High School Soil Study Guide PDF

Soils Resource Manual High School Revised August 2023 Updated 8/27/24 1 Key Concept 1: Soil Genesis What is Soil Soil Forming Factors Soil Forming Processes Key Concept 2: Soil Ecology Nutrient Cycles Energy Transfer Ener...

Soils Resource Manual High School Revised August 2023 Updated 8/27/24 1 Key Concept 1: Soil Genesis What is Soil Soil Forming Factors Soil Forming Processes Key Concept 2: Soil Ecology Nutrient Cycles Energy Transfer Energy Flow Soil Ecosystem Key Concept 3: Soil Properties Soil Physical Properties Water Movement in Soils Soil Chemical Properties Soil Horizons Key Concept 4: Soil Classification Soil Surveys Applications Land Capability Classification Soil Recycling Key Concept 5: Conservation and Management of Soil Erosion Problems Facing Farmers Best Management Practices Importance of Soils 2 Soil: A natural, three-dimensional body at the earth's surface. It is capable of supporting plants and has properties resulting from the integrated effect of climate and living matter acting on earthy parent material, as conditioned by relief and by the passage of time. Soil is more than just dirt. Soil is a living, dynamic natural resource that makes up the outermost layer of our planet. There are tens of thousands of different kinds of soil throughout the world. To understand the environment, it is important to study the soil—how it is formed, what it is made of and how it is used. Soil is a record of the geological and climatic history of the region where it is found. It reflects the ancient presence of rocks, or rivers, or oceans. It reveals past climate and ecosystems that historically flourished there. It can also serve as a crystal ball used to view a specific geographic area’s future. Learning Objectives: Gain a basic understanding of the soil ecosystem. Recognize basic soil forming factors. Recognize basic soil properties. Gain a basic understanding of soil biology. Learn how to use a soil survey to find specific information on soils and their use and management. Gain a basic understanding of the Land Capability Classification System. Identify various types of soil erosion. Identify or recommend various types of Best Management Practices for the control of accelerated soil erosion. Learn how basic soil science knowledge is used to make environmentally sound land use decisions when given a set of known facts. Know the importance of soils and be able to give examples. What is soil? Though soil may seem lifeless, it is actually a dynamic mixture, containing organisms ranging from simple bacteria to more advanced forms of life like earthworms and rodents, which supports plant life. Soil has biological, chemical and physical properties that are always changing. It is a naturally occurring mixture of mineral and organic ingredients with a definite form, structure, and composition. In general, the average soil sample is 45 to 50 percent minerals (clay, silt, sand, gravel, stones), 50 percent pore space (air and water, the relative amounts will change depending upon the moisture content of the soil), and 0 to 5 percent organic matter (living and dead organisms). 2 The solid portion of soil is composed primarily of minerals. These minerals are classified according to size. The 3 soil size particles are sand, silt, and clay. These mineral particles give soil its texture. Sand particles range in diameter from 2 mm to 0.05 mm, are easily seen with the unaided eye, and feel gritty. Silt particles are between 0.05 mm and 0.002 mm and feel like flour when dry. Clay particles are smaller than 0.002 mm and cannot be seen with the unaided eye. Wet clay usually feels sticky. Pores Pores – The spaces The voids between the mineral particles are called pores. These small spaces between the rocks are filled with water and air and are essential to the life of the soil organisms and and organic material plants that take their nutrients from the soil. that make up the soil. These spaces are important Two types of pore spaces generally occur in soils, macropores and micropores. because they allow The macropores are the larger pores and they allow air and water to move air and water to through the soil. In micropores, the air movement is greatly slowed, and water penetrate the soil movement is restricted mainly to slow capillary action. In soils with many and reach the roots interconnected macropores, such as a sandy textured soil, the movement of air of plants. and water can be rapid. Fine textured soils with lots of clay particles allow for relatively slow air and water movement. Soil Formation Soil genesis – the An understanding of soil formation processes is a valuable tool used in part of soil science interpreting soils for specific uses. A soil’s properties result from the integrated that deals with the factors and effects of climate and living organisms acting upon parent material as processes of soil conditioned by topography and time. formation. The study of soil formation is called soil genesis. Interpreted literally, soil genesis means explaining the origin of soils. Soil is a natural body. This means that it is formed by the actions of nature. The factors affecting soil formation do not act independently. The type of soil formed is the net result of all five soil forming factors working together or against each other. The 5 factors responsible for soil formation are: Parent Materials Climate Relief or Topography Biological Factors Time/Exposure Parent Materials Parent materials are defined as the material underlying the soil (in the C horizon) from which, in most cases, the soil develops. Parent material influences both the physical and chemical properties of the soil. Most of the mineral matter that makes up soil was ultimately derived from hard rocks. Soils may also form in organic materials such as peat. 3 Parent material can consist of many different materials. Many soils form from weathered bedrock while others form from material that has been transported and deposited by wind, water, glaciers, or volcanoes. Moderately developed and well-developed soils are made when a parent material is changed both chemically and physically over time. These processes proceed more rapidly in parent material containing high proportions of less resistant minerals. In all soils, the availability of nutrients depends on the amount of decomposed