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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. The area of the soil profle known as the “C Horizon” unique characteristics of soil allow soil scientists is comprised of parent material. Parent materials to classify soils into different categories via Soil attribute to both the chemical and physical Taxonomy (see Soil Classifcation section). properties of a soil and can affect soil drainage, porosity, and plant available water, among other things. Parent material in any location can vary greatly even in areas adjacent to one another. For instance, Illinois contains eight different soil regions that are a result of glaciations, wind- blown materials, and continuous weathering of 0” parent material. O: organic matter 2” A: mineral soil mixed with some organic Climate. In early soil profle development, matter; topsoil 10” parent material gives way to the major physical and chemical properties of a soil. As the soil

B: zone of illuviation; becomes more developed (through horizon accumulation of clay transported from formation and increased soil structure over time), above its characteristics depend more heavily on the climatic conditions to which it is exposed. Climate

30” plays a signifcant role in the formation of soil and any number of climate-related occurrences (e.g.,

C: partially precipitation and temperature) may infuence soil weathered parent development. material Biota. Soil profle features related to biotic (plant 48” R: bedrock; and animal) activity such as burrows, mounds, unweathered parent root channels, and worm castings contribute to material soil profle development because each of these Figure 1. Master horizons identifed in a typical soil processes change the porosity of the soil. The profle. Source: USDA NRCS; adapted. burrowing of animals, much like old root channels,

Journal of Contemporary Water researCh & eduCation UCOWR 24 Schoonover and Crim creates large pores for rapid movement of water, Topography. Topography (slope and aspect) gases, and solutes through the soil. The structure also has a strong infuence on soil characteristics. of some surface horizons is formed entirely by Slope can infuence erosion and deposition along animal activity (earthworms, ants, termites, and a hillslope (Figure 2). Steep soils are susceptible other organisms). Earthworms are capable of to accelerated erosion and generally have a consuming their own body weight in food daily shallower A horizon and overall less development. (Minnich 1977). They are also responsible for the Conversely, soil development in fat areas is ‘sinking’ of objects through the soil profle over heavily infuenced by soil drainage, where well- time (Darwin 1897). Charles Darwin devoted his drained soils tend to have greater development last book, “The Formation of Vegetable Mould compared to poorly drained soils. A fat, poorly- Through the Action of Worms” to the process of drained soil retains water, leaving the soil profle bioturbation, the process in which plants and saturated, which slows soil profle development. In animals facilitate the mixing, or rearrangement, of well-drained soils, E horizons can develop above the soil profle. a well-developed B horizon due to the eluviation Much like animals, plants can have a strong (transport of materials via water) of clay from the infuence on soil properties. For example, upper soil horizons. Additionally, the aspect (or whether a soil is formed under forest cover or horizontal direction) on which a soil is formed prairie vegetation can greatly infuence the carbon affects profle development. South-facing slopes inputs into the system and ultimately how the receive more intense solar radiation than north- soil is classifed (see Soil Classifcation section). facing slopes, which affects soil temperature Plant roots increase infltration, break up dense and moisture conditions. In the Appalachians, soil layers, and can pull nutrients and moisture south-facing slopes are dominated by coniferous from deep within the soil profle. Further, species while north-facing slopes are dominated windthrow (uprooting of trees by wind) can create by hardwood species due to differences in a pit and mound topography that helps physically microclimate. Furthermore, topographic position weather the soil by dislodging and breaking rock. (i.e., summit, shoulder, backslope, and toeslope) Windthrow can also expose deeper soils to surface infuences the development of soils. Generally, conditions that may lead to accelerated chemical summits are well-drained and have strong horizon weathering. development, whereas shoulder and backslopes

Figure 2. A comparison of hillslope soil profle development in poorly-drained and well-drained conditions. Surface erosion may occur in poorly-drained soils, exposing subsurface soils and transporting soil from upper horizons to the slope bottom. Source: http://www.britannica.com/ EBchecked/media/19383/Soil-profiles-on-hillslopes-The-thickness-and-composition-of-soil (Encyclopedia Britannica, Inc. 1999) .

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 25 are infuenced by erosion and have shallower distribution of soil orders across the United States topsoils and less infltration. Toeslopes typically is shown in Figure 3. The defning characteristics accumulate organic matter and topsoil from both of the broadest levels of classifcation are based alluvial (deposited by streams) and colluvial on soil-forming processes and parent materials, (primarily deposited by gravity) inputs, leading whereas the narrower levels become much to thicker A-horizons overlaying relatively young more specifc and consider the arrangement of soils. horizons, colors, textures, etc. The soil-forming factor “climate” has a predominate role in Soil Time. The duration of time a soil has undergone Taxonomy classifcation, followed by parent development is determined largely by the material, and biota; topography and time are not degree of weathering of the soil. Time has two utilized in defning taxa (Bockheim et al. 2014). separate meanings in terms of soil formation. Soil is infuenced by both chronological and Soil Orders physiological time. The ‘age’ of a soil commonly refers to the degree or amount of weathering the Entisols are the ‘youngest’, or most recently soil has undergone. This age is not referring formed soils of all the soil orders. Characteristics to chronological time, but the physiological of Entisols include weak profle development characteristics attributed to the soil via the where very little, if any, horizonation can be weathering process. documented. Entisols sometimes contain a weakly formed A or Ap (plow layer) horizon. These soils Soil Classifcation can be found on steep slopes with severe erosion, on foodplains that receive alluvial deposits, and Around the globe, many soil classifcations any number of scenarios in between. systems have been developed to categorize Another soil order with notably weak profle soils into groups based on morphological and/ characteristics is the Inceptisol order. When or chemical properties. The most widely-used Soil Taxonomy was frst established in 1975, classifcation system is the Soil Taxonomy Inceptisols were commonly referred to as the system that was made known by the United States ‘wastebasket soil order’. These soils generally did Department of Agriculture (USDA) (Soil Survey not ft into the other soil orders at the time. When Staff 1999). This system is a morphogenetic additional soil orders were introduced, many system that utilizes both quantitative factors and Inceptisols were reclassifed and the ‘wastebasket’ soil genesis themes and assumptions to guide title no longer applies. Inceptisols are somewhere soil groupings (Buol et al. 1997). “Keys to between the stages of no profle development Soil Taxonomy”, a free publication distributed and weak profle development. Inceptisols are by the USDA, is a great resource for in-depth commonly found along major rivers and streams classifcation of soils (Soil Survey Staff 2014a). due to the weak profle development. Over time, The USDA has also published a free resource both Entisols and Inceptisols have the potential titled “Illustrated Guide to Soil Taxonomy” that is to develop more horizons, at which time they similar to “Keys of Soil Taxonomy”, but written would likely be reclassifed into a different soil for a broader audience (Soil Survey Staff 2014b). order. The features of ‘young’ soils like Entisols Both resources can be found on the USDA website and Inceptisols are more heavily infuenced by listed in the Soil Resources section of this article. their parent material, whereas ‘old’ soils are more The Soil Taxonomy system is a hierarchical infuenced by climate and vegetation factors. scheme consisting of 6 classifcation levels. In Gelisols are also ‘young’ soils in regard to order from broadest to narrowest, the levels of geologic time and were developed under cold classifcation are: 1) Order, 2) Suborder, 3) Great temperatures or frozen conditions. These soils Group, 4) Subgroup, 5) Family, and 6) Series. are often associated with permafrost conditions Currently, there are 12 soil orders, 65 suborders, and cryoturbation (frost churning) in places like 344 great groups, ~ 18,000 subgroups, and over Canada and Alaska. Permafrost is a perennially 23,000 soil series (Bockheim et al. 2014). The frozen soil horizon (SSSA 2008).

Journal of Contemporary Water researCh & eduCation UCOWR 26 Schoonover and Crim

Mollisols are commonly referred to as the (Brady and Weil 2007). Thus, an illuvial layer of ‘prairie soil’. These soils were formed primarily humus and Fe/Al oxides form. Like the Alfsols, under grassy prairies and are characterized by their an E horizon is commonly found in Spodosol soil high organic matter content, dark color, and deep profles. In many cases, a Spodosol will have a A horizon. The A horizon must be greater than 8 white E horizon on top of a bright red B horizon. inches in depth and requires at least a 50 % base Spodosols are associated with loamy or sandy saturation (at least 50 % of the cation exchange soil conditions and can be found in Wisconsin, sites are occupied; see Soil Chemistry section for Michigan, the northeastern United States, and on more information). Mollisols are common in the the coastal plains of the eastern and southeastern midwestern United States where native prairies United States. once dominated the landscape. Aridisols are commonly associated with semi- Alfsols, formed under deciduous forests, are arid and arid regions. These regions have a low also very common in the midwestern United mean annual rainfall. The lack of moisture in the States. Alfsols are generally found in humid soil affects the soil development and weathering regions of the world and often contain an E process. Therefore, these soils are primarily horizon in the soil profle. These soils must have affected by physical weathering, not chemical a base saturation of at least 35 %. weathering (weathering processes discussed Spodosols generally originate from coarse- in the Soil Chemistry section). Aridisols are textured (i.e., increased sand content), acidic characterized by a high base saturation percentage parent materials. Spodosols are formed under of ~ 100 %. These soils can be found throughout forest vegetation, especially coniferous forests due the deserts or drier areas of the western United to the buildup of pine needles that inherently have States. high acidic resins. When pine litter decomposes, Ultisols can be found in humid and warm strongly acidic compounds are leached through the regions such as the southeastern United States. coarse materials, transporting Fe, Al, and humus Ultisols have a high amount of clay mineral

Figure 3. Distribution of soil orders across the United States.

