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Textural Classification of Soil: Soil Texture Triangle

A triangular classification chart has been developed by making use of grain size limits for sand, silt, and clay as shown in the figure for classifying mixed soils. The first step in the classification of soil is to determine the percentage of sand, silt, and clay-size materials in a given sample by mechanical analysis.

What is Soil Texture?

Texture means the visual appearance of the surface of materials such as fabric or cloth. The visual appearance of soil is called its texture.  The texture depends on the particle’s size, the shape of the particle, and the gradation of the particle. The textural classification incorporates only particle size as it is difficult to incorporate the other two.

Textural Classification of soil
Fig.1. Textural Classification of soil

Soil texture triangle

A point is located on a triangular chart as shown in the figure. The designation given on the chart for the area in which the point falls is used as the classification of the sample. This method of classification does not reveal any properties of the soil other than grain-size distribution. 

Interesting for You: Field Identification of Soil | Gravel V/S Sand, Silt & Clay

The triangular classification system suggested by the US Bureau of Public Roads is called a textural classification system. According to the textural classification system, the percentage of sand (size 0.05-2) mm, silt (0.005-0.05) mm, and clay (size less than 0.005 mm) are plotted along the three sides of an equilateral triangle.

Soil texture Triangle
Fig.2. Soil texture Triangle

The table below shows Soil fractions as per the U.S. Department of Agriculture:

Soil fractionDiameter in mm  
Gravel  >2  
Sand  2-0.05  
Silt  0.05-0.002  
Clay  < 0.002  

The U.S. Department of Agriculture developed 12 (twelve) sections for soil texture classification that’s are given below: 

1. sand

2. loamy sand

3. sandy loam

4. sandy clay loam,

 5. Loam,

6. silt loam,

7. silt,

8. silty clay loam,

9. clay,

10. clay loam,

11. sandy clay and

12. silty clay.

The equilateral triangle is divided into 12 zones each zone indicates a type of soil. A key is given that indicates the direction in which the lines are to be drawn to locate the point.

For example, if a soil contains 80% sand, 20% silt, and 50% clay, it is shown by point P in the figure.

A textural class that combines the proportions of these three particle size classes that are used to describe soil texture. Soil texture classifications include:

Loam: This is a well-balanced mixture of sand, silt, and clay often considered ideal for most crops and gardening. Also, it provides good water retention and drainage.

Sandy clay: Contains more sand than loam, so it drains faster but holds less water and nutrients.

Silty loam: Contains more silt than loam, which makes it more water-holding but less draining.

Clay loam: Contains more clay than loam, which makes it good for water and retaining nutrients but can cause problems when draining.

Different variations, such as silty clay loam, sandy clay, and sandy clay, describe soils with different proportions of silt, clay, and sand.

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