Soil is a mixture of mineral matter, organic material, air and water. The mineral portion of soil is made up of particles which vary in size from stones to powder. These particles are called soil separates. The United States Department of Agriculture recognizes three major groups of soil separates: sand (2.0-0.050 mm), silt (0.050-0.002 mm) and clay (<0.002 mm). The relative proportions of these different sized particles determine a soils texture (see figure).
Fine textured soils have a high percentage of clay and are very sticky when wet and hard when dry. The term heavy soil was coined a long time ago when people found clay soils very difficult (heavy) to plow. Coarse textured soils have a high percentage of sand and are loose and friable. Coarse textured soils are not sticky when wet and are easy to plow, hence the term light soil texture was coined.
Texture is an important characteristic of soil and affects water holding capacity, drainage properties, root development and more. Because texture has a great affect on water movement through a soil, it can also affect the rate at which pesticides and nutrients move through the soil. Soil particles, such as clay, can also bind up certain pesticides causing differences in pesticide activity on soils with different soil textures.
In the laboratory there are several different methods used for determining soil texture. The most common method for determining soil texture is the hydrometer method. In this method the soil particles are dispersed with a sodium metaphosphate (calgon) and then agitated. After dispersion, the amount of each particle group (sand, silt, clay) are determined by using a hydrometer (see picture). The hydrometer measures the amount of particles in suspension. The principal of Stokes law, which states that particles will fall out of suspension at different rates over time, based on particle size, is used to determine the amount of each particle size present in a soil. The amount of each particle fraction, sand, silt and clay, determines the soil texture.
There are other methods of determining soil texture. The pipet method is used when the quantity of a sample is too small to be analyzed by the hydrometer method. The pipet method also uses particle settling rate to determine the percentage of sand, silt and clay in each soil sample. There is also a European soil texture classification system that is different than the USDA classification system. In the European method, hydrometer measurements are take at different time intervals from the USDA method. The classification of soil texture categories and the texture names for the European classification system are also different that the USDA method.
AGVISE Laboratories is proficient in all methods of soil texture determinations. If you don’t know specifically which method should be used on your soil characterization samples, our staff of professional Soil Scientists can help you.