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What Soils are Made of and How They Develop

What Soils are Made of and How They Develop

By: Gerald Miller, Department of Agronomy Emeritus, Iowa State University  

Getting Into Soil & Water 2013   

Soil is defined as media that supports plant growth and plant development. A soil consists of solids and spaces between the solids (voids). The solids include inorganic and organic materials. Inorganic materials include minerals such as quartz, feldspars, and mica. Organic materials are decayed products that originated as plants or animals. Voids are occupied by gases and liquids, primarily air and water. All soils contain solids and voids in varying ratios. Changes in a soil are driven by physi- cal, chemical and biological processes which act on the organic and inorganic components resulting in different ratios of solids and voids over time. Not only does the ratio change, the size of the components also change. 

To be classified as soil material the size of the inorganic material must be less than 2 mm (~25 mm = 1inch). The largest particles are sand, which range in size from 2 to 0.05 mm, while silt-sized particles range from 0.05 to 0.002 mm. Particles smaller than 0.002 mm are clay. Inorganic material greater than 2 mm are either gravel, cobbles, or boulders as size increases. 

Soil development refers to the changes that occur over time in soil material extending from the soil surface to a given depth below the ground surface, generally 60 to 80 inches. The vertical section is a soil profile. The de- velopment process requires two ingredients. First, a raw parent material in which the soil forms. In Iowa, this parent material can be deposits from glaciers, water or wind or bedrock such as limestone, sandstone or shale. The second ingredient is differentiation of one or more layers, called horizons, within the soil profile. 

Jenny (1941) in his classic work identified five soil fac- tors that influence soil development. These are: 1) parent material, 2) climate, 3) living organisms, 4) topography and slope, and 5) time. The interaction of these factors results in soils with different physical, chemical and bio- logical properties as one traverses across the landscape. These different properties allow for the differentiation of soil horizons within the soil profile (Figure 1). The primary soil horizons found in Iowa soils are: A, topsoil a dark colored organic enriched horizon; B, the subsoil where products of weathering accumulate, and C, the parent material, a horizon containing loose, non weathered material deposited by glaciers, wind or streams. Other soil horizons that occur in some Iowa soils include: O horizon, representing an accumulation of organic debris above the A horizon; E, a surface or subsurface horizon formed in soils that experienced development under forest vegetation; and R horizons where the parent material is bedrock.

Other researchers have developed additional soil form- ing models built on Jenny’s model. One of these models, which is a widely accepted model for soil development, was proposed by Simonson (1959). He proposed four major processes that change parent material into a soil. He focused on horizon differentiation stating that the processes in soil formation involved: 1) additions, 2) losses or removals, 3) transfers or translocations, and 4)transformations. An example of additions is the accumulation of organic material. Other examples are dust deposits from wind and sed- iments deposited on the soil surface from flooding. 

Losses or removals involve the infiltration of water into the soil profile which dissolve carbonates and salts. Percolating water move these soluble materials downward in the soil profile. The decay of plant residues into organic matter results in the loss of carbon as a gas in the form of carbon dioxide. The movement of sand, silt, and clay particles across the soil surface as the re- sult of erosive wind and water is another example of losses and removals. 

Transfers include the downward movement of clay sized particles located in the topsoil horizon to the subsoil. This involves the infiltration of water in the topsoil and its downward movement carrying fine clay particles to the B horizon. 

Transformations occur when fine silt sized inorganic particles weather and form smaller clay sized particles. Another trans- formation defines the form of iron in the soil. The presence or absence of aerobic conditions in a soil will determine the form of iron oxides that occur, either ferric (oxidized) or ferrous (reduced). 

The overall process of soil development and horizon differentiation can be characterized as weathering. Weathering is alteration of the parent material and inorganic component of the soil by the interaction of physical, chemical and biological action. The weathering process determines how fast or slow a soil develops.

Figure 1. A generalized soil profile showing the six primary horizons (USDA NRCS, 2007, p. 5) 

A young or recently developed soil may consist of only an A and C horizon. Other soils may have an A, a weakly devel- oped B horizon with minimal clay accumulation characterized by color and structure differences. Most Iowa soils occupying stable landscapes have an A horizon and strongly developed B horizon characterized by clay accumulation, strong color dif- ferences in contrast to above and below horizons, and aggrega- tions of the solid material which form discernible structural features. Profiles developed under the influence of forest veg- etation contain observable E horizons as well as developed B horizons. 

A common question asked is how old are most Iowa soils. In order to answer that question one must remember that some physical, chemical and biological actions result in soil properties that formed very recently. Generally, soil scientists consider observable and measurable properties found in the A, E, and B horizons when considering the age of a soil profile. 

The age of the soil profile and the age of the parent ma- terial are not the same. For example, Iowa parent materials range from the most recent flood deposit of thick sediments to glacial deposits that occurred more than 600,000 years before present. Since soil profiles are a product of the soil forming factors that influenced their development, the age of Iowa soils range from less than 100 years in age to 3,000 to 5,000 years old. An horizon on a stable landscape can develop from raw parent material in less than 100 years (Hallberg, et al., 1977). Also, soil on a steep slope that has experienced acceler- ated surface erosion may have a very young A horizon and a recently modified E and/or B horizon. In Iowa, upland soils occupying stable landforms have proper- ties that reflect the influence of climate and vegetation that has been present during the past 3,000 to 5,000 years (Ruhe, 1969). 

References 

Jenny, Hans. 1941. Factors of Soil Formation – A Sys- tem of Quantitative Pedology. McGraw-Hill, New York. 

Hallberg, George R., Nyle C. Wollenhaupt, and Ger- ald A. Miller. 1978. A Century of Soil Develop- ment in Spoil Derived from Loess in Iowa. Soil Science Society of America Journal 42: 339-343. 

Ruhe, R. V. 1969. Quaternary Landscapes in Iowa. Iowa State University Press, Ames. 255 p. 

Simonson, Roy W. 1959. Outline of a Generalized Theory of Soil Genesis. Soil Science of America Proceedings 23: 152-156. 

USDA Natural Resources Conservation Service. 2007. From the Ground Down: An Introduction to Iowa Soil Surveys. 17 p. ftp://ftp-fc.sc.egov. usda.gov/IA/news/GroundDown.pdf 

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