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 27 weathering and translocation that leads to a with a climatic connotation of the soil. Great subsurface accumulation of clays (Brady and Weil groups account for the most signifcant properties 2007). They have a base saturation of < 35 % and of the soil as a whole, including the type and are naturally less fertile than Alfsols or Mollisols. arrangement of soil horizons, temperature However, Ultisols respond favorably to nutrient regimes, and moisture regimes. Scientists use management and are cultivated in many regions of subgroups to further classify soils by assessing the world. These soils are characterized by a high the degree of similarity between particular soils degree of weathering and are typically more acidic and grouping them accordingly. These are than Alfsols, but less acidic than Spodosols. intergrades that refect transitions to other orders, Oxisols are the most highly weathered soil order suborders, or great groups. The family grouping in the U.S. classifcation system. Oxisols get their has similar physical, chemical, and mineralogical name from being oxidized. They are dominated properties, which often relate to plant growth. by high clay content and Fe/Al hydrous oxides A soil series is the lowest level of taxonomy, or that typically give the soil a red hue. Oxisols the most specifc to the soil in question. The soil can be found in tropical and sub-tropical regions series classifcation narrows characteristics of of the world such as Hawaii, Puerto Rico, South similar soils down to a local level where not only America and Africa. Oxisols are generally formed physical, chemical, and mineralogical properties in wetter environments, but can be found in areas matter, but also management, land-use history, that are presently drier than during the time the vegetation, topography, and landscape position. soils were formed (Brady and Weil 2007). Most soil series are named based on the location Vertisols are soils that lack profle development where the series was frst discovered. An example due to the expansion and contraction of clay-rich of the taxonomic classifcation for the Illinois state soil. These processes cause the soil to mix, which soil is provided (Table 1). does not allow for clear soil profle development. Pedologists (scientists who study the origin During dry conditions, the soil shrinks and cracks and composition of soils as a component of form that can extend to ~ 80 cm deep and 2 to 3 cm natural systems) continue to learn more about wide. Vertisols are found in the southern United soil morphology, chemistry, mineralogy, and States in places like southeast Texas and eastern biota, thus tighter limits on groupings of soils Mississippi where smectite, montmorillonite, and are being defned. Soil scientists are constantly vermiculite clays are present. reevaluating soil taxonomy as technology and The Andisol order was developed in 1990. Previously, these soils were grouped with the Inceptisol order. Andisols are soils of recent Table 1. Example of the taxonomic classifcation according to U.S. Taxonomy for the state soil of origin (young) that are developed from volcanic Illinois, USA. materials. They are found in Hawaii and the northwestern United States in places like Taxonomic Level Example for IL State Soil Washington, Idaho, and Oregon. Histosols are the only organic soil order in the Order Mollisol classifcation system. Histosols are comprised of several different subhorizons within the O horizon Suborder aquoll and contain at least 20 % organic matter. Histosols occupy a small total area, but are found in various Great Group Endoaquoll places in the United States and Canada such as Wisconsin, Minnesota, the Florida Everglades, Subgroup Typic Endoaquoll and along the Gulf Coast. Fine-silty, mixed, superactive, Family Suborder, Great Group, Subgroup, Family, mesic Typic Endoaquoll and Series Series Drummer Suborders categorize properties associated

Journal of Contemporary Water researCh & eduCation UCOWR 28 Schoonover and Crim science progress. It is important to remember soil sample into a ball and squeezing it between that soils are dynamic, living environments. Soils the thumb and index fnger to make a ribbon. are ever-changing through both chemical and Texture is determined by ribbon length (if one can physical weathering processes and can vary across be formed at all) and grittiness or smoothness of the landscape, resulting in corresponding changes the sample. A step-by-step guide to the “Feel” in soil classifcation over time. Method is available on the NRCS website (see Soil Resources section). With practice, the “Feel” Soil Physical Properties Method provides an immediate and accurate way to determine textural class, and even percent sand, In this section, soil physical properties will silt, or clay, while in the feld. Laser Particle Size be introduced, examples for measurement will Analysis (LPSA) has become more commonly be provided, and the applicability in the feld used in labs across the country. Although this will be discussed. Soil physical properties have method is accurate, it requires training/skill to a profound effect on how soils infuence soil use the analytical instrument and the initial cost quality and productivity. Oftentimes soil quality is high. is driven by soil physical properties that determine nutrient and moisture levels in soils. The physical Table 2. Particle diameter (mm) of sand, silt, and clay properties of soil include soil texture, bulk density, based on the United States Department of Agriculture water holding capacity, organic matter content, System. soil structure, soil color, and soil consistence. A Texture Size Class Particle Diameter (mm) useful feld guide for describing soil properties is “Field book for describing and sampling soils” (Schoeneberger et al. 2012). Very coarse sand (vcos) 1.0 - 2.0

Soil Texture Coarse sand (cos) 0.5 - 1.0 Soil texture is determined by the amount of sand, silt, and clay in a soil sample. Table 2 Medium sand (s) 0.25 - 0.5 shows the size comparison of sand, silt, and clay particles based on the United States Department Fine sand (fs) 0.1 - 0.25 of Agriculture (USDA) System, one of the most commonly used among several particle size classifcation systems. Clays have a particle size Very fne sand (vfs) 0.05 - 0.1 diameter of < 0.002 mm, silts between 0.002 and 0.05 mm, and sands 0.05 to 2.0 mm. Percentages Silt (si) 0.002 - 0.05 of sand, silt, and clay categorize soil into different textural classes. Particle size percentages can Clay (c) < 0.002 be compared to the USDA soil textural triangle (Figure 4) to determine soil texture. For instance, a soil with 18 % clay, 60 % sand, and 22 % silt Bulk Density would be considered a sandy loam. Soil texture Bulk density is the mass of the soil in relation can be determined in the lab via particle-size to a known volume of soil and is often used as analysis using the hydrometer method; this method an indicator of soil compaction. Soil compaction determines particle-size distribution by measuring decreases the ability of a plant’s roots to penetrate the time it takes for soil particles in water to settle through the soil profle. Bulk density is related out of suspension. The hydrometer method can to soil textural class and soil porosity. Soils be time consuming, requires certain equipment, containing a high percentage of porosity will have and is not optimal for most people. However, a lower bulk density. Typically fne-textured soils anyone can determine textural class using the have lower bulk densities than coarse-textured “Feel” Method. This involves forming a moist soils because of increased pore space both

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 29

Figure 4. Soil textural triangle. Texture class is based on particle size percentages. Source: USDA NRCS soil textural triangle; adapted. between (interstructural pores) and within (matrix to texture, land cover and management also pores) soil aggregates. Ideal bulk densities for impact bulk density. Typically, bulk density plant growth range from < 1.10 g cm-3 for clays readings in a forest are much lower than readings to < 1.6 g cm-3 for sands (Table 3). Bulk density in an agricultural feld or an urban area. Forest is also affected by grain size and arrangement of soils tend to have a higher amount of porosity coarse-textured soils. If sandy soils are loosely- because of tree rooting, increased biotic activity, packed and of uniform size, porosity is higher increased organic matter on the soil surface, and therefore bulk density is lower than sandy and less anthropogenic disturbances. Bulk soils with tightly-packed aggregates of different density readings, soil texture data, infltration grain sizes. If a textural class has a bulk density capacity readings, and penetration resistance reading that exceeds the growth-limiting factor data can be compiled to explain soil porosity or for that class, then conditions for plant life are degree of compaction. Compaction affects both not optimal. Daddow and Warrington (1983) root penetration and the soil’s ability to allow determined growth-limiting bulk densities precipitation to infltrate into the soil profle. according to particle size. Generally, the greater Bulk density is commonly measured using a core the clay percentage is within a soil, the lower the method where a known volume of soil is collected growth-limiting bulk density of the soil because with a cylindrical soil core, then oven-dried, and of the increased pore space in clays. The growth- weighed. Results are reported as a mass per unit limiting bulk density is ~ 1.40 g cm-3 for soils volume (e.g., g cm-3). Bulk density can also be with > 80 % clay and ~ 1.70 g cm-3 for soils with determined via the clod method, where a clod (or < 20 % clay (Daddow and Warrington 1983). The ped) of soil is coated in paraffn wax and placed impact of bulk density on root growth according in water to determine the exact volume based to textural class is shown in Table 3. In addition on displacement. The clod is then oven-dried,

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Table 3. Relationship between bulk density and root growth based on soil texture. Source: Arshad et al. 1996. Textural class Ideal bulk density Bulk density that may Bulk density that (g cm-3) affect root growth restricts root growth (g cm-3) (g cm-3) Sands, loamy sands < 1.60 1.69 > 1.80

