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PHYSICAL PROPERTIES OF SOIL

The physical properties of a soil are those characteristics which can be seen with the eye or felt between the thumb and fingers. They are the result of soil parent materials being acted upon by climatic factors (such as rainfall and temperature), and affected by topography (slope and direction, or aspect) and life forms (kind and amount, such as forest, grass, or soil animals) over a period of time. A change in any one of these influences usually results in a difference in the type of soil formed. Important physical properties of a soil are color, texture, structure, drainage, depth, and surface features (stoniness, slope, and erosion).

The physical properties and chemical composition largely determine the suitability of a soil for its planned use and the management requirements to keep it most productive. To a limited extent, the fertility of a soil determines its possible uses, and to a larger extent, its yields. However, fertility level alone is not indicative of its productive capacity, since soil physical properties usually control the suitability of the soil as growth medium. Fertility is more easily changed than soil physical properties.

Color
When soil is examined, color is one of the first things noticed. It indicates extremely important soil conditions. In general, color is determined by: (1) organic matter content, (2) drainage conditions, and (3) degree of oxidation (extent of weathering).

Surface soil colors vary from almost white, through shades of brown and gray, to black. Light colors indicate a low organic matter content and dark colors can indicate a high content. Light or pale colors in the surface soil are frequently associated with relatively coarse texture, highly leached conditions, and high annual temperatures. Dark colors may result from high water table conditions (poor drainage), low annual temperatures, or other conditions that induce high organic matter content and, at the same time, slow the oxidation of organic materials. However, soil coloration may be due to the colors imparted by the parent material. Shades of red or yellow, particularly where associated with relatively fine textures, usually indicate that subsoil material has been incorporated in the surface layer.

Subsoil colors, in general, are indications of air, water, and soil relationships and the degree of oxidation of certain minerals in the soil. Red and brown subsoil colors indicate relatively free movement of air and water allowed by the soil. If these or other bright colors persist throughout the subsoil, aeration is favorable. Some subsoils that are mottled (have mixed colors), especially in shades of red and brown, are also well-aerated.

Yellow-colored subsoils usually indicate some drainage impediment. Most mottled subsoils, especially those where gray predominates, have too much water and too little air (oxygen) much of the time. The red-to-brown color of subsoils comes from iron coatings under well-aerated conditions. In wet soils with low oxygen levels, the iron coatings are chemically and biologically removed, and the gray color of background soil minerals shows.

Texture
Texture refers to the relative amounts of differently sized soil particles, or the fineness/coarseness of the mineral particles in the soil. Soil texture depends on the relative amounts of sand, silt, and clay. In each texture class, there is a range in the amount of sand, silt, and clay that class contains.

The coarser mineral particles of the soil are called sand. These particles vary in size. Most sand particles can be seen without a magnifying glass. Sand particles feel gritty when rubbed between the thumb and fingers. Relatively fine soil particles that feel smooth and floury are called silt. When wet, silt feels smooth but is not slick or sticky. When dry, it is smooth, and if pressed between the thumb and finger, will retain the imprint. Silt particles are so fine that they cannot usually be seen by the unaided eye and are best seen with a microscope. Clays are the finest soil particles. Clay particles can be seen only with the aid of a very powerful (electron) microscope. They feel extremely smooth when dry, and become slick and sticky when wet. Clay will hold the form into which it is molded. Soils high in clay content often show pronounced surface cracking when dried.

Soil textural classes take their names from the particle size categories (sand, silt, and clay) and also from the category called loam. Loam is a textural class of soil that has moderate amounts of sand, silt, and clay. Loam contains approximately 7% to 27% clay, 28% to 50 % silt, and 23% to 53% sand. It is smooth to the touch when dry, but when moist, it becomes somewhat slick/sticky.

Most surface soils fall into five general textural classes. Each class name indicates the size of the mineral particles that are dominant in the soil. Intermediate texture soils are called loams. Texture is determined in the field by rubbing moist-to-wet soil between the thumb and fingers. These observations can be checked in the laboratory by mechanical analysis which separates particles into clay, silt, and various-sized sand groups.

Estimating Soil Texture
Soils can be classified by their texture or particle size distribution. Practically speaking, a soil particle must pass through a 2mm sieve (No. 10, US) to be called a soil particle.

Soil particles are either organic or inorganic. Texture is used only to describe the distribution of the inorganic fraction. In the laboratory, the organic matter is either removed prior to mechanical analysis. In the field, organic matter is disregarded when the texture is determined.

An estimation of texture can be made in the field by using the following method:

1. Place about a tablespoon of soil in the palm of your hand.
   
2. Mix it with water and form a moist ball. The soil is at the correct consistency when the ball does not leave soil on the palm of your hands when it is rolled around (the consistency of modeling clay.)
3. Press the moistened soil ball between the thumb and forefinger in an attempt to form a ribbon with the soil. As the thumb and forefinger are pressed together the soil will extrude forming the ribbon. The motion should be repeated several times to test the cohesiveness of the ribbon, attempting to form a continuous ribbon.
4. Ribbons can be classed into three broad categories:
   • Good Ribbon —The ribbon does not break and has very little cracking along the sides.
   • Medium Ribbon —The sides of the ribbon crack deeply and eventually the ribbon will break and fall off.
   • Poor Ribbon —No ribbon formed (no cohesiveness) or the ribbon breaks with the first applied pressure and does not cohere.
     
5. The sample is then further wetted and mashed between the thumb and forefinger. This is to determine the amount of grittyness or smoothness. The soil should be wet enough to feel individual particles. If the soil feels like sand, then it is called "gritty." If it feels like flour then it is called "smooth." There is a category between the two that is both smooth and gritty but no descriptive term is used.