Soil Properties

Soils develop horizons or layers of distinct characteristics through soil forming processes. They are commonly referred to as the A, B, and C-horizons. The A-horizon is a layer that has been leached of soluble salts and clay but has accumulated organic matter through its high biological activity. The higher concentration of organic matter gives it a darker appearance than other horizons. The B-horizon occurs between the A and C. It commonly has an accumulation of clay compared to the A and C horizons. The A and B horizon are considered the solum or true soil. The C-horizon, also called parent material, shows little biological activity or soil development other than mineral decomposition of rock.

Climate and biological factors generally produce broad geographic patterns of soils (zonality). Parent material or geology from which soils form also affects the patterns of occurrence at regional and local levels. Topographic patterns (landscape) add further complexity, affecting both the time of exposure to processes of soil formation and the kinds of process (Soil Survey Staff, 1993).

The scale most useful for management of soils is a farmer's field or a building site. Using this scale, land managers have identified natural transitions between contrasting characteristics and developed classification systems to pigeonhole soils with similar properties. The USDA soil classification system, Soil Taxonomy, defines pedon as the

Smallest body of one kind of soil large enough to represent the nature and arrangement of horizons and variability in the other properties that are preserved in samples. A pedon extends down to the lower limit of a soil. It extends through all genetic horizons and, if the genetic horizons are thin, into the upper part of the underlying material. The pedon includes the rooting zone of most native plants. For purposes of most soil surveys, a practical lower limit of the pedon is bedrock or a depth of about 2 m, whichever is shallower. A depth of 2 m provides a good sample of major soil horizons, even in thick soil. It includes much of the volume of soil penetrated by plant roots, and it permits reliable observations of soil properties.

The surface of a pedon is roughly polygonal and ranges from 1 m² to 10 m² in area, depending on the nature of the variability in the soil. Where the cycle of variations is less than 2 m and all horizons are continuous and nearly uniform in thickness, the pedon has an area of approximately 1 m². Where horizons of other properties are intermittent or cyclic over an interval of 2 to 7 m, the pedon includes one-half of the cycle (1 to 3.5 m). If horizons are cyclic over an interval greater than 7 m, each cycle is considered to contain more than one soil. The range in size, 1 to 10 m², permits consistent classification by different observers where important horizons are cyclic or repeatedly interrupted over short distances (Soil Survey Staff, 1993, p. 18-9).

Soil maps delineate areas that have high percentages of the same soils as defined by the map's classification system and legend. Rarely do they delineate units that are 100% pure. Soils maps can provide useful information for planning if the scale and classification criteria are appropriate. However, if the map is used for other than its intended purpose, final decisions such as project design and good management will depend on site investigations of important soil properties and knowledge of how they relate to management.

There are many soil properties that are important to use and management. Some of them are listed below with a few of the more important ones described later.

• chemistry: cation exchange capacity, pH, availability of plant macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur) and micronutrients (iron, manganese, copper, zinc, boron, molybdenum, chlorine, and cobalt), sodium content, clay mineralogy, percent base saturation, percent organic carbon, etc.

• physics: texture, water holding capacity, saturated and unsaturated hydraulic conductivity, water infiltration rate, bulk density or apparent specific gravity, particle density or true specific gravity, porosity, moisture content, Proctor density curve, structure, shrink-swell potential, modulus of rupture, compressive strength, plasticity index, cohesion, matric and osmotic potentials of water, etc.

• biology: yield per hectare (tons of maize, wheat, cotton, etc.), soil seed bank, biodiversity (mycorrhizae, nitrogen fixation bacteria, etc.).

Texture

Texture, or the grain size composition of minerals that help make up a soil, is one of the most important properties of a soil because it is related to many other properties. Knowing the texture of a soil gives one an idea about its other properties. The USDA system separates texture into four major classes: coarse fragments (gravels, stones, and boulders; greater than 2 mm in diameter), sand (the stuff beaches and dunes are mostly made of; 2 mm to 0.05 mm in diameter), silt (one can't see the individual grains, the main component of dust and feels like flour; 0.05 mm to 0.002 mm in diameter), and clay (pure moist clay looks and feels like fresh axle grease and 30 to 40% clay in the soil will make it plastic like modeling clay; less than 0.002 mm in diameter).