Sandy loams, loams < 1.40 1.63 > 1.80

Sandy clay loams, clay loams < 1.40 1.60 > 1.75

Silts, silt loams < 1.30 1.60 > 1.75

Silt loams, silty clay loams < 1.40 1.55 > 1.65 Sandy clays, silty clays, some < 1.10 1.49 > 1.58 clay loams (35 - 45 % clay) Clays (> 45 % clay) < 1.10 1.39 > 1.47 weighed, and reported on a mass/volume basis like the core method. 3-D scanners are also used to determine accurate clod volumes. Water Holding Capacity Water holding capacity is the amount of water held by soil. The water holding capacity of a soil is reliant on soil texture, structure, organic matter content, and arrangement of soil pores. Organic matter has a high degree of microporosity, which allows it to retain more water. Therefore, soils with a higher amount of organic matter and/or a large percentage of micropores (e.g., fne-textured soils like clays) generally have a higher water Figure 5. Water holding capacity stages within the soil. holding capacity. Soil compaction also impacts Source: Department of Agriculture Bulletin 462, 1960. water holding capacity since soil compaction weakens soil structure and collapses pores, thereby Soil Structure decreasing the soil’s ability to hold water. There are several different terms used to discuss water Soil structure is the description of how individual capacity in soils. Available water is the amount soil particles (sand, silt, and clay) are arranged into of water that is readily available for plant uptake; soil aggregates (also called peds) and refects both conversely, unavailable water is water that plants physical and chemical weathering. Several factors cannot utilize. Gravitational water (water that infuence soil structure, including soil texture, soil drains freely until feld capacity is reached) is a moisture, organic matter content, compaction, form of unavailable water. Field capacity refers to the activity of soil organisms, and management the water held by the soil following 24 to 48 hours practices. Soil structure is easiest to observe in of free drainage and is available for plant uptake. dry soil. When characterizing soil structure, the The permanent wilting point is the point at which shape, size, and grade of the structural units (peds) plants wilt and are unable to recover due to a lack are defned. The primary soil structure shapes are of plant available water. Lastly, saturation occurs granular, platy, blocky, prismatic, and columnar. when all pores are flled with water (Figure 5). Granular soil structure contains peds that are

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 31 generally small and round and are commonly found fngers) after pressure is released, while plasticity in horizons near the surface where high amounts describes the malleability of wet soil. Further of root activity are present and porosity is greater. reading on the topic, including the scale used to Platy soil structure consists of peds that are plate- describe consistence at different moisture contents, like (fat and thin) and usually oriented horizontally. can be found in chapter 3 of the Soil Survey Platy soil structure may occur throughout the soil Manual (Soil Survey Division Staff 1993). Soil profle, but is common in E horizons or compacted consistence is one of many soil physical properties soil. Blocky soil structure consists of sharp- used to determine site suitability for agriculture or edged peds arranged in square or angular blocks engineering purposes. and is typically found in the subsoil, especially in humid regions. Prismatic soil structure contains peds that are longer vertically than horizontally and have fat tops. Columnar soil structure is similar to prismatic, but peds have distinct rounded tops. Both prismatic and columnar soil structure are commonly found in the subsoil of arid and semi-arid regions. Soils can also be defned as “structureless” where soil structure is single- grained (typically sand particles that do not stick together) or massive (aggregation is not present). Aggregate size is typically described as very fne, fne, medium, coarse, or very coarse and the size limits of these classes depend on the aggregate shape. Grade describes the distinctness of the peds and is classifed as weak, moderate, or strong. More information on size and grade can be found in the Soil Survey Manual (Soil Survey Division Staff 1993). A soil structure description is written as “grade, size, shape”, such as “moderate medium subangular blocky structure.” Soil structure greatly impacts soil porosity, thereby infuencing soil water movement. General infltration rates (rapid to slow) associated with soil structure shapes are shown in Figure 6. Soil Consistence Soil consistence is a measure of the response of soil to applied pressure at various moisture contents. In other words, how well does the soil hold together when under applied stress? Soil consistence is measured separately for dry, moist, and wet soil because moisture content impacts how a soil responds to pressure. In the feld, soil consistence is measured by testing the ease with which the soil is crushed between the thumb and forefnger, or underfoot. Additionally, the terms stickiness and plasticity are often used to describe wet consistence. Stickiness refers to how Figure 6. Water infltration rates associated with typical well wet soil adheres to other objects (like your soil structure shapes. Source: USDA; adapted.

Journal of Contemporary Water researCh & eduCation UCOWR 32 Schoonover and Crim

Soil Color bare mineral soil is much more susceptible to Soil color is important in determining soil accelerated erosion processes. Since billons of classifcation. The Munsell color chart, a book tons of soil in the world are displaced every year, it containing standard color chips similar to paint is important for soil organic matter to remain intact chips found at a hardware store, was developed whenever possible. Additionally, organic matter as the standard system for determining soil color. acts as a binding agent for nutrients and potential Color is determined from three characteristics: hue, contaminants, and therefore aids in reducing inputs value, and chroma. Hue refers to the degree of of these contaminants to waterbodies. Depending redness or yellowness of soil. Value refers to the on environmental conditions, organic matter can lightness or darkness of soil. Chroma refers to the be stored in the soil for long periods of time. brightness or dullness of soil. A freshly exposed Warm, humid conditions promote breakdown face or ped is used to determine color. Using moist of organic matter by microorganisms, whereas soil is most common and dry soil may be misted cooler, drier climates limit decomposition and with water. In direct sunlight, the moist soil ped is soils act as a carbon reservoir. Tillage also impacts compared to the Munsell color chart to determine organic matter storage in agricultural soils. the hue, value, and chroma. Soil color varies with Tillage generally reduces organic matter content, topography, climatic factors, and soil depth, among as it improves conditions for decomposition other variables. Drainage characteristics of a soil by increasing pore space and moisture content can have a large impact on soil color; thus, soil and by exposing organic matter adsorbed to soil color can reveal insights into the local hydrologic aggregates; converting to no-till agriculture has regime. Soils that are well-drained tend to be been shown to increase organic matter content in brighter than poorly-drained soils. Poorly-drained soils (Schlesinger and Andrews 2000). soils create anaerobic conditions in which the Fe in the soil is reduced, resulting in very dull colors. Soil Water Movement Soils with extremely reduced conditions and a Soil water is the amount of water present in the chroma < 2 are referred to as “gleyed” soils. vadose zone, or the zone of unsaturated fow, of the soil profle. The term groundwater refers to Organic Matter the area of saturated fow in the soil. Water enters Organic matter content has a profound infuence the soil profle through the process of infltration, on both soil processes and soil quality. In the and then moves through the soil profle via feld, high organic content may be recognized by percolation. These processes depend on various a dark soil color in the surface horizons. In the soil properties that range from soil porosity to lab, organic matter content is quantifed by Loss the shape and arrangement of soil peds (Figure on Ignition, a process in which a soil sample is 7). Water moves through the available pore space exposed to high temperatures (360° C) and the more readily and at a faster rate in soils with amount of weight lost after exposure is assumed granular structure when compared to the longer to be organic carbon. Organic matter in soils fow path that platy soil structure provides (Figure promotes biotic growth since it serves as a food 7). The percentages of micropores, mesopores, source for earthworms and other organisms. and macropores in the soil horizons also infuence Organic matter has a high water infltration how quickly water enters into and moves through capacity, a high moisture holding capacity (it can the soil profle. Micropores, or capillary pores, hold between 80 and 90 % of its weight in water), are the smallest soil pores and contain most of the and contains many plant-essential nutrients. The plant-available water in soil. Water is held in these increased water holding capacity allows more water pores by the combined matric forces of adhesion to be available to plants over a longer timeframe. and cohesion. Adhesion is the ability of a water Along with aiding in the retention of soil moisture, molecule to adhere to a soil particle. Cohesion is organic matter protects the soil against the kinetic the ability of a water molecule to stick to itself (or energy of raindrops and also acts as an insulation other water molecules). Mesopores are medium- layer for the soil surface. Without organic matter, sized pores formed by numerous processes like

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 33

Figure 7. Water movement through soils with differing soil structure. Source: USDA NRCS, adapted. the shrinking and swelling of clays, earthworm from areas of high energy to areas of low energy activity, and freeze-thaw cycles. These pores within the soil. are primarily drained during the period of free Soil water has three types of energy: gravitational, drainage (gravity-driven drainage immediately matric, and osmotic. Gravitational potential following a rain event), although some water refers to the energy related to the elevation of drained by mesopores is eventually plant available water within the soil; water at the soil surface has if the drainage network leads into micropore a high gravitational potential, but once it reaches spaces. Macropores are the largest of the soil pore the water table the gravitational potential is zero. classifcation. Some macropores are formed by the Matric potential is the energy associated with burrowing of animals (non-matrix pores) while adhesion and cohesion within the soil matrix (or others exist because tree roots that once occupied profle) and is a negative potential since it opposes the soil have rotted away. Drainage in macropores gravitational and osmotic potential. Osmotic is gravity-driven and rapid. The water in these pores potential measures dissolved chemicals within is quickly replaced by air; therefore, macropores the soil; the higher the concentration of dissolved cannot supply plants with needed water. Soils chemicals, the less room water has to move, though containing a range of pore sizes will have good typically this has little contribution to overall soil drainage and air available to plants (from large water movement except in saline soils. Soil water pores), in addition to water plants can access (from potential is presented as the following simplifed small pores). Ideal pore distribution is generally formula: found in a well-structured soil with the majority Ψ = Ψ + Ψ + Ψ of small pores within soil aggregates (matrix T G M O pores) and large pores between soil aggregates where ΨT is total soil water potential, ΨG is

(interstructural pores) (Kimmins 1997). Forest gravitational potential, ΨM is matric potential, and soils contain a larger percentage of organic matter ΨO is osmotic potential. This energy status refects that results in better soil aggregation and porosity; the amount of energy required to extract water thus, moisture availability in forests is usually from the soil. greater than in agricultural or urban environments. Soil water potential is typically measured in bars or kilopascals of pressure. It is important to note Soil Water Potential that soil water potentials in the vadose zone are Soil water potential is the measure of the energy recorded as negative pressures. Positive pressure status of the soil water and refects the amount of values would indicate that the soil is saturated and water available for plant uptake. Water moves when this occurs, it means that the water being

