How to Determine Soil Texture – Using the Textural Triangle or Pyramid:
The textural triangle represents all possible combinations of soil separates. There are 12 primary classes of soil texture defined by the USDA (Soil Survey Division Staff, 1993). The textural classes are defined by their relative proportions of sand, silt, and clay as shown in the USDA NRCS textural triangle below (Figure 1).
Figure 1: USDA NRCS illustration of the textural triangle represent increasing or decreasing percentages of sand, silt and clay particles.
How to Test Soil Texture:
There are three kinds of soil particles organized by size: Clay (less than 0.002 mm), Silt (0.002-0.05mm), and Sand (0.05-2mm), and the percentage of each of these makes up your Soil Texture. Soil also contains other objects such as gravel, sticks, twigs and maybe worms. When taking your soil sample for the Soil Texture Test, you will need to screen your sample to eliminate these objects Clark, E. 2015 (1).
When running your Soil Texture Test, you will need a straight-sided tube or jar with some water, shake and let settle. Then get out your ruler and measure the thickness of each layer. Do a little math, and you can figure out your soil type (Figure 2) Clark, E. 2015 (1).
Figure 2: Clark, E.. eTilth Blog posted on February 3, 2015.Understanding Soil Tests – Pt 1, Medium Loam - ideal soil texture content.
The R-Squared Diagnostic's Soil pH and Texture Test Kit:
R-Squared Diagnostics offers a Soil pH and Texture Test Kit. The kit contains the following:
3 - Air tight protective tubes containing 15 pH papers each
1 - Soil pH and texture sample reaction vessel (blue cap)
1- pH and soil texture instruction leaflet
1 - pH scale color chart
1 - Soil textural triangle chart
1 – Ruler
Other optional items not included with kit:
- Wax or grease pencil for marking S and W line on Sample Reaction Vessel and drawing "separate" lines on Soil Textural Triangle
- Non-foaming dishwasher detergent
Soil Texture Test Instructions:
- Collect soil or compost samples, with several handfuls, at random locations and mix together well, in a bucket or small container, to form a “composite sample”. Remove debris (large pieces of leaf, wood or rock). Use a garden sieve if available.
- The kit contains a Sample Collection Vessel (with blue cap) and a ruler.
- Using the ruler, measure the Soil (S) and Water (W) lines on Sample Collection Vessel. Add soil to S line and clean water (filtered or distilled) to W line indicated by the ruler.
- After running the pH test, as an optional step, add one small drop of non-foaming dishwasher detergent to help disperse the soil particles.
- Shake vigorously for 1 minute so that the soil is thoroughly mixed with the water.
- Wait for the soil to settle, allowing a minimum of 2 hours. Heaviest particles will settle first (sand, silt and clay) and their layers will become visible. Sand, Silt and Clay settle in successive order, beginning with Sand. Sand at bottom, Silt in the middle and Clay at the top. A clear solution, comprising Organic Matter, is above the Clay.
- Using the ruler supplied in the kit, measure the height of the total soil sediment (vessel base to top of sediment). The soil sample below shows a total soil sediment height of 2.5 cm (Figure 3).
Figure 3: Using the ruler, measure the height of the sediment(from bottom of Sample Reaction Vessel to top of sediment line)
8. Using the ruler, measure the depth of each separate (sand, silt and clay) in Figure 4:
Figure 4: Use a wax (grease) pencil to mark each separate (clay, silt and sand) layer. Using the ruler to measure separate depth.
9. Calculate each soil separate (clay, silt and sand) as a percentage of the total sediment content:
- Total Soil Sediment: 2.5 cm = 100%
- Total Sand separate: 1.0 divided (/) by 2.5 cm = 40%
- Total Silt separate: 1.0 / 2.5 cm = 40%
- Total Clay separate: 0.5 / 2.5 cm = 20%
Determine the Soil Texture Using the Soil Textural Triangle (supplied with the kit):
The USDA NRCS provides an online calculator and MS Excel spreadsheet to calculate and determine the Soil Texture based on the Soil Textural Triangle shown above. Click here: USDA NRCS Soil Texture Calculator - to access the online calculator USDA NRCS, 2015 (2).
The Soil Textural Triangle is easy to use once it is understood. The three sides of the textural triangle represent increasing or decreasing percentages of sand, silt and clay particles, called “separates”. Using a soil texture composition that is 20 % clay, 40 % silt and 40 % sand, the triangle will determine the soil texture, based on the % of each separate (Figures 5, 6 and 7).
Clay Composition (Figure 5): From the lower left corner to the top of the triangle, the percent clay increases from 0 to 100 percent. Move the ruler, supplied with the kit, along the left side of the triangle until you reach 20 % clay. Then draw a line at 20 % clay parallel to the bottom of the triangle.
Silt Composition (Figure 6): From the top right corner to the lower right of the triangle, the percent silt increases from 0 to 100 percent. Now, using the ruler, draw a line at 40 % silt that is parallel to the left side of the triangle.
Sand Composition (Figure 7): From the lower right corner to the lower left corner, the percent sand increases from 0 percent to 100 percent. Move the ruler along the bottom of the triangle until you reach 40 % sand. Draw a line at 40 % sand that is parallel to the right side of the triangle.
Result: The point at which the three clay, silt and sand lines intersect will define the soil’s texture. In the example above, the soil texture is the ideal soil mixture - Medium Loam.
Hand Texturing "Ribbon" Method:
Hand texturing is another practical way to estimate soil texture. The practice is commonly called the "Ribbon Method". By using samples of known texture, and a lot of practice, the soil texture (sand and clay percentages) can be estimated. The procedure involves moistening soil and working it between the thumb and fingers to form a ribbon (Table 1) Plant & Soil Sciences eLibrary, 2015 (3).
Table 1: Soil texture as defined by soil textural class and estimated by hand
Clark, E. eTilth Blog posted on February 3, 2015.Understanding Soil Tests – Part 1.
USDA NRCS web page. 2015. Soil Survey / Soil Texture Calculator
Plant & Soil Sciences eLibrary. 2015. Soils Part 2: Physical Properties of Soil and Soil Water. USDA, NIFA, UC-Davis, NSF, UNE