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The Soil-Forming Factors

Five soil-forming factors are generally recognized: parent material, organisms, climate, topography, and time. It has been shown that Bt and Bhs horizon development is related to the clay and sand content within the parent material and/or the amount of clay that is formed during soil evo­lution.

Grass vegetation contributes to soils with thick A horizons because of the profuse growth of fine roots in the upper 30 to 40 centimeters of soil. In forests, organic matter is added to soils mainly by leaves and wood that fall onto the soil surface. Small-animal activities contribute to some mixing of organic matter into and within the soil. As a result, organic matter in forest soils tends to be incorporated into only a thin layer of soil, result­ing in thin A horizons.

The climate contributes to soil formation through its temperature and precipitation com­ponents. If parent materials are permanently fro­zen or dry, soils do not develop. Water is needed for plant growth, for weathering, leaching, and translocation of clay, and so on. A warm, humid climate promotes soil formation, whereas dry and or cold climates inhibit it.

The topography refers to the general nature of the land surface. On slopes, the loss of water by runoff and the removal of soil by erosion retard soil formation. Areas that receive runoff water may have greater plant growth and organic matter content, and more water may percolate through the soil.

The extent to which these factors operate is a function of the amount of time that has been available for their operation. Thus, soil may be de­fined as: unconsolidated material on the surface of the earth that has been subjected to and influenced by the genetic and environmental factors of par­ent material, ctimale, organisms, and topography, all acting over a period of time.

Soil Bodies as Parts of Landscapes

At any given location on the landscape, there is a particular soil with a unique set of properties, including kinds and nature of the horizons. Soil properties may remain fairly constant from that location in all directions for some distance. The area in which soil properties remain reasonably constant is a soil body. Eventually, a significant change will occur in one or more of the soil-forming factors and a different soil or soil body will be encountered. The dark-colored soil in the fore­ground receives runoff water from the adjacent slopes. The light-colored soil on the slopes devel­oped where water runoff and erosion occurred. Distinctly different management practices are re­quired to use effectively the poorly drained soil in the foreground and the eroded soil on the slope.

The boundary between the two different soils soils is easily seen. In many instances the bound­aries between soils require an inspection of the soil, which is done by digging a pit or using a soil auger.