Physical Properties of Soil
Soil Texture
Soil texture is perhaps its most fundamental inherent characteristic, as it affects many of the important physical, biological, and chemical processes in a soil. Soil texture is the proportion of three mineral particles—sand, silt and clay. These particles are distinguished by size and shape, and make up the fine mineral fraction. Clays have a particle size diameter of < 0.002 mm, silts between 0.002 and 0.05 mm, and sands 0.05 to 2.0 mm. Particles greater than 2 mm diameter are known as pebbles (2 mm–7.5cm), cobbles (7.5–25cm), stones (25–60cm), and boulders (> 60cm).
Soil Textural Classes
Soils are rarely composed of a single-size class of mineral particles (sand, silt and clay); they are mixtures of different size classes. Soil textural class names convey a more precise idea of the size distribution of particles in a soil, and indicate the general physical properties of that soil. The range in percentages of sand, silt, and clay for the 12 major textural classes are shown in the triangle in Figure 4.3. Soil textures are classified based on the percentage composition of sand, silt, and clay in a soil. Particle size distribution can be determined by laboratory analysis, with the results shown in percentages. Soil textural class names can then be determined from this triangle if the percentages of any two size fractions are known. Soil classifications are typically named for the primary constituent particle size or a combination of the most abundant particles sizes, e.g., “sandy clay” or “silty clay.”
Groupings of Soil Texture Classes
The need for fine distinctions in the texture of the soil layers results in a large number of classes and subclasses of soil texture. It commonly is convenient to speak generally of broad groups or classes of texture. Table 4.1 provides an outline of three general soil texture groups and five subgroups.
Nature of Soil Separates
The percentages of sand, silt, and clay determine the texture. As mentioned, the three groups are divided by their particle size. Sand and gravel particles can be seen with the naked eye, although some of the smallest sands may better be seen with a hand lens. Silts are microscopic. This puts them in the same general size range as most microbes, such as bacteria and fungi. Clay particles are too small to be visible under a standard microscope, and can only be seen with the aid of an electron microscope. Humus particles, which are derived from organic matter, are in the same size range as clay.
Sand
Sand particles are those smaller than 2 mm but larger than 0.05 mm. They may be rounded or angular, depending on the extent to which they have been worn down by abrasive processes during soil formation. The coarsest sand particles may be rock fragments containing several minerals, but most sand grains consist of a single mineral, usually quartz or other primary silicate minerals. Sand feels gritty between the fingers and the particles are generally visible to the naked eye. As sand particles are relatively large, so too are the voids between the particles which promote water infiltration (rate at which water enters the soil) and aeration.
Silt
The silt particles are intermediate between sands and clays. Particles smaller than 0.05 mm but larger than 0.002 mm in diameter are classified as silt. Individual silt particles are not visible to the unaided eye, nor do they feel gritty when rubbed between the fingers. Silt is a non-plastic or low plasticity (malleability) material due to its fineness. What little plasticity, cohesion (stickiness), and adsorptive capacity some silt fractions exhibit is largely due to a film of adhering clay.
Clay
Particles smaller than 0.002 mm are classified as clay and have a very large specific surface area, giving them a tremendous capacity to adsorb water and other substances. Unlike most sand and silt particles, clay particles tend to be shaped like tiny flakes or flat platelets. The large surface area of clay and its mineral composition make it the storehouse of plant nutrients. Clay size particles are so small that they behave as colloids. A colloid is a mixture is two or more substances mixed together but not chemically combined (they can be separated). Soil colloids are the most active constituent of the soil and it determine the physical and chemical properties of the soil. They are important because their surfaces attract soil nutrients dissolved in soil, water as positively charged mineral ions, or cations.
Soil Texture Affect on Soil Behavior
Soil texture has a profound effect on the behavior of soils. Soil texture is interrelated with the soil fertility and quality in the long term. The soil texture is associated with soil porosity, which in turn regulates the water holding capacity, gaseous diffusion, and water movement that determines the soil health.
Air-Water Relationships
Soil texture influences many physical properties, such as water-holding capacity and drainage (Table 4.2). Coarse-textured sandy soils generally have high infiltration rates but poor water holding capacity. Coarse-textured soils are also leached very easily. Silt particles are much smaller than sand, have a greater surface area, and water-holding capacity.
Soil Fertility
Soil texture influences the soil’s inherent fertility. Fine-textured soils are more fertile than coarse-textured soils. Fine-textured soils have higher contents of weatherable minerals. They contain more organic matter and exchangeable bases. Fine-texture
Determining Soil Texture
Soil texture can be determined in the laboratory. A sample of soil is completely dispersed in water and passed through a set of sieves to separate out the sand fractions. A sedimentation procedure is then used to ascertain the quantities of silt and clay that had passed through the sieves. Because soil particles are denser than water, they tend to sink, settling at a velocity that is proportional to their size. This method can be time consuming, requires certain equipment, and is not optimal for most people. In recent years laser diffraction is beginning to replace sedimentation as the preferred technique in determining particle size distribution in small samples of soil dispersed in water.
Click on the following topics for more information on physical properties of soil.
