Physical Properties

M. B. Russell.

The physical properties of a soil largely determine the ways in which it can be used. On the size, shape, arrangement, and mineral composition of its particles and the volume and form of its pores depend other important physical properties.

The flow and storage of water, the movement of air, and the ability of the soil to supply nutrients to plants are examples of properties determined by the size and arrangement of the soil particles.

The proportions of the four major components of soils inorganic particles, organic material, water, and air vary greatly from place to place and with depth. The amount of water and air in a soil often fluctuates widely from season to season. The physical characteristics of the primary solid components of soil, however, are essentially unchanging.

Inorganic soil particles occupy about one-half of the total volume of most surface soils. Some of the particles can be seen very easily, but others can be examined only with an electron microscope. For many purposes it is convenient to divide the particles into size groups called separates.

Particles more than 2.0 millimeters in diameter are classed as gravel or stones and are not usually included in analyses of particle size. Particles under 2.0 mm. are divided into three major separates, each of which may be further subdivided.

Sand has particles between 0.05 and 2.0 mm. in diameter. The percentage of sand is determined by screening a thoroughly dispersed soil.

Silt has particles 0.002 to 0.05 mm-in size. Clay has particles less than 0.002 mm in diameter. The amount of silt and clay usually is determined indirectly from measurements of the speed of fall of the individual particles which have been well dispersed in water. The size of the individual particles is calculated from the settling speed.

THE PERCENTAGES of sand, silt, and clay determine the texture of the soil. The percentages in each of the several classes of texture are summarized in the illustration.

1. The texture triangle shows the percentage of sand, silt, and clay in each of the textural classes.

The physical properties and the chemical composition of the large and small particles differ greatly. The coarse separates the stones, gravel, and sand act as individual particles. They are composed mainly of rock fragments or such primary minerals as quartz.

Because of their size, these large particles have low specific surface (surface area per unit mass). Most of the important chemical and physical-chemical reactions in soils take place at the surface of the particles. The amount of such surface therefore strongly affects the ability of soils to react chemically.

The density of the larger soil particles usually is near 2.65 grams per cubic centimeter (gm./cc.). Soils containing large amounts of gravel or sand exhibit little or no plasticity and cannot retain large amounts of water or nutrients. They can easily transmit water and air, however, because large voids occur between their particles.

2. Specific surface is important in determining the reactivity of soils. The amount of surface varies inversely with the size of the soil particles.

The properties of silt particles are intermediate between those of sand and the clays, but mineralogically silts are more like sand because they are composed largely of primary minerals.

Silt particles have greater chemical activity because of their higher specific surface. Silts also exhibit more plasticity and cohesion than the coarser soil separates. The amount of chemical and physical-chemical activity in the silts, however, is not enough to give desirable physical behavior to soils that contain large amounts of such particles but little or no clay.

The clay fraction is the one that controls most of the important properties of a soil. In soils of the temperate regions, it is composed chiefly of secondary crystalline alumino-silicates, which are platy in form. Hydrated sesquioxides of iron and aluminum are the main components of the clay in the more completely weathered soils typical of many parts of the Tropics.

The clay minerals are secondary hydrated alumino-silicates, in which isomorphous substitutions may have occurred. These minerals are platelike in structure and show marked basal cleavage. Their crystalline structure is determined by the spatial arrangement of oxygen atoms, which by weight constitute roughly half of the mineral content, although on a volume basis the clays are roughly 90 percent oxygen.

The oxygen tetrahedron, which consists of four oxygen atoms in a close-packed arrangement around a silicon atom, is one of the basic structural units of the clay minerals. Such tetrahedra, by sharing oxygens, form sheets having the composition (Si₂O₅)n.

Silica sheets are characteristic of the micas and the clay minerals. In these minerals the sheets are bonded through common oxygen atoms with sheets of aluminum or magnesium octahedra.

Kaolinite, a major clay mineral in many mature soils, especially in the Southeastern States, consists of silica and alumina sheets in a 1: 1 ratio. Two other important types of clay minerals, montmorillonite and illite, are composed of silica and alumina sheets bonded together in a 2:1 ratio.

Ionic substitution of Al +++ for Si++++and Mg++ or Fe++ for Al +++ are common in minerals of the 2:1 type. Such substitution destroys the balanced-charge condition of the idealized structure and gives the clay mineral crystal a negative charge. Some negative charge also comes from unsatisfied bonds at the edges of clay mineral crystals and from the dissociation of H+ from the surface-exposed hydroxyl groups.

The negative charge of the clay minerals causes the clay particles to react with other charged particles, ions, and with dipolar molecules, such as water. The attraction between the negatively charged clay and such positive ions as H+, Ca++, Mg++, and K+ is one of the most important properties of a soil.

The attracted ions are held in a state of dynamic equilibrium with similar ions in the soil solution and can be replaced or "exchanged" from the soil particle in response to changes in concentration in the soil solution. This process of ionic exchange is a process of fundamental importance in soil management and plant nutrition.

3. Kaolinite crystals are composed of pairs of silica and alumina sheets held together by hydrogen bonds. The space between the crystal units is fixed and is largely inaccessible for surface reactions.

The charged clay surfaces together with their associated exchangeable ions also react with water molecules, which become oriented when they are present in the strong electric field near the charged surfaces. The resulting layers of oriented water molecules give the characteristic properties of plasticity, cohesion, and shrinkage to clays and soils that contain large amounts of clay.