Journal of Contemporary Water researCh & eduCation UCOWR 34 Schoonover and Crim evaluated is not true vadose zone soil water since content (TAW) can be obtained by multiplying the the vadose zone is unsaturated. The most common AWC by the depth of the plant root zone (Rd) or: device used for measuring soil water potential in TAW = AWC * Rd. the feld is a water-flled tube called a tensiometer. Water fows from a higher potential (inside the Soil Chemistry and Plant Uptake tube) to a lower potential (within the soil matrix) until an equilibrium is reached and the soil water Soil chemistry plays a key role in vegetative potential is displayed as a negative value (range productivity and species composition and is of 0 to -85 kPa) on the tensiometer vacuum gage. largely determined by weathering of rock, rock The closer the value is to 0, the closer the soil is to type, the cation exchange capacity of the soil, saturation. Smaller values (closer to -85 kPa on acid production resulting from microbial and root the negative scale) mean drier soil and that more respiration, and management strategies of the soil. energy is required to extract water. The soil provides nutrients necessary for plant Texture and structure may vary from one horizon growth. The sources of plant nutrients range from to another, which can infuence the movement of biogeochemical cycling inputs, decaying organic water through the soil profle. Soil texture affects matter, amendments added by humans (e.g., how water moves through the soil and how much fertilizers and pesticides), and nutrients naturally water can be stored in the soil. Soil structure occurring within mineral soil. affects the ability of roots to penetrate the soil, the amount of water a plant can uptake, and water Weathering movement through the soil. A poorly structured There are two processes associated with soil soil may have as little as 35 % total porosity, while weathering: physical and chemical weathering. a well-structured soil of the same texture may Physical weathering is the establishment of have 65 % total porosity (Kimmins 1997). A well- suffcient stress on a rock so that it physically structured soil will be more effcient at soil water breaks the rock. Mechanisms that fall within the movement and plant uptake. Additionally, because category of physical weathering include erosion, small pores have more matric potential (adhesion freeze-thaw cycles, bioturbation (plants and and cohesion forces) than large pores, fne-textured animals physically disturb the soil and plant roots soils that contain more micropores generally hold may physically crack rock apart), and formation of more water than coarse-textured soils containing cracks or gaps in soils. Parent material weathers a greater percentage of macropores. By the time to release primary minerals to soils. Chemical feld capacity (FC) (soil is not saturated, but wet) weathering is defned as a change in the chemical is reached, most of the gravitational water is lost, nature of rock, resulting in a change in mineral larger pores become flled with air, and water is structure, reactivity, surface area, and particle size; held within smaller pores. At FC, most water is it is driven by the instability of primary minerals plant available and soil water potential values at the earth’s surface. The minerals must be typically show ~ -10 to -30 kPa (Brady and Weil exposed to the environment for weathering to take 2000). The permanent wilting point (PWP) is place. Primary minerals include quartz, feldspars, the potential at which soil water is unavailable muscovite, and biotite. These minerals have for plant uptake (~ -1500 kPa). At PWP, plants persisted through geologic time and their mineral cannot recover from water stress and will remain composition shows only negligible differences wilted. However, there is still a small amount from their original state. Chemical weathering of hygroscopic water in the smallest pores. The is generally responsible for the formation of available water content (AWC) is considered to secondary minerals, which are derivatives of be the soil water retained between FC and PWP primary minerals. Secondary minerals are the fne and is presented as the formula: materials that make up clay particles in the soil. AWC = FC - PWP. The presence of clay in a soil signifes an active history of weathering. An example of a primary From this formula, the total available water mineral weathering to form a secondary mineral

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 35 is the breakdown of feldspar (primary) into clay with limited rainfall. Soils may become acidic (secondary). Chemical weathering reactions through the weathering of rocks rich in silica, increase in warm, humid climates and are also production of acids from organisms, and through enhanced by the presence of water and oxygen, the release of acids from decaying organic material. as well as biological agents including the acids Alkaline soils are a result of the weathering of produced by microbial and plant-root metabolism rocks such as limestone that contain large amounts (Brady and Weil 2007). of calcium carbonate (salts) and from the dust The four primary chemical weathering processes inputs of salts resulting from the evaporation of are oxidation, reduction, hydration, and hydrolysis. drainage basins, primarily in arid regions. Soil pH Oxidation occurs when the oxygen supply is high affects nutrient solubility and decomposition rates and an element loses electrons; an example is when in soil and thereby has a profound effect on the ferrous Fe combines with oxygen to form ferric Fe availability of nutrients to plants. A slightly acidic oxide (or rust). Reduction occurs when an element pH of between 6 and 7 appears to provide optimal gains electrons and generally occurs in anoxic nutrient availability to plants, though there are (oxygen-depleted) environments. Hydration is a exceptions (Kimmins 1997). Most macronutrients result of the association of water molecules onto are available within this range. Knowledge of the the mineral structure (e.g., anhydrite and water soil pH profle is especially important in making forms gypsum). Lastly, hydrolysis is essentially crop management decisions as some plants may an attack on the silicate structure by hydrogen ions, require a set pH range for optimal growth. Soil pH meaning water breaks down the rock. Chemical can be measured in the lab using a pH meter or in weathering weakens the rock structure and makes the feld with quick test strips. it more susceptible to additional weathering. Nutrients Cation Exchange Capacity There are a number of essential elements Clay and organic matter have what is referred required for plant growth that are categorized as to as cation exchange capacity (CEC), which is macronutrients or micronutrients. The difference the total sum of exchangeable cations that a soil lies in the quantities needed for plant growth. can adsorb. Clay and organic matter are negatively Macronutrients are elements that are needed in charged, thereby possessing the ability to adsorb and greater quantities, whereas micronutrients are hold cations (positively charged ions) onto what is required in smaller portions. All are required known as the cation exchange complex. Cations, in varying quantities for optimal growth. such as K+, Na+, and Ca2+, can be adsorbed onto soil Macronutrients include nitrogen (N), potassium or organic colloids (very small chemically reactive (K), calcium (Ca), magnesium (Mg), phosphorus particles with a large surface area per unit mass), (P), and sulfur (S). Micronutrients include making the cations available for plant uptake by chlorine (Cl), iron (Fe), boron (B), manganese preventing cation leaching from the system (Brady (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), and Weil 2007). Soils high in organic matter and/ cobalt (Co) and nickel (Ni). Macronutrients can or clay generally have a greater CEC than soils further be classifed into primary and secondary with little humus or clay (i.e., sandy soils), though nutrients. Generally N, P, and K are considered CEC can vary greatly depending upon the type of primary nutrients because they are most often clay and amount of organic matter present. CEC the nutrients limiting plant growth. Ca, Mg, and must be measured in a lab. S are rarely limiting nutrients and as such are considered secondary nutrients. Primary nutrients Soil pH are discussed in greater detail below. Soil pH is a measure of the hydronium ion in Nitrogen. Nitrogen is the nutrient needed in the the soil solution, which determines the acidity or greatest quantities by plants and is generally one alkalinity of the soil. Soil pH varies by region and of the most limiting to plant growth due to lack acidic soils are typically found in wet climates, of environmental availability. The atmosphere whereas alkaline soils are generally found in areas contains a large reservoir of nitrogen gas (~79 %

Journal of Contemporary Water researCh & eduCation UCOWR 36 Schoonover and Crim of the atmosphere), most of which is unavailable time. Decomposition returns P to the soil, but if to plants and animals. Both inorganic and organic crops are removed, the P is also removed from the forms of N are found in soils; however, only the system. For these reasons, P is often a limiting inorganic form is available for plant uptake. Most nutrient in the soil. Phosphorus defciency in of the soil N (> 95 %) exists in organic form plants is recognized by bluish-green foliage, sparse - and is therefore unavailable. Nitrate (NO3 ) and fowering, and stunted, thin stems. Because plants + ammonium (NH4 ) ions are the two inorganic require large quantities of P and there is little natural forms utilized by plants. Nitrate is held primarily available P from weathering, crop harvesting in solution and is readily available for plant uptake. removes the majority of P from the feld. Without Ammonium ions are mostly held on the cation P amendments, the subsequent crop will likely be exchange complex. Fortunately, soil microbes P defcient, which has led to the over-fertilization can break down organic N (NH2) and convert it to of felds and an accumulation of P in the soil. + - forms usable by plants (NH4 and NO3 ) in a process Unlike nitrate, which is mobile downward through known as nitrogen mineralization. Immobilization the soil profle, P binds readily to sediment and is of N can also occur when inorganic N is converted easily transported in this bounded state to streams to unusable organic form. and rivers. Excessive P in runoff can promote the Sources of N are primarily from the atmosphere, eutrophication (over-abundance of algae growth biological fxation, and fertilization. Biological caused by excessive nutrient accumulation) of fxation accounts for the majority of N inputs in downstream water bodies. The resulting algal most ecosystems (Kimmins 1997). In agricultural growth decreases the dissolved oxygen levels in systems, annual biological N fxation is estimated the water, which may lead to substantial fsh kills. at 50 - 70 million metric tons of N globally, With the Mississippi River Watershed draining primarily from the symbiotic relationship between much of the nation’s cropland, the Gulf of Mexico leguminous crops (dominated by soybeans) and consistently has one of the largest dead zones in rhizobia (nitrogen-fxing bacteria) (Herridge et al. the world due to eutrophication; the average size of 2008). Nitrogen defciencies in plants are typically the dead zone over the last 5 years is ~ 5,500 mi2. noticed by the yellowing of foliage and stunted For this reason, P management in watersheds is of growth. Because N defciencies are widespread primary concern. and can lead to poor crop yields, N is often applied Potassium. Potassium is available in greater in excess. Nitrate is extremely mobile and readily quantities than any other soil macronutrient and leached from the soil if not used by plants. High is essential in providing protection against crop NO - concentrations (> 10 mg L-1) in drinking 3 disease. However, most of the K within the soil water are a danger to public and ecosystem health, exists as a mineral and is not readily available for thus nitrate in drinking water is regulated by the plant uptake. The K-containing minerals, such as Environmental Protection Agency. Additionally, feldspars and micas, are resistant to weathering; excess N in the soil can lead to incomplete this leads to minimal K inputs from weathering denitrifcation, which generates nitrous oxide into during a growing season. Only ~ 2 % of soil K the atmosphere. Nitrogen is a nutrient requiring is readily available for plant uptake and is either careful management, critical for both successful in solution form or exchangeable form (Brady crop production and environmental quality. and Weil 2007). Potassium in the soil solution is Phosphorus. Phosphorus is second only to nitrogen immediately available to plants. Exchangeable in the amount needed for optimal plant growth. K is a part of the cation exchange capacity of The majority of soil P is derived from mineral soils and is adsorbed on the soil colloid. The weathering and organic matter decomposition, but dissolved and exchangeable K concentrations are concentrations of plant-available P are generally in equilibrium. For instance, when plants remove very low in soils. Most soil P is in insoluble K from the soil solution, K is released from the forms that are unavailable to plants and when cation exchange complex into the soil solution soluble forms of P are added via fertilization, until K equilibrium is reached. Potassium also they can be converted to insoluble forms over occurs in nonexchangeable or fxed form. Fixed

