Determining Soil Texture

What is Soil Texture?

Soil texture is a measure of the relative amounts of sand, silt, and clay particles (or separates) in the fine earth fraction. The amounts are conventionally given as percent (mass basis), so that sand, silt, and clay add up to 100%. The mineral (inorganic) fraction of soil is made up of grains of different sizes called soil separates (or particles). Depending on its size each particle will fall into the coarse fragments (gravels, cobbles, boulders or stones) or fine earth fraction (sand, silt, and clay) categories. Coarse fragments diameter is greater than 2 mm whereas fine earth fraction particles have a diameter smaller than 2 mm. For the purposes of determining soil texture, we are primarily interested in the fine earth fraction.

(1)   Sand particles (S): 2.0 - 0.05 mm

Sand particles are those with a diameter between 2 and 0.05 mm. Sand particles are the only particles of the fine earth fraction that may be large enough to be seen with the naked eye. Sand feels grainy when rubbed between the fingers. Because of their large size particles of sand have relatively low specific surface area (the surface area for a given volume or mass of particles). Most sand particles consist of a single mineral, usually quartz (SiO₂). Predominantly sandy soils have low total porosity (f), but large individual pores. Consequently, sand can hold little water and is prone to drought. In addition, sandy soils have low capacity to adsorb nutrients.

(2)   Silt particles (Si): 0.05 - 0.002 mm

Silt particles are smaller than 0.05 mm, but larger than 0.002 mm. Individual silt particles are not visible with the naked eye. Silt feels soapy or powdery (like flour) when rubbed between the fingers. Silt particles are essentially micro-sand particles with quartz as the predominant mineral. Soils having predominantly silt particles do not hold together well when wet, though they are more cohesive than sandy soils.

(3)   Clay particles (C): <0.002 mm

Clay particles are smaller than 0.002 mm and they behave as colloids (if suspended in water they do not readily settle out). Clay feels smooth when dry and sticky when wet. Because of their small size particles of clay have very high specific surface area. Clay has high total porosity (f), but small individual pores. Consequently, soils with a high number of clay particles have a very high water and nutrient holding capacity. Soil colloids and their different types are described in greater detail, here.

Silt and sand comprise essentially primary minerals that originate from the physical breakdown of rocks and their minerals. Clay is usually made of secondary minerals, which are formed in the soil by the chemical recombination of silicon, aluminum, and oxygen atoms to form new types of minerals. Some examples of primary and secondary minerals can be examined on this page.

Why is Soil Texture Important?

Soil texture is an important characteristic to be able to classify. Soil texture is commonly the first and most common soil property determined by agrologist, engineers, and foresters. For examples, a soil's proportional makeup of particles from different size fractions influences the ease at which water moves through the soil. As such, it has agricultural applications as different crops are better suited to certain soil environments. Knowing a soil's texture also helps with being able to foresee how it will respond to various environmental and management conditions such as flooding or calcium (lime) requirements. Particle size and distribution of soils also affect their ability to hold water and nutrients, and thus chemical and physical properties are closely related to texture. While sandy soils have large pores that allow lots of water movement and leaching, finer soils have a higher capacity for water and nutrient retention.

Classification

Soil texture triangles are commonly used to determine a soil's textural composition when a sample's percentage sand, silt and clay are known. The Canadian and US soil texture triangles used for classifying soils are very similar. They both have three sides with an axis for % clay, % sand, and % silt. One difference is that the "clay" segment within the US texture triangle is broken down into two segments in the Canadian one: "clay" and "heavy clay". Naming is based on the primary constituent particle size or a mixture of the most abundant particle sizes. Loam describes soil combinations that have equal properties of sand, silt and clay.

Canadian Texture Triangle

The Canadian's version of the soil texture triangle can be found on p. 12 within this document. The thirteen classifications are heavy clay, silty clay (SiC), clay, sandy clay, silty clay (SiCL), clay loam, sandy clay loam, silt loam, loam, sandy loam, loamy sand, sand, and silt.

American Texture Triangle

The American soil texture triangle is below. The twelve classifications are sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay.

How is Soil Texture Measured?

There are several methods available for determining particle size distribution and textural class, where the determination of soil texture in the laboratory is called particle size analysis or mechanical analysis. These following are just a few of the possible methods, and differ in accuracy, efficiency, equipment requirement, etc. Some methods are not suited for all particle sizes.

References

1. BC Ministry of Environments, Lands, and Parks and the BC Ministry of Forests. 1998. Field methods for describing terrestrial ecosystems. Land management handbook no. 25. Victoria, BC.
2. Food and Agriculture Organization of The United Nations. (n.d.). Soil Texture. Retrieved July 21, 2020, from http://www.fao.org/tempref/FI/CDrom/FAO_Training/FAO_Training/General/x6706e/x6706e06.htm
3. Soil texture. (2020, June 16). Retrieved July 21, 2020, from https://en.wikipedia.org/wiki/Soil_texture