand, silt, clay and organic matter components of the soil. Using the millimeter scale on your ruler, measure the overall height of the bottom three layers and write it down. Although organic matter is critical to good soil health, it is not part of the soil structure classification and is omitted. Measure the height of the individual sand, silt and clay layers and note the readings.

5. Divide a layer's height by the overall height. Move the decimal to the right two times--this gives you the percentage of the soil component. For example, if the clay layer is 20mm thick and the overall sample height is 100mm, the calculation would be 20 divided by 100 equals 0.2. Moving the decimal place to the right two times gives 20 percent. Therefore, the soil sample has 20 percent clay content. Repeat this for all three layers.

6. Look at the USDA soil triangle (see Resources). You can either view it online or print out a copy. On the side marked "% Clay," find the percentage that equals your clay content. Trace along the entire length of the line. For example, if you have 20 percent clay content, draw a line from left to right near the bottom part of the triangle. The numbers along each side of the triangle are underlined to show you which way to draw. Repeat this for all three soil components. The point where all three lines meet will land within a labeled soil type--this is your soil structure. For example, a soil with 40 percent sand, 40 percent silt and 20 percent clay falls into the "loam" category.

Tips & Warnings

• If you would like to determine your soil's organic content, divide the thickness of the organic layer by the height of the entire sample (all four layers.) Move the decimal to right two times to find the percentage. Organic matter should make up approximately 5 percent of your soil to provide a healthy environment for your plants.

• This test does not provide any information about the nutrient content or pH of your soil. Use do-it-yourself kits or consult a local agricultural labs for chemical analysis.

References

• Purdue University Cooperative Extension Service: Landscape Plants for Sandy Soils
• Theodore Payne Foundation: Plants for Heavy Soils

Resources

• USDA: Soil Texture Triangle