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 37

K is trapped between clay particles and remains (> 2 mm) includes animals such as groundhogs, unavailable until it converts to exchangeable K. moles, earthworms, centipedes, ants, and termites; This conversion usually does not occur within the mesofauna (0.1 - 2.0 mm) includes springtails and span of one growing season; therefore, fxed K is mites; microfauna (< 0.1 mm) includes species considered a slowly available form of K. such as rotifers, nematodes, and other single-celled Plants uptake large amounts of K in their organisms. Soil fora includes organisms as small aboveground biomass, often more than needed as diatoms and algae up to the size of tree roots. for growth. Because of this excessive uptake, An important member of soil fora is mycorrhizae, harvesting crops or forests can remove large fungi that form a symbiotic (mutually benefcial) amounts of K from the system each year. Potassium relationship with plant roots. Most plants defciency is normally seen frst on the lower plant contain roots infected with mycorrhizal fungi. leaves because K is translocated from older to Mycorrhizae enhance water and nutrient absorption younger tissue. Defciency symptoms include by increasing root surface area and accelerate yellow scorching along the leaf edge, slow growth, mineral weathering which releases nutrients to the and weak stalks. Like nitrate, K is extremely mobile soil (Fisher and Binkley 2000). Collectively, soil and readily lost via leaching. Applying substantial biota carries out enzymatic and physical processes amounts of K fertilizer can lead to excessive plant that decompose organic matter, build soil humus, uptake and loss to leaching. In addition to the soil and make nutrients available for plants. Table 4 itself as a natural K source, leaving crop residue provides a description of the microfauna numbers on the soil can return a signifcant portion of K to found in a teaspoon of soil. the soil. Decomposition is one of the most critical roles that soil biota play in an ecosystem. Without Role of Soil Biology effcient decomposition, organic material would One critical function of soil is to provide a home accumulate on the soil surface and nutrients would for organisms. Soil biota plays an integral role in be bound within the material. Decomposition is soil ecosystems by decomposing leaves, downed initiated immediately when a leaf, twig, or fruit logs, and animals, and also providing the primary hits the ground. Once on the ground surface, biota nutrient source for vegetation. Soil biota includes begins to physically break down the material, both fora (plants) and fauna (animals). Soil fauna creating more surface area to which fora can subsists on a wide variety of energy sources, adhere. including: living plant material (herbivores), animals (carnivores), dead material (detritivores), Soils and Roots fungi (fungivores), and bacteria (bacterivores). Roots are a critical component in the soil The size of soil fauna also varies. Macrofauna environment. Plants rely on roots for structure,

Table 4. Brief description of the organisms in a teaspoon of soil. (Source: Melendrez 1975).

Organism Number of Individuals per Teaspoon of Soil Primary Role

5 to 500 million in agricultural soils; Bacteria Consume readily decomposable materials 20 million to 2 billion in forest soils

Fungi Up to 40 miles of fungal hyphae Consume hard to decompose organic matter

Protozoa 100 to 100,000 Feed on bacteria and each other

Nematodes 5 to 500 with high variability among soils Feed on bacteria, fungi, and plant roots

Microarthropods Several species Feed on fungi, plants, and organic matter

Journal of Contemporary Water researCh & eduCation UCOWR 38 Schoonover and Crim support, water, and nutrient uptake. The relationship works, cut a well-watered plant close to the ground, between roots and mycorrhizal fungi increases quickly attach a manometer to the stem and observe nutrient availability and absorption. Roots also how the pressure changes on the gauge. The cut act as a reservoir for food storage (starches) and stem will exude water, suggesting pressure from sometimes synthesize growth hormones for the the roots to the stem is being released. Although plant. Root growth is controlled by soil moisture, most water is absorbed through roots, some plants compaction, structure, texture, temperature, and have developed the ability to absorb water through chemistry. Once roots decay, the channels left their leaves. Along the California coast, ~ 80 % behind improve air and water movement within of the redwood species, Sequoia sempervirens, the soil. absorb water deposited by fog through their leaves Nutrients are taken up by roots by the processes (Limm et al. 2009). This adaptation is especially of root interception, mass fow, and diffusion. useful in areas prone to drought, like California. Root interception occurs when roots grow Plants have both primary roots and secondary towards nutrients in nutrient-rich soil so that roots (Figure 8), which come in many different they can be utilized by the plant. Since the roots shapes and sizes. Plant root networks are must continually grow in undepleted soil for heterogeneous and no two root systems will be root interception to occur, this process is limited. identical. Roots adapt and grow in response to Mass fow occurs when nutrients are transported environmental conditions. There are three common with soil water to a root that is actively extracting root systems of North American tree species water from the soil. This process is most (Figure 9). Tap root systems have a prominent tap effective in periods of rapid transpiration with root and smaller (secondary) lateral roots that grow high concentrations of nutrients in the soil water off the side of the dominant primary root. Tap solution. Diffusion occurs when nutrients move root systems are effective in areas where access from high concentration areas (nutrient saturated) to water is located deep within the soil profle so to low concentration areas (nutrient depleted) near water can be accessed in periods of drought. Thus, the root surface. Rates of uptake via diffusion many tree species with tap roots are well-adapted depend on concentration gradients, soil water for upland and dry site conditions. Heart root contents, ion size and charge, soil temperature, systems occur in both upland and bottomland tree and root adsorption rates. Water enters the root species and are adapted for mesic (moderately through root hairs or the cortex. Osmotic pressure moist) site conditions. Flat roots are shallow root (movement of water from areas of high to low systems that often occur in bottomland species. concentration) causes the water to enter into the Tree species with fat root systems are commonly plant cell. The expansion and contraction of the uprooted in high windstorms when the ground is roots cause water to move up through the cortex, near saturation. Contrary to popular belief, ~ 90 through the xylem vessels, the stem, and into the to 95 % of tree roots are found within 1 m of the rest of the plant. A manometer, an instrument soil surface and most tree root systems extend out used for measuring the pressure of a fuid, is used ~ 2 times the width of the crown. Absorption of to measure the pressure of a root system. Root nutrients occurs mainly in fne roots (< 2 mm in pressure is the pressure exerted on the liquid diameter) which are concentrated in the surface contents of cortical cells in roots. Cortical cells, horizons (Kimmins 1997). Trees rely on these roots which aid in the transport and storage of water for access to water and nutrients. Root systems of and nutrients, are either turgid (expanded) or various tree species are shown in Table 5. faccid (contracted). Turgor pressure is the actual hydrostatic pressure developed inside a cell. This Soil Erosion and Conservation pressure is due to endosmosis, or the inward fow of water from outside of the cell. Flaccidity is Soil erosion is a major concern around the when the cell undergoes exosmosis, the opposite globe. In order to properly prevent and manage of endosmosis, where water is lost and the cell erosion, it is important to understand erosion becomes limp. To understand how root pressure concepts. Soil erosion is both naturally-occurring

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 39

Table 5. Common root systems for various tree species in the United States.

Tap root Heart Root Flat root

Hickory Red Oak Fir

White Oak Honey Locust Spruce

Butternut Basswood Sugar Maple

Walnut Sycamore Cottonwood

Hornbeam Pine Silver Maple

Figure 8. Common root system shapes for North American tree species. Drawing by Robin L Quinlivan.

Figure 9. Major features of a root. Drawing by Robin L Quinlivan.

