Soil Formation

     ¹Hans Jenny, a pioneer of modern soil studies, asserts that any individual soil or soil property results from the interaction of four interdependent soil-forming factors: parent material, climate, biotic factors, and topography.
     ²Parent material is the uncombined mass from which soils form. ³It results from parent rock or from transported material. ⁴Rocks are residual or in-place parent material. ⁵They may be igneous, sedimentary, or metamorphic. ⁶The makeup of all these rocks largely determines the chemical makeup of the soil. ⁷Other parent materials are transported by wind, water, glaciers, and gravity. ⁸Because of the diversity of materials, transported soils are commonly more fertile than soils derived from in-place parent materials.
     ⁹Climate influences the development of soil. ¹⁰Temperature and rainfall, which vary with elevation and latitude, govern the rate of weathering of rocks, decomposition of minerals and organic matter, and leaching and movement of weathered material. ¹¹Further, climate influences the plant and animal life in a region, both of which are important in soil development.
     ¹²Biotic factors—plants, animals, bacteria, and fungi—all contribute to the formation of soil. ¹³Vegetation is largely responsible for the organic matter in the soil and the color of the surface layer. ¹⁴It reduces erosion, and it influences the nutrient content of the soil. ¹⁵Animals, bacteria, and fungi decompose organic matter, mix it with mineral matter, and aid in exposing soil material to air and dripping water through it.
     ¹⁶Topography, the contour of the land, influences the amount of water that enters the soil. ¹⁷More water runs off and less enters the soil on steep slopes than on level land. ¹⁸Therefore, soil is less developed on steep slopes, and layers are often indistinct and shallow. ¹⁹On low and flat land, extra water enters the soil, and the subsoil may be wet and grayish in color. ²⁰Topography also influences the rate of erosion and downhill transport of soil material.

—Adapted from Smith and Smith, Elements of Ecology, 4th ed., p. 110

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