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
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.
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.
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.
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.Text by Joe Tabor