How to use the soil texture triangle
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Soil texture can be identified using the soil texture triangle and the proportions of clay, silt, and sand in a sample. Knowing the soil texture can give us information about factors such as permeability and water-holding capacity. These qualities impact what types of plants can grow as well as erosion rates. Using soil texture analysis has applications in a variety of areas including conservation, agriculture, and construction.
The proportions of particle types are found by suspending a sample in water (put soil and water in a jar and shake it), allowing the components to settle (this can take up to 24 hours), and then measuring the layers formed. Sand, the largest particles, form a layer at the bottom of the container. Silt forms the middle layer, and clay, the smallest particle, is at the top. There are twelve different possible soil texture designations when using this method.
Other methods of determining soil texture are the ribbon method and feel test. These methods involve physical manipulation of the soil. While they can easily be carried out quickly during fieldwork, they require more expertise to produce accurate results.
Using the triangle
First, you need the percentages of each particle type. Measure the height in the container of each particle layer (click for a description of the setup). Then divide the layer's height by the total sample height to find the percent. For example, if the height of the sand layer is 48mm, silt is 37mm, and clay is 35 mm, then the proportion of sand is 40%.
Total = 48mm + 37mm + 35mm = 120mm
% Sand = (48mm/120mm) x 100 = 40%
You need the percentages for two of the three particle layers to use the soil triangle but it’s worth finding the percentage for all three so that you can confirm that they equal 100% (thereby checking your math).
For the Soil Texture simulation, the math is simple because the total height of the sample is always 100 “millimeters” (not to scale). For the sample in the picture below, the sand is 66mm (66%), the silt is 12mm (12%) and the clay is 22mm (22%).
On the triangle, the soil texture type is identified by finding the intersection of the percentages of the three particles. The tricky part is knowing how to use the many intersecting lines on the diagram. The lines running side to side correspond to the clay values (shown in the image in red). The lines running upwards from right to left are the sand values (shown in blue). The lines running downward from right to left are the silt values (shown in gray).
The area that contains the point at which the three types intersect is the soil type. The boundaries on this diagram are marked in black, but some soil triangles use more colors to designate the areas defining each type. For this example, the type is sandy clay loam.
Now you're ready to practice identifying samples using the Soil Texture simulation!
What do you do if the lines intersect on a border between two types?
The short answer is...hope it doesn't come up.
Anyone making an assessment shouldn't have any questions where the result is on a border, so a student should not have to worry about this happening in an exam situation. In both the simulation and in real life, it could happen, though. The numbers used for the triangle can be found in a 1987 USDA Soil Mechanics training document. The boundaries for each type are specifically defined. Note that in the function used in the simulation to define the correct soil type (in the image below), there are many places using greater/less than or equal to (<= and >=) to denote which boundaries are inclusive to which texture type.
This information isn't represented in the triangle. Should it come up, here is a triangle with the borders marked to identify the type...