Journal of Contemporary Water researCh & eduCation UCOWR 40 Schoonover and Crim

(geologic erosion) and infuenced by humans foodplains, felds, wetlands, lakes/reservoirs, (accelerated erosion). Accelerated erosion can rivers, and streams. In agricultural felds, sediment be 10 to 1000 times more damaging than geologic may also be deposited in front of control structures erosion (Brady and Weil 2007). Examples include such as dry dams, terraces, or vegetative flters disturbance of soil or natural vegetation by grazing (e.g., riparian areas and grass waterways). livestock, deforestation, tillage, and construction, Soil erosion is divided up into different each of which are exacerbated by high rainfall and/ categories: rill erosion, interrill or sheet erosion, or steep slopes. and gully erosion. Rill erosion is the formation of Globally, ~ 15 % of total land area is eroded, of tiny channels across a soil surface that often occurs which over half is severely eroded (Blanco-Canqui in sloping agricultural felds. Interrill erosion is and Lal 2008). Soil erosion rates are greater than the removal of a somewhat uniform layer of soil soil formation rates, posing a threat to sustainable which is attributed to the splash effect of raindrop agriculture (Pimentel 2006). Productive land impact and also sheet fow. If rill erosion persists, is < 11 % of earth’s total land area (Eswaran et it is likely that gully erosion will occur. Gully al. 2001), yet it must supply food to the world’s erosion is the erosion process that removes large population (> 7 billion people). Pimentel (2006) amounts of soil and results in a narrow, deep suggested that soil is being lost at a rate that is channel. 10 to 40 times faster than soil formation and ~ 10 Eventually, high amounts of erosion will million ha of cropland is lost yearly to erosion. destroy soil quality, which in turn, compromises Agriculture accounts for ~ 75 % of soil erosion long-term food and timber production. The Dust worldwide (Pimentel 2006). Soil erosion is lowest Bowl of the 1930s raised awareness to the need for in the U.S. and Europe (~ 10 metric tons ha-1 yr-1 soil conservation and prompted implementation soil lost) and much more severe in the rest of the of conservation practices. Several government world (~ 30 – 40 metric tons ha-1 yr-1 soil lost) programs geared towards soil conservation are in where resources and incentives for implementing place to minimize and prevent unnaturally high conservation measures are inadequate (Blanco- occurrences of soil erosion by wind and water. Canqui and Lal 2008). The Food and Security Act of 1985 included Erosion is defned by the Soil Science Society conservation measures that helped reduce erosion of America (SSSA 2008, 19) as: “(i) The wearing from U.S. cropland by 1.3 billion metric tons away of the land surface by rain or irrigation water, between 1982 and 1997 (Montgomery 2007). For wind, ice, or other natural or anthropogenic agents example, the Conservation Reserve Program (CRP) that abrade, detach, and remove geologic parent (included in the Food and Security Act of 1985) material or soil from one point on the earth’s surface provides economic incentives to landowners who and deposit it elsewhere, including such processes implement soil conservation measures on highly as gravitational creep and so-called tillage erosion. erodible cropland. In 2010, 10.7 metric tons ha-1 (ii) The detachment and movement of soil or rock of cropland was lost in the U.S., whereas CRP land by water, wind, ice, or gravity.” reported a total of 1.7 metric tons ha-1 lost to wind Essentially, erosion occurs in a three-step process and water erosion (USDA 2013). Since the CRP of detachment, transport, and deposition. The frst was implemented in 1986, soil erosion has been step, detachment, is the removal of soil material reduced by > 7 billion metric tons (USDA 2010). from the soil mass by raindrops, running water, Additionally, recent adoption of notill farming wind, or human/animal activities. The detached has contributed to a reduction in soil erosion. In soil materials are then transported downhill by a review of studies comparing conventional and method of splashing, foating, dragging, or rolling. notill methods, Montgomery (2007) found that Sand particles are heavier than silt or clay, and notill practices reduced soil erosion by 2.5 to > therefore cannot be transported as far or at the same 1,000 times compared to conventional methods. velocity. Silts and clays often become suspended Although adoption of conservation practices in water and may be transported long distances. is increasing, there are several obstacles that land The third step, deposition, usually occurs in managers interested in soil conservation must

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 41 overcome. At times, the initial implementation orchard grass) that retain groundcover in the winter of conservation practices can be more costly than versus annually harvested crops (e.g., soybeans, the economic return. The fear of the unknown corn) will improve the C factor, as will planting will sometimes steer people away from adopting cover crops during times the feld would normally new conservation technologies. It is also diffcult be fallow. The conservation practices factor (P) to convey the importance of soil conservation can be changed by installing various conservation to every individual, whether it is due to lack of structures and implementing contour cropping interest or unavailable information. wherever necessary. An example of a conservation The Universal Soil Loss Equation (USLE), structure would be installing a water bar after a a model based on extensive erosion data from timber harvest. Water bars are used to divert water small plot studies across the U.S., was created by away from logging roads and into vegetated areas, countless federal and university scientists in the decreasing erosion of these roads. mid-1900s to predict soil loss. The USLE has been used worldwide to estimate soil erosion and guide Soil Quality soil conservation efforts. The USLE equation accounts for slope, rainfall, soil series, topography, The basic defnition of soil quality is the crop system, and management practices on a capacity of a soil to function (Karlen et al. 1997). landscape. The equation has been revised over This functional defnition balances the physical, time and is now referred to as Revised Universal biological, and chemical components of soil. The Soil Loss Equation, Version 2 (RUSLE2), a expanded version of this functional defnition is computer-based equation. However, to understand “the capacity of a specifc kind of soil to function, the principles of estimating soil erosion rates, it is within natural or managed ecosystem boundaries, important to analyze the original formula: to sustain plant and animal productivity, maintain or enhance water and air quality, and support USLE: A = R * K * LS * C * P human health and habitation” (Karlen et al. 1997). where A is annual soil loss (tons ac-1 yr-1), R is What constitutes good soil quality may be different erosivity factor, K is erodability factor, LS is according to land use and/or geographic region. slope length and slope steepness factor, C is cover For this reason, Karlen et al. (1997) suggests a and management factors, and P is conservation relational approach to evaluating soil quality, practices factor. as opposed to an absolute approach. Similarly, The erosivity factor (R) assesses the total Doran et al. (1996) suggest that soil quality should rainfall, intensity, and seasonal distribution of be evaluated based on how well a soil functions rainfall in a geographic location. The R factor within its specifc ecosystem (agriculture, urban, accounts for the driving force of rill and interrill etc.). For example, in an agricultural feld, the erosion. In other words, the R factor recognizes capacity of a soil to function at sustaining crop that high intensity rainfall will cause more growth would depend on several soil characteristics catastrophic damage than a lower intensity rainfall including bulk density, soil moisture, infltration, event. The erodability factor (K) assesses how and biological activity, to name a few. Many of susceptible the soil is to erosion, which includes a these properties can be changed by management soil’s infltration capacity, texture, and its structural (e.g., infltration and organic matter content) stability/integrity. and soil quality can be improved according to Only three of the factors in the equation can be its function. As seen throughout this article, manipulated. The slope length factor (LS) can soil also affects water quality, air quality, and be changed by reshaping the land into terraces, biotic quality. Protecting and/or improving soil dry dams, or other erosion control structures. The quality can provide a stepping stone to improving cover factor (C) is probably the easiest factor to environmental quality as a whole. For example, change. In forests, limiting harvesting and road planting cover crops when a feld would otherwise structures can greatly improve this value. In be bare, helps reduce soil erosion and aids in agriculture felds, perennial crops (e.g., alfalfa, soil and nutrient retention on site, limiting its

Journal of Contemporary Water researCh & eduCation UCOWR 42 Schoonover and Crim transportation to waterways where water quality for nutrients and bacteria. In these areas the would be affected. vegetative uptake often is much lower than the supply of nutrients. Nitrogen, for instance, either Land Use and Soils leaves the site through groundwater or surface runoff or is denitrifed under optimal conditions. Cultivated and Grazed Soils Oftentimes in heavily grazed land, the carbon In cultivated areas, soils are mechanically supply for completed denitrifcation is lacking, worked to break up compaction and amended with thus incomplete conversion of NO3 to N2 gas fertilizers, herbicides, and pesticides to enhance occurs, resulting in nitrous oxide pollution into the crop production and provide weed and pest control. atmosphere. These processes can expose bare mineral soil and Forest Soils increase the risk of water and wind erosion. Under improper management, agriculture can devastate Forest soils tend to be on land unsuitable for the landscape. For example, intense cotton farming row-crop agriculture or grazing. Thus, forests in the southeastern United States was blamed for are typically in areas of steep topography, poor the loss of 10 to 30 cm of topsoil (with an average drainage, or where soils are too rocky for farming. of ~ 18 cm), which led to the accumulation of large Contrary to agricultural soils, forest soils typically volumes of legacy sediment in the foodplains have an intact litter layer that can protect soils from (Trimble 1974). Similarly, poor farming practices temperature and moisture fuctuations and from coupled with a drought, caused millions of acres wind and water erosion. of farmland to become lost during the time Forest Harvesting. Forest harvesting is a common known as the “dust bowl” or “dirty thirties” era. occurrence in the United States and around the Fortunately, recent farming practices have adopted world. Harvesting provides the nation with wood environmentally conscientious methods to reduce products needed in our daily lives. Along with soil erosion and other impacts on the environment. these benefts come potential damages to our Farmers now understand that crop selection and environment. The majority of compaction in a farm management decisions should be based on forest is caused by forest harvesting procedures. soil characteristics and land conditions. Farmers Surprisingly, the primary source of erosion during seek to maximize yields while simultaneously a harvest results from the construction of roads creating a sustainable farming system. into the forest and not from the actual logging Land use change can have signifcant impacts practices, as one might expect. It is the job of on soil quality. If cultivated areas are abandoned, foresters and ecologists to conduct sustainable shrubs and trees quickly reoccupy the site; however, forest harvesting using best management practices carbon pools and nutrient dynamics within the soil in an effort to reduce site impacts. Harvesting may be slow to recover. For example, following a timber has a variety of different effects on the soil, 30-yr succession in North Carolina, most available depending on the kind of harvest and caution used carbon was allocated to aboveground standing while harvesting. The harvesting process opens up biomass and not towards soil organic matter the canopy, which in turn allows for more light to accumulation (Richter et al. 1995). hit the forest foor. This increase in light exposure Impacts of grazing on soils fall into two changes the chemical processes occurring in the categories: 1) physical impacts from animals and soil. These changes can then shift the plant and 2) chemical and biological impacts from animal animal species composition found on the site. waste. Livestock, such as cattle, compact the The exposure of mineral soil from harvesting can soil structure and typically consume vegetation increase the potential for erosion and leaching of in heavily used areas, such as feeding or watering nutrients. sites. As a result, the compacted bare areas are Loggers use a variety of methods for harvesting more susceptible to erosion and surface runoff. timber. Common equipment includes log skidders, Waste associated with livestock is also a problem. feller-bunchers, and cable logging, each with their Urine and feces from livestock can create hotspots own pros and cons. Heavy machinery equipped

