Soil, Physical Characteristics of

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Soil, Physical Characteristics of

Soil physical properties are those related to the size and arrangement of solid particles, and how the movement of liquids and gases through soils is affected by the particles. Soil mineral particles are derived from the weathering of rocks and minerals. Soil organic matter is the product of microbial decomposition of the remains of plants and animals.

Soil Texture

Soil texture refers to a particular soil's distribution of mineral particles within certain size ranges. (Organic matter is removed before soil texture is determined.) Soil texture is an intrinsic property of a soil, which may be influenced by geologic processes such as erosion, but generally does not change appreciably within a human life span or as a result of human activities. Different groups use different classification schemes for particle sizes. A commonly used scheme is that of the U.S. Department of Agriculture (USDA). Although soil particles are rarely spherical, the USDA classification system is based on particle diameters.

Using the USDA classification system, gravel is between 2 and 75 millimeters, cobbles 75 to 254 millimeters, and stones greater than 254 millimeters. Soil is considered to consist of particles less than 2.0 millimeters in diameter. Sand-sized particles have diameters between 0.05 and 2 millimeters. The smallest sand particles are nearly invisible to the eye. Silt-sized particles range between 0.002 and 0.05 millimeter. Root hairs, nematodes, and fungi are also in this size range. Clay-sized particles are less than 0.002 millimeter in diameter. They are in the size range of bacteria and viruses.

In any soil analysis, the total amount of sand, silt, and clay in a soil always adds up to 100 percent. There are twelve soil textural classes defined by the percentages of these size groups. Along with an analysis of the percentages of sand, silt, and clay in a soil sample, a diagram called a soil textural triangle is used to determine a soil's textural class. For example, soils containing equal amounts of sand, silt, and clay are classified as clay loams. Although the term loam refers to a soil with a particular textural composition, loam is commonly used by nonsoil scientists to mean a fertile soil with a texture neither too sandy nor too clayey. Sandy soils are also called coarse-textured, clayey soils are referred to as fine-textured, and soils with a balance of sand, silt, and clay may be called medium-textured.

Soil Structure and Porosity

The arrangement of primary particles, particularly clays, into clumps or aggregates is referred to as soil structure. Soil organic matter is generally involved in binding particles into stable aggregates. The amount and arrangement of aggregates determines the total porosity of a soil. Total porosity of a soil can be determined from the soil's bulk density, the weight of a fixed volume of dried soil. The individual mineral particles in soil have an average density of about 2.7 g/cm³, and the organic matter has a much lower density in the range of 1.2 to 1.5 g/cm³. A volume of dried soil contains mineral particles, organic matter, and pore space; although, of course, only the mineral and organic fraction contribute to the weight. The higher the bulk density, the lower the total pore space available for air and water within a soil. Soil bulk density ranges from about 0.1 to 0.7 g/cm³ for highly organic soils and 0.9 to 1.8 for mineral soils. Sandy soils have higher bulk density values than those with more clay. Bulk density values higher than about 1.4 g/cm³ indicate possible limitations to root growth and penetration; typical bulk densities for cultivated soils are 1.0 to 1.25 g/cm³.

Porosity influences both gas diffusion and water movement in soil. As a generalization, a medium-textured soil with good aggregation contains about 50 percent pore space and 50 percent solid particles by volume. Porosity values can range from 25 percent in compacted soils to 60 percent in highly organic, well-aggregated soil. Macropores, also called aeration pores, are the larger pores between soil aggregates that allow relatively rapid water movement through a soil profile. More macropores in a soil means faster infiltration of water into the profile. Micropores are pores within aggregates; although they may represent a significant fraction of a soil's total porosity, water does not move rapidly through these small-diameter pores.

Porosity greatly influences water relations in soils. Soils with high clay content usually have a greater total porosity than sandy soils. However, a high percentage of the total pores are micropores that do not permit rapid water movement. Therefore water infiltrates slowly into, and out of, these high-clay soils. During a rain event, water may run off the surface of these soils more rapidly than it moves downward into them. Once they become wet, they dry out slowly. Sandy soils, with a higher percentage of macro-pores, have a high water infiltration rate. Water moves rapidly through the profile, and the soils generally dry out rapidly after rain.

Soil Tilth

Soil tilth, used more commonly by the general public than by soil scientists, is a general term for the physical condition of a soil. Soil tilth is influenced both by soil texture and soil structure. A soil with good tilth offers little resistance to penetration by plant roots during their growth. It also provides ample oxygen and water for plants. The presence of both macropores and micropores is important for good tilth. Macropores permit infiltration and drainage of water; micropores store water for future plant needs. Both the pore distribution and the amount of rainfall received influence whether the pores in any soil contain water or air. Capillarity refers to the ability of small pores to retain water against the force of gravity and results from the adhesive forces between water molecules and the particles in soil and from the cohesive forces between water molecules. Because small pores tend to be water-filled due to capillarity, fine-textured soils with little structure and large amounts of micropores may have inadequate oxygen for plant growth. Oxygen diffuses rapidly through air-filled pore space and slowly through water-filled pore space. Mechanized agricultural practices tend to destroy soil structure and compact soils, resulting in poor tilth. Compaction results from the pressure exerted on soils by heavy equipment moving over them. Agricultural practices also tend to destroy organic matter in soils that is needed to maintain structure.

see also Agriculture, Organic; Compost; Decomposers; Plant Community Processes; Roots; Soil, Chemistry of.

M. Susan Erich