Exploring Soil Texture

Contributed by our friends at LaMotte, this is an excerpt from the LaMotte Soil Microbe Hunter. 

The Microbe Hunter Activities Series is a fun and safe way to bring STEM-based learning into the classroom through the culturing and presumptive identification of common microbes on innovative BioPaddles. For more information visit the LaMotte website.

Exploring Soil Texture

The look and feel of a soil is called the soil texture. Soil texture is determined by the size and proportion of the particles that make up the soil. 

Every soil can be separated into three fractions – sand, silt, and clay. Sand particles are the largest and make the soil feel gritty. Silt particles are medium sized and give a smooth, floury feeling to the soil. Clay particles are the smallest and make the soil feel sticky.

Soil texture defines the nature and size of the pore spaces in soil so it determines the water holding capacity and the rate at which water can move through the soil. If pores are too large water will not be retained. If pores are too small, water will not pass through. Most soils have a combination of the three types of soil particles. Loam soils - with 20% clay, 40% silt and 40% sand – have the combination of water handling qualities that are best for growing crops.

Water moves quickly through sandy loam soil so it is beneficial for irrigated agricultural areas. Clay loam soils store more water. If irrigation is not available, soils with more clay retain water longer.

Activity 1: Determining Soil Texture by Feel

Rub some moist soil between your fingers.

• ‍Sand will feel gritty.
• ‍Silt will feel smooth.
• ‍Clay will feel sticky.

Note: Clay has a large influence on the feel of a soil. As little as 20% clay will give the soil a clay-like feel. Sandy soil is described as soil material that contains 85% or more sand where the percentage of silt plus 1.5 times the percentage of clay does not exceed 15%. Silt soil is described as soil material that contains 80% or more silt and less than 12% clay. Clay soil is described as soil material that contains 40% or more clay, less than 45% sand and less than 40% silt. Every soil can be classified by using the Soil Texture Triangle.

Activity 2: Determining Soil Texture by Settling and the Texture Triangle

This procedure will separate soil into its three basic mineral fractions: sand, silt, and clay. The amount of time required for the soil particles of various sizes to settle in a graduated cylinder forms the basis for this test. The sand fraction will settle first, then the silt fraction, followed by the clay fraction. The approximate percentage of each fraction can be determined in the original soil sample.

Materials

• ‍Graduated cylinder, 100 mL
• Plastic wrap
• Rubber bands
• Ruler (metric)
• Powdered dishwasher detergent, 1/8 teaspoon
• Measuring spoons
• Soil Sample - Spread the sample out and allow it to dry overnight. Pick out any stones, twigs and leaves.

Procedure

1. ‍Add the soil sample to the 100 mL graduated cylinder until it is even with 50 mL line. Note: Gently tap the bottom of the cylinder on a firm surface to pack the soil and eliminate air spaces.
2. Add tap water to the 100 mL line.
3. Add about 1/8 teaspoon powdered dish washer detergent.
4. Cover the top of the graduated cylinder with plastic wrap. (Double or triple the plastic wrap to make a stronger cover.) Secure the plastic wrap with a rubber band.
5. Mix the soil and water thoroughly by inverting the graduated cylinder. Allow the graduated cylinder to stand for 15 minutes.
6. Mix the contents of the graduated cylinder again by inverting it slowly for 5 minutes. 
7. Record the total volume of soil in the graduated cylinder.
8. Allow the graduated cylinder to stand undisturbed for 24 hours or longer.
9. Look at the layers of the settled soil in the graduated cylinder. The clay will be on the top, the silt will be between the clay and the sand, and the sand will be on the bottom. The measurement at the top of the clay level will also be the total volume of the soil sample.
10. Determine how many milliliters of each fraction are in the graduated cylinder. Record the following data:
    Total volume of soil sample:__________mL
    Volume at top of silt column:__________mL  (same as bottom of the clay column)
    Volume at top of sand column:__________mL  (same as bottom of the silt column) 
11. Calculate the percentages of sand, silt, and clay. 
    % sand =top of sand column, mL – bottom of sand column, mL x 100 / Total volume of soil sample, mL
    % silt =top of silt column, mL – bottom of silt column, mL x 100 / Total volume of soil sample, mL
    % clay =100 - % sand - % silt
    Note: Due to the colloidal nature of clay in solution and its tendency to swell and form a gel, the portion of clay in the graduated cylinder is not used to determine the clay fraction present in the soil. The clay fraction is calculated by adding the sand and silt fractions and subtracting this total from the initial volume of soil used for the separation.
12. Use the texture triangle to determine the texture of the soil sample.