21 Universities CoUnCil on Water resoUrCes JoUrnal of Contemporary Water researCh & edUCation issUe 154, pages 21-47, april 2015 An Introduction to Soil Concepts and the Role of Soils in Watershed Management *Jon E. Schoonover1 and Jackie F. Crim1 1Department of Forestry, Southern Illinois University, Carbondale, IL, USA *Corresponding Author Abstract: Soil is a non-renewable dynamic natural resource that is essential to life. Water movement, water quality, land use, and vegetation productivity all have relationships with soil. This article introduces many important soil concepts including development, classifcation, properties (physical, chemical, and biological), quality, and conservation. A general understanding of soil concepts and these interwoven relationships is essential to making sound land management decisions. Keywords: soil conservation, soil developoment, soil disturbance, soil management, soil properties, soil quality, soil taxonomy oil plays a vital role in sustaining life on the since soil nutrients and soil physical properties planet. Nearly all of the food that humans can directly impact yields. In urbanized areas, Sconsume, except for what is harvested from soil plays a vital role in reducing runoff through marine environments, is grown in the Earth’s soils. infltration and nutrient attenuation. The value of Other obvious functions that soils provide humans soil is easily overlooked until soil quality becomes include fber for paper and clothing, fuelwood degraded and the critical services the soil once production, and foundations for roads and provided are diminished. buildings. Less obvious functions that soils serve are providing a medium to attenuate pollutants Soil Overview and excess water, groundwater recharge, nutrient cycling, and habitat for microorganisms and biota. Soil Defned Soils also have many secondary uses such as The defnition of soil is relative to the function ingredients in confectionaries, insecticides, inks, it provides to the person(s) defning it. From a paints, makeup, and medicines; uses of clays morphological stance, the Natural Resource range from drilling muds, pottery, and artwork, Conservation Service (NRCS) defnes soil as: to providing glossy fnishes on various paper “a natural body comprised of solids (minerals products. and organic matter), liquid, and gases that Soil is a critical component of nearly every occurs on the land surface, occupies space, and ecosystem, but is often taken for granted. Soil can is characterized by one or both of the following: be thought of as the ecosystem foundation, as soil horizons, or layers, that are distinguishable from productivity determines what an ecosystem will the initial material as a result of additions, losses, look like in terms of the plant and animal life it transfers, and transformations of energy and matter can support. For example, in forest ecosystems, or the ability to support rooted plants in a natural soils can determine species composition, timber environment” (Soil Survey Staff 2014a). The productivity, and wildlife habitat, richness, and Soil Science Society of America (SSSA) defnes diversity. The role soil plays in forests is also soil in terms of its genetic and environmental critical to maintaining water quality and long- factors: Soil is “[T]he unconsolidated mineral term site productivity. In cultivated felds, soil or organic matter on the surface of the Earth that quality plays a signifcant role in crop productivity has been subjected to and shows effects of genetic Journal of Contemporary Water researCh & eduCation UCOWR 22 Schoonover and Crim and environmental factors of: climate (including Soils under intense cultivation will incorporate water and temperature effects), and macro- and materials that would normally be considered part microorganisms, conditioned by relief, acting on of the O horizon. These organic materials also parent material over a period of time. A product- contribute to the A horizon leading to a higher soil differs from the material from which it is organic content than other horizons. derived in many physical, chemical, biological, The E horizon (eluvial layer) is a common and morphological properties and characteristics” mineral horizon in forest soils that is distinguished (SSSA 2008). by its lack of clay, iron (Fe), or aluminum (Al). The loss of the above materials is known as The Soil Profle eluviation, which entails that these substances The vertical section of soil that shows the and dark minerals have been stripped from the soil presence of distinct horizontal layers is known as particles. Clay, Fe, and/or Al are removed from the the soil profle (SSSA 2008). The term horizon E horizon via leaching, which causes its light color refers to the individual or distinct layers within the compared to the adjacent horizons. Leaching is soil profle. Most soils are composed of several the loss of nutrients from the root zone due to the horizons (Figure 1). Typically, horizons of a soil movement of water through