The textural triangle is used to determine textural classes of soil for particles less than 2 mm in diameter (sands, silts, and clays). Each of its three axes ranges from 0 to 100% for each size fraction. To determine the textural class, estimate the percentages of the two easiest fractions sand, silt or clay and find where the intersection of the two occurs on the triangle. The following are descriptions of how some textural classes feel.

• Sand: Sand is loose and single-grained. The individual grains can readily be seen or felt. If squeezed when moist, it will form a cast (a coherent mass or body), but will crumble when touched.

• Sandy loam: Sandy loam is a soil containing much sand but which has enough silt and clay to make it somewhat cohesive. The individual sand grains can readily be seen and felt. Squeezed when dry it will form a cast which will readily fall apart, but if squeezed when moist a cast can be formed that will bear careful handling without breaking.

• Loam: Loam is a soil having a relatively even mixture of different grades of sand, silt, and clay. It has a somewhat gritty feel, yet is fairly smooth and slightly plastic (malleable like modeling clay). A squeezed moist cast can be handled quite freely without breaking.

• Silt loam: Silt loam is a soil having a moderate amount of fine grades of sand and only a small amount of clay. Over half the particles are of the size called "silt." When dry it may form clods that can be easily broken, and if pulverized it feels soft and floury. When wet the soil readily disperses and flows into puddles. Either dry or moist it will form casts that can be freely handled without breaking. When moistened and squeezed between thumb and finger it will not form a "ribbon" (a thin ribbon shaped mass of soil which easily forms if the soil is very cohesive) but will give a broken appearance.

• Clay loam: Clay loam is a fine textured soil which usually breaks into clods or lumps that are hard when dry. The moist soil is plastic and will form a cast that will bear much handling. When kneaded in the hand, it does not crumble readily but tends to form a heavy compact mass.

• Clay: Clay is a fine textured soil that usually forms very hard lumps or clods when dry and is quite plastic and sticky when wet. When the moist soil is pinched out between the thumb and fingers it will form a long, flexible ribbon (Soil Survey Staff, 1975, p. 471).

Cation Exchange Capacity

Cation exchange capacity (CEC) refers to the negatively charged sites on the soil minerals and organic matter which attract and hold positively charged ions, including plant nutrients (K⁺¹, Ca⁺², NH₄⁺¹, Mg⁺², etc.) for uptake by roots. CEC is measured in milliequivalents (meq) per 100 grams of soil. Good agricultural soils in the US generally have CECs that range from 15 to 50 meq/100 g. Sandy, aridic soils commonly have CECs around 1 meq/100 g or less because the sand and silt of these soils contribute very little to the exchange capacity. Clay and organic matter contribute the most. Organic matter contributes 150 to 300 meq/100 g and clay contributes 8 to 100 meq/100 g depending on the type of clay mineral (kalonite, montmorillinite, etc.).

Water Holding Capacity

Water holding capacity (WHC) of the soil is mainly influenced by texture. A very sandy soil (greater than 80% sand) can hold 8 to 12% water, depending on the size of the sand particles. Any additional water would percolate out of the root zone or runs off the surface. A soil low in sand (less than 20%) can hold 18 to 20% water. Non-sandy soils allow plants more time between rainfall events or irrigations before they become stressed from lack of water.

References

Brady, N.C. (1974). The Nature and Properties of Soils. New York: MacMillan. 639 pp.

Buol, S.W., F.D. Hole, and R.J. McCracken (1973). Soil Genesis and Classification. Ames, Iowa: The Iowa State University Press. 360 pp.

Soil Survey Staff (1975). Soil Taxonomy. USDA Agriculture Handbook No. 436. Washington, D.C.: U.S. Government Printing Office. 754 pp.

Soil Survey Staff (1993). Soil Survey Manual. USDA Handbook No. 18. Washington, D.C.: U.S. Government Printing Office. 437 pp.

Spangler, M.G. and R.L. Handy (1982). Soil Engineering. New York: Harper & Row. 819 pp.

URL:http://cals.arizona.edu/OALS/soils/composition.html
Last revised: Last revised: 27 August 2001
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