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 43 with wide tracks instead of pressured wheels documented that a historical fre (20 years earlier) reduces the degree to which the machine compacts exhibited long-term effects on soil nitrogen and the soil. Types of machinery used often depend tree nutrition. Vegetation following a 500 km2 upon the region. In places like the Midwest burn showed defciencies of both nitrogen and where there are large tracts of agricultural land phosphorus in foliar analyses. and few tracts of forestland, access to state-of- Using properly executed prescribed fre may the-art logging equipment is limited. Detailed actually increase nutrient uptake and provide guidelines on Best Management Practices (BMPs) greater surface roughness through the establishment associated with forest harvesting are described in of dense vegetation post-burn. The increased “Guiding Principles for Management of Forested, surface roughness from the newly established and Agricultural, and Urban Watersheds” (Edwards et vigorous vegetation growth can reduce surface al. 2015, this issue). runoff velocities, in turn reducing the risk of soil Long-term site productivity continues to be a erosion. Additionally, the fush of vegetation can concern to natural resource managers. An increase increase nutrient uptake, thereby retaining the in the industries of biofuel production, whole-tree nutrients on site. Conversely, uncontrolled fre harvesting, woodchip utilization, and wood pellets can be detrimental to a forest ecosystem. Under have researchers and managers studying how conditions where all vegetation is consumed, the long forests can sustain the continuous removal risk of soil erosion is increased, nutrient uptake of biomass from forests. A potential short-term by plants may be reduced, and high stream solution to this problem could be the introduction temperatures may result. As vegetation is removed, of fertilization into forest ecosystems to replenish evapotranspiration in the watershed is reduced, nutrients exported from the site via whole-tree thus providing a greater volume of water that can removal. Large-scale fertilization of forests has produce surface runoff. Increased overland fow increased in the last decade. However, it is unclear combined with a consumed litter layer can lead how large-scale fertilization affects long-term site to serious erosion problems and result in higher productivity because a longer monitoring period is stream discharges. To compound the problem, needed to assess the changes in site quality. intense burns can produce hydrophobic conditions Fire. Fire plays a key role in shaping the structure in the soil that ultimately reduce infltration and of a forest community; however, fre can also be increase surface runoff. Hydrophobic conditions both benefcial and detrimental to forest soils. are generally produced during moderate to high Fire can alter the chemical and physical status of severity burns (175° - 200° C), in coarse textured soils, impact hydrology, increase soil erosion, and soils, and at lower soil water contents (DeBano et infuence the biotic structure of soils. The severity al. 1998). Both nutrient runoff and erosion can of fre impacts generally depends on burn intensity, be exacerbated in areas of steep slopes and high climatic conditions following the burn, landscape, rainfall. and soil characteristics. Following intense burns, Soil heating is the primary mechanism by nutrient cycling can be disrupted and lead to which fres impact a soil’s physical and biotic leaching, volatilization, and/or transformation. structure (Neary et al. 1999). The degree of soil When vegetation is consumed by fre, some of the heating during a fre is highly variable across a litter-incorporated N, P, K, Ca, Mg, Cu, Fe, Mn, site, but depends on fuel characteristics (size, and Zn are volatilized and lost from the system, arrangement, and moisture status), soil properties while metallic nutrients such as K, Ca, and Mg are (texture, structure, and both the moisture and converted into oxides and accrued in ash (DeBano organic content), and fre behavior (intensity and et al. 1998). The long-term effects of fre on the duration). Generally 8 - 10 % of the aboveground soil nutrient status can be signifcant (Williard et heat can be transferred to the soil, resulting in al. 2005). Repeated low intensity fres, such as higher soil temperatures (DeBano et al. 1977). prescribed fres or single large fres, can reduce Fuels differ in terms of maximum temperatures soil nitrogen pools signifcantly (Gagnon 1965; produced and the duration that they can burn. Carreira et al 1994). In fact, Gagnon (1965) For example, downed logs and stumps can burn

Journal of Contemporary Water researCh & eduCation UCOWR 44 Schoonover and Crim for days following the passage of the fre front, a soil-forming factor that plays an important role which will induce a more sustained soil heating. in reducing the impacts of urbanization on soil Further, fner textured soils (i.e., increased clay characteristics. content) tend to transfer heat better than quartz- Knowledge of urban soil properties can lead to based soils. better management of water resources in terms of Organic content of soils can also be impacted both quantity and quality. Duration of irrigation by fre. The consumption of soil organic matter, in urban areas should be based on the water which is a cementing agent in soils, can result holding capacity of the soil. Organic matter can in a loss of soil structure. Reduced structure be incorporated into the soil to increase pore space can reduce porosity, increase bulk density, and and soil aggregate stability, thereby increasing decrease infltration and percolation. Structure infltration and nutrient retention and reducing takes time to rebuild, and under severe conditions runoff. Soil texture should be considered when where the soil biota was impacted, the recovery determining fertilizer management. For instance, can be even greater since soil invertebrates play a sandy soils exhibit poor nutrient retention and key role in maintaining soil porosity. The degree nutrients that leach below the rooting zone to which soil invertebrates are impacted relates to may eventually travel to ground and surface soil temperature and moisture. Biota can tolerate waterbodies, impacting water quality. Nitrate is higher temperatures in dry soils compared to moist especially mobile and nitrate that is not used by soils. plants or microbes is easily transported from the soil water to groundwater. Urban Soils Soils are modifed by anthropogenic activities Evaluating Your Soil in urban environments. The impacts of urban infrastructure on the environment create changes This article has provided an introduction to soils in the physical, chemical, and biological properties and stressed the importance of considering soil of urban soils. Notably, pore space is reduced characteristics when making land management due to soil compaction from building and road decisions. How does one learn about the soil engineering, thereby increasing soil bulk densities. specifc to a given watershed in order to make Additionally, the loss of pore space and soil sound management decisions? There are many structure combined with increased impervious helpful internet sites available to land and surfaces leads to reduced infltration and increased watershed managers; one place to start is a soil surface runoff. Compaction may also lead to survey. The USDA NRCS operates the Web Soil poor establishment of plant communities in urban Survey (Soil Survey Staff). Users obtain soils areas. Pouyat et al. (2007) found that K, P, bulk information by entering coordinates, interactively density, and pH were higher in urban residential selecting a feld area with a map interface, or by soils compared to forest soils, likely a result of importing a shapefle. An example of soil series lawn fertilizers and intensity of use. However, data produced from a user-defned area of interest other soil variables measured were related to in Jackson County, IL, is provided in Figure 10. parent material rather than land use, including soil Soil surveys contain information on the suitability texture, Al, Fe, and other micronutrients (Pouyat and limitations of soils in the specifed geographic et al. 2007). While urbanization can impact some area and are created from on-site surveys of the soil characteristics, especially bulk density and soil profle collected by soil scientists. These infltration, other surface soil characteristics are surveys can help determine if a site is suitable for still predetermined by parent material. Urban soils a basement, prone to fooding, or a good site for can recover over time. Scharenbroch et al. (2005) a septic system, among many other applications. found that as time since the initial disturbance It should be noted that spatial variability is of urban soils increased and allowed for soil inherent in soils and a 100 hectare site may contain development, bulk densities decreased and organic several different soil series with contrasting soil matter and biological activity increased. Time is properties. If interested in the nutrient, organic

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 45

Figure 10. Soil survey data of an area in Jackson County, IL. Produced from Web Soil Survey (Soil Survey Staff). matter content, or pH of your soil for gardening on the land conditions. In terms of soil quality, or farming, soil samples can be taken or shipped management decisions must consider the capacity to various laboratories around the country that will of the soil to function within the framework of the provide basic to extensive soil information. These ecosystem. soil data can be useful to develop appropriate land management practices. Soil Information Resources Conclusion Web Soil Survey: http://websoilsurvey.nrcs.usda.gov/ Watershed management, from urban to rural environments, must incorporate the functionality “Feel” Method Guide to Soil Texture: of the soils within the watershed. Understanding http://www.nrcs.usda.gov/Internet/FSE_ watershed soil characteristics allows for a greater MEDIA/nrcs142p2_050352.jpg understanding of the processes controlling erosion, water movement and storage, pollutant runoff, and Soil Taxonomy: site productivity, among others. Soil is a fnite http://www.nrcs.usda.gov/wps/portal/nrcs/ resource and as such it is important for watershed main/soils/survey/class/taxonomy/ managers to consider both soil quality and conservation in watershed management plans. In Offcial Soil Series Descriptions: terms of conservation, it is important to minimize http://www.nrcs.usda.gov/wps/portal/nrcs/ erosion through structural and/or vegetative detail/soils/survey/geo/?cid=nrcs142p2_053587 measures. Measures should not only be cost- effective, but also sustainable and practical based

