Soil texture is the relative proportion of sand, silt, and clay in a soil. Sand is the 2.0 to 0.05 mm soil fraction, and according to the USDA system is subdivided into five classes (very coarse sand 2.0-1.0 mm, coarse sand 1.0-0.5 mm, medium sand 0.5-0.25 mm, fine sand 0.25-0.1 mm, and very fine sand 0.1-0.05 mm). Silt is the 0.05 to 0.002 mm soil fraction, and clay is the soil fraction which has diameter less than 0.002 mm. Texture changes slowly with time.

Soil containing high amounts of swelling clay favouring the formation of wide and deep cracks during the dry period (photo by C. Kosmas)

Soil texture profoundly affects soil drainage, water holding capacity, soil temperature, soil erosion as well as fertility and productivity. Wind erosion is a major problem when sandy soils are used for crop production in regions with dry seasons. Wind erosion is accelerated when the vegetative cover is removed. Clay holds more water available for plant growth than sandy soils and the presence of water considerably modifies the heat requirements of the soil. Temperature of wet clay soils responds more slowly than that of sandy soils to changes in air temperature in spring and autumn. Clay soils have poor drainage of excess water and may become waterlogged. Soil texture affects soil resistance to erosion. The coarser the soil texture, the smaller the active surface area of the soil particles, and the smaller is the resistance of the soil to erosion. Soil texture usually changes with soil depth but the texture of the soil surface horizon is considered as most important.

Soil texture has a major influence on the form, stability and resiliency of soil structure. As the clay content increases the characteristics of the soil matrix (including stability and form) are increasingly dominated by the characteristics of clay, the nature and quantities of cementing materials.

Soil crusting is affected by several factors including soil texture. Differential swelling breakdown increases as clay content increases, while slaking decreases as clay contents increase favouring crusting. Mineralogy of the soil clay plays an important role in crust formation. Soils containing high amount of silt, such as those formed on marl deposits, are sensitive to crust formation generating high surface water runoff and sediment loss.

Soils are classified according to their texture in classes, and each textural class have a given range of sand, silt and clay. For practical value in agriculture 12 classes were designated: Sand (S), loamy sand (LS), sandy loam (SL), loam, (L), silt loam (SiL), silt (Si), clay loam (CL), sandy clay loam (SCL), silty clay loam (SiCL), clay (C), silty clay (SiC), and sandy clay (SC). There are small but generally unimportant variations in soil properties within each class and significant differences between classes. The texture is expressed with the use of class name. Four broad groups of classes are recognized: Sands, Silts, Clays and Loams. Sands contain at least 80% sand particles and 15% or less clay particles by weight. Silts contain at least 80% silt and 12% clay particles, respectively. Clays contain at least 35% of clay particles. Loams are mixtures of sand, silt and clay particles that exhibit the properties of those particles in equal proportions. Loam soils have the best combination of physical and chemical properties in terms of cultivation and crop growth.

The USDA textural triangle showing the limits of sand, silt and clay contents of the various textural classes.

• L, SCL, SL, LS, CL
• SC, SiL, SiCL
• Si, C, SiC
• S

Zachar D., 1982. Soil erosion :Developments in soil science 10. Elsevier Scientific Publishing Company, Amsterdam, 547 pp.

Sumner M., Handbook of soil science. CRC Press, New York

Agricultural University of Athens, Laboratory of Soils and Agricultural Chemistry, Iera Odos 75, Athens 11855, Greece

Dr Constantinos Kosmas
email: lsos2kok@aua.gr