the soil profle. The E profle will follow the topography of a landscape. horizon is comprised of concentrations of quartz, Designation of horizon boundaries also comes silica, or other minerals that are less susceptible from measurements of soil color, texture, structure, to leaching. consistence, root distribution, effervescence, rock The B horizon, known as the “zone of fragments, and reactivity. accumulation”, occurs below the O, A, and/or The uppermost layer, the O horizon, consists E horizons, if present. The B horizon receives primarily of organic material. Forested areas deposits of illuviated materials such as clay usually have a distinct O horizon. However, in particles, Fe and Al oxides, humus (organic matter some settings such as a grassland or cultivated formed from the decay of plant and animal matter), feld, there may be no O horizon present. Factors carbonates, gypsum, and silicates leached from the such as erosion or constant tillage contribute to overlying horizons. The common presence of Fe the lack of organic matter. The O horizon has and Al oxide coatings often give the B horizon a three major sub-classifcations, or subordinate redder or darker color than the adjacent horizons. distinctions (designated by the lowercase letter): The C horizon is the soil layer that generally hemic (Oe), fbric (Oi), and sapric (Oa). The hemic sees little infuence from pedogenic weathering layer consists of decaying material that is slightly processes and is therefore comprised of partially decomposed, yet the origin is still identifable. The weathered parent material. The C horizon fbric layer is composed of organic material that is represents a transition between soil and bedrock. slightly more decomposed and unidentifable, but As the upper portion of the C horizon undergoes is not decayed entirely. The sapric layer consists weathering, it may eventually become part of the of fully decomposed material whose origin is overlaying horizons. There is an obvious shift in completely unidentifable. soil structure between strongly developed B and The A horizon is a mineral horizon that is C horizons that aids in identifying the horizon formed at or just below the soil surface. It is boundary in the feld; however, the structure shift commonly referred to as the “surface soil.” Some may be more subtle in weakly developed soils. characteristics of an A horizon may include Under the C horizon comes the R horizon, or the accumulation of organic matter and/or the bedrock. Depending on the geographic location, presence of a plow pan. A plow plan (or plow environmental conditions, and landscape position, layer) is a common characteristic of soils that bedrock may be found in excess of 100 feet deep have undergone conventional tillage at some point or merely centimeters from the soil surface. in recent time. The darkness of the A horizon Bedrock is a consolidated layer of rock material can sometimes be attributed to the movement that gave way to the soil properties found on the of organic matter from the overlying O horizon. site. Bedrock is occasionally disrupted or broken UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 23 up by tree roots, but roots generally cannot cause Soil Formation enough stress on the rock to fracture it, so much of The 5 soil-forming factors that infuence the the deeper bedrock weathering is biochemical in development of soil were frst termed by Hans nature. The layer of freshly weathered material, Jenny, an American soil pedologist in the early- to in contrast to the solid rock (i.e., bedrock), is mid-1900s. The factors he determined as essential generally termed saprolite/saprock. in the formation of soil include: parent material, The division of the soil profle termed the climate, biota, topography, and time (Jenny regolith is defned as “the unconsolidated mantle 1994). Hans presented these factors as a formula: of weathered rock and soil material on the Earth’s s = f (PM, Cl, O, R, T). The formula roughly surface which extends from the soil surface to the translates to soil is a function of parent material, bottom of the parent material” (SSSA 2008). So climate, organisms, relief, and time. This section basically, the regolith is the heterogeneous material will briefy discuss the role of each of the soil- that lies on top of solid rock. The soil solum is the forming factors and how each factor aids in soil weathered soil material in the upper soil horizons development. (typically A, E, and B horizons) located above the parent material (C horizon). Not every soil Parent material. Parent material is the profle is comprised of the same horizons. Some unconsolidated and chemically-weathered mineral profles will contain O, A, E, B, C, and R horizons or organic matter from which the soil is developed. while another soil profle may only be composed It consists of any number of combinations such as of a C and R horizon. These differences in limestone, sandstone, or even volcanic rock. The horizonation are what make soils unique.