Journal of Contemporary Water researCh & eduCation UCOWR 46 Schoonover and Crim

Acknowledgment texture. Report WSDG-TN 00005. Watershed Systems Development Group, USDA Forest This work was supported by funds from the USDA Service, Fort Collins, CO. McIntire Stennis Cooperative Forestry Research Darwin, Charles. 1897. The Formation of Vegetable Program. Mould, through the Action of Worms with Observations on Their Habits. D. Appleton and Author Bio and Contact Information Company, New York, NY. DeBano, L.F., P.H. Dunn, and C.E. Conrad. 1977. Jon e. schoonover is an Associate Professor of Forest Fire’s effect on physical and chemical properties Soils and Physical Hydrology at Southern Illinois of chaparral soils. General Technical Report WO- University Carbondale. His current research interests 3. USDA Forest Service. focus on the infuence of agricultural best management practices on soil and water quality. More specifcally, Debano, L.F., D.G. Neary, and P.F. Ffolliot. 1998. his work focuses on riparian buffer vegetation, nutrient Fire’s effects on ecosystems. John Wiley and Sons, management strategies, and cover cropping systems. He Inc., New York. is also interested in the propagation and restoration of Doran, J.W., M. Sarrantonio, and M.A. Liebig. giant cane in Southern Illinois riparian areas. He can be 1996. Soil health and sustainability. Advances in contacted at [email protected] or Department of Forestry, Agronomy 56: 1-54. 1205 Lincoln Drive, Southern Illinois University, Edwards, P., J.E. Schoonover, and K.W.J. Williard. Carbondale, IL 62901-4411. 2015. Guiding principles for management of JacKie F. crim is a Researcher III in the Department forested, agricultural, and urban watersheds. of Forestry at Southern Illinois University Carbondale. Journal of Contemporary Water Research 154: She received her M.S. in Forestry with a water resources 60-84. emphasis from Auburn University, AL. Encyclopedia Brittanica. 1999. “hillslope: hillslope profle”. Art. Encyclopedia Britannica Online. References Available at: http://www.britannica.com/ EBchecked/media/19383/Soil-profiles-on- Arshad, M.A., B. Lowery, and B. Grossman. 1996. hillslopes-The-thickness-and-composition-of- Physical tests for monitoring soil quality. In: soil. Accessed March 30, 2015. Methods for assessing soil quality, J.W. Doran Eswaran, H., R. Lal, and P.F. Reich. 2001. Land and A.J. Jones (Eds.). Soil Science Society of degradation: An overview. In: Responses to America Special Publication 49, SSSA, Madison, land degradation. Proceedings of the 2nd WI, pp. 123-142. International Conference on Land Degradation Blanco-Canqui, H. and R. Lal. 2008. Soil and water and Desertifcation, Khon Kaen, Thailand. E.M. conservation. In: Principles of Soil Conservation Bridges, I.D. Hannam, and L.R. Oldeman (Eds.). and Management. Springer, New York. pp 1-19. Oxford, New Delhi. Bockheim, J.G., A.N. Gennadiyev, A.E. Hartemink, Fisher, R.F. and D. Binkley. 2000. Ecology and and E.C. Brevik. 2014. Soil-forming factors and Management of Forest Soils. 3rd edition. John soil taxonomy. Geoderma 226-227: 231-237. Wiley and Sons, Inc., New York. Buol, S.W., F.D. Hole, R.J. McCracken, and R.J. Gagnon, J.D. 1965. Nitrogen defciency in the York Southard. 1997. Soil Genesis and Classifcation, River burn, Gaspe, Quebec. Plant and Soil 23: 49- 4th edition. Iowa State University Press, Ames, 59. IA. Herridge, D.F., M.B. Peoples, and R.M. Boddey. 2008. Brady, N.C. and R.R. Weil. 2007. The Nature and Global inputs of biological nitrogen fxation in Properties of Soils, 14th edition. Prentice-Hall, agricultural systems. Plant and Soil 311: 1-18. Upper Saddle River, New Jersey. Jenny, H. 1941. Factors of Soil Formation: A System of Carreira, J.A., F.X. Niell, and K. Lajtha. 1994. Quantitative Pedology. McGraw-Hill, New York. Soil nitrogen availability and nitrifcation in Karlen, D.L., M.J. Mausbach, J.W. Doran, R.G. Mediterranean shrublands of varying fre history Cline, R.F. Harris, and G.E. Schuman. 1997. Soil and successional stage. Biochemistry 26: 189-209. quality: A concept, defnition, and framework Daddow R.L., and G.E. Warrington. 1983. Growth- for evaluation (A Guest Editorial). Soil Science limiting soil bulk densities as infuenced by soil Society of America Journal 61: 4-10.

UCOWR Journal of Contemporary Water researCh & eduCation An Introduction to Soil Concepts and the Role of Soils in Watershed Management 47

Kimmins, J.P. 1997. Forest Ecology. A Foundation Soil Survey Division Staff. 1993. Soil Survey Manual. for Sustainable Management. 2nd edition. Soil Conservation Service, U.S. Department of Prentice Hall, Inc., Upper Saddle River, New Agriculture Handbook 18. Jersey. Soil Survey Staff. 1999. Soil Taxonomy: A Basic Limm, E.B., K.A. Simonin, A.G. Bothman, and T.E. System of Soil Classification for Making Dawson. 2009. Foliar water uptake: A common and Interpreting Soil Surveys, 2nd edition. water acquisition strategy for plants of the Natural Resources Conservation Service. U.S. redwood forest. Oecologia 161: 449-459. Department of Agriculture Handbook 436. Melendrez, M.M. 1975. Soil ecology and the soil Soil Survey Staff. 2014a. Keys to Soil Taxonomy, food web: Nature helping science restore the 12th edition. U.S. Department of Agriculture, natural process of the soil food web. Soil Natural Resources Conservation Service, Secrets, LLC., Los Lunas, NM. Washington, DC. Minnich, J. 1977. The Earthworm Book. Rodale Soil Survey Staff. 2014b. Illustrated Guide to Soil Press, Emmaus, PA. Taxonomy. U.S. Department of Agriculture, Montgomery, D.R. 2007. Soil erosion and agricultural Natural Resources Conservation Service, sustainability. Proceedings of the National National Soil Survey Center, Lincoln, NE. Academy of Sciences 104: 13268-13272. Soil Survey Staff. Natural Resources Conservation Neary, D.G., C.C. Klopatek, L.F. Debano, and P.F. Service. U.S. Department of Agriculture. Ffolliott. 1999. Fire effects on belowground Web Soil Survey. Available online at http:// sustainability: a review and synthesis. Forest websoilsurvey.nrcs.usda.gov/. Accessed October Ecology and Management 122: 51-71. 14, 2014. Pimentel, D. 2006. Soil erosion: A food and Trimble, S.W. 1974. Man-induced soil erosion on the environment threat. Environment, Development Southern Piedmont: 1700-1970. Soil and Water and Sustainability 8: 119-137. Conservation Society of America. Ankeny, IA. Pouyat, R.V, I.D. Yesilonis, J. Russell-Anelli, and USDA. 2010. Conservation reserve program: Annual N.K. Neerchal. 2007. Soil chemical and physical summary and enrollment statistics. FY 2010. properties that differentiate urban land-use and USDA. 2013. Summary Report: 2010 National cover types. Soil Science Society of America Resources Inventory. Natural Resources Journal 71: 1010-1019. Conservation Service, Washington, DC, and Richter, D.D., D. Markewitz, J.K. Dunscomb, P.R. Center for Survey Statistics and Methodology, Heine, C.G. Wells, A. Stuanes, H.L. Allen, Iowa State University, Ames, IA. Available B. Urrego, K. Harrison, and G. Bonani. 1995. at: http://www.nrcs.usda.gov/Internet/FSE_ Carbon cycling in a loblolly pine forest: DOCUMENTS/stelprdb1167354.pdf. Accessed Implications for the missing carbon sink and the March 30, 2015. concept of soil. In Carbon forms and functions in Williard, K.W.J., D.R. DeWalle, and P.J. Edwards. forest soils. W.W. McFee and J.M. Kelly (Eds.). 2005. Influence of bedrock geology and Soil Science Society of America, Madison, WI. tree species composition on stream nitrate Scharenbroch, B.C., J.E. Lloyd, and J.L. Johnson- concentrations in mid-Appalachian forested Maynard. 2005. Distinguishing urban soils with watersheds. Water, Air, and Soil Pollution 160: physical, chemical, and biological properties. 55-76. Pedobiologia 49: 283-296. Schlesinger, W.H. and J.A. Andrews. 2000. Soil respiration and the global carbon cycle. Biogeochemistry 48: 7–20. Schoeneberger, P.J., D.A. Wysocki, E.C. Benham, and Soil Survey Staff. 2012. Field book for describing and sampling soils, Version 3.0. Natural Resources Conservation Service, National Soil Survey Center, Lincoln, NE. SSSA. 2008. Glossary of Soil Science Terms. Soil Science Society of America. Madison, WI.

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