material that remains in the soil or is removed in drainage water. Minerals and rocks are important to soils because they are the most abundant materials that weather or break down to form soil. Minerals A mineral is a naturally occurring inorganic body that has a definite internal structure and composition that results in definite physical and chemical properties. A rock is simply a complex mineral aggregate. The mineral groups that are of prime importance in soil development are: Feldspars (photo, orthoclase feldspar) Amphiboles and pyroxenes Micas Silicas Iron oxides Carbonates (photo, calcite) Calcite Rocks Though minerals are the main component of rocks, most soils are actually formed from materials that originated in rocks rather than pure mineral deposits. Rocks are classified by placing them into one of three groups depending on how they were formed. These three groups are igneous, sedimentary, and metamorphic. Knowing how the rocks in a soil sample were formed can help a soil scientist make a broad soil interpretation. Igneous rocks are formed from the cooling of molten materials that have been pushed upward from deep within the earth. These materials fall into one of two categories depending upon where they cooled and became solid rock: extrusive or intrusive. Extrusive igneous rocks are formed from magma (molten rock) that was forced out onto the surface of the earth as either lava or volcanic ash. Once exposed to the air, these molten materials cool rapidly and produce a rock with fine crystals. These crystals are often too small to see without the aid of magnification, giving the rock a nearly uniform color. Scoria, basalt, and tuff are examples of extrusive rocks. Scoria (photo) is a volcanic rock, frequently found on the flanks of volcanoes. Basalt is a dark 4 colored rock that usually forms from magma upwelling in areas of ocean floor spreading. Tuff is an example of an extrusive rock that formed from fine-grained volcanic ash. Intrusive igneous rocks are formed when molten materials are pushed part of the way to the surface of the earth. This molten material or magma often cools very slowly producing larger size crystals in the rock. These crystals are easily visible giving the rock a multicolored appearance. Granite (photo), diorite, and gabbro are examples of intrusive rocks. Sedimentary rocks are formed when sediments and small rocks are cemented together, either chemically or by compression. Although these rocks comprise only five percent of the rock volume of the outer ten miles of the earth’s crust, they cover 75 percent of the Earth’s surface. The coloration of these rocks depends upon the color of the original sediments. Sandstone, shale (photo), and limestone are examples of sedimentary rocks. The deposition of sediments is called sedimentation. The agents responsible for this process are wind, running water, and precipitation. For example, if a strong wind passes over a barren area it may pick up sand and soil particles. The wind will carry these particles as long as it maintains its velocity. However, as weather patterns change, the wind may dissipate. As it slows it will first drop the largest particles it is carrying and as it gradually begins to drop smaller and smaller particles until the wind stops. If this wind pattern prevails for a long period of time, the deposits from it may become quite thick. As the deposits become thicker, they may be cemented into sedimentary rocks. Sedimentary deposits are also formed when running water transports materials. This works much the same way as wind, but water is capable of moving much larger particles than wind. During severe floods, water running at high velocities can move boulders short distances while small rocks and coarse gravel can be carried great distances. As the velocity of the water decreases, the larger particles are left behind. This is why sediments deposited by water are commonly sorted by size. Whenever the sedimentary products of wind, water, or precipitation are cemented together, a sedimentary rock is formed. The composition of the rock formed will depend on the type of material that was transported. A few sedimentary deposits can also result from leaching. Leaching takes place when water moves downward, or precipitates, through the earth's surface and dissolves minerals such as calcite, dolomite, and gypsum. As long as fresh water is moving through the soil from the surface, these minerals will keep dissolving. If, however, the water stops moving, the dissolved materials may become so concentrated that they precipitate out and form a sedimentary deposit. Metamorphic rocks are igneous or sedimentary rocks that have been subjected to heat, chemical activity, and/or pressure to radically alter their characteristics. These processes, called metamorphism, are usually associated 5 with volcanic activity during magma formation or during times when the earth’s crust is changing shape and mountains are being formed. The soils formed from metamorphic rocks are very similar to those formed from the original igneous and sedimentary parent rocks. Gneiss, schist (photo), marble, and slate are examples of metamorphic rocks. Seven General Categories of Parent Material: Residual material Alluvial and marine deposits Colluvial deposits Eolian deposits Organic deposits Glacial deposits Volcanic deposits Residual Material Bedrock such as granite, gneiss, schist, limestone, sandstone, shale, slate, and many others break down into residuum through the weathering process. This residuum becomes the parent material of the soil and imparts some of the parent characteristics into the resulting soil profile. Soils that form in place from the underlying bedrock are known as residual soils. Residual soils are very common in the Piedmont and mountains of North Carolina. Alluvial and Marine Deposits When a soil forms in material that was deposited by water the parent material is known as alluvium. Alluvial deposits usually refer to sediments carried by and deposited in freshwater. Marine deposits are sediments deposited in the ocean. The types of alluvial deposits that are common parent materials are alluvial fans, floodplains and terraces, lacustrine deposits and deltas. Flood Plains The most common alluvial deposit is in floodplains. Streams and rivers commonly overflow their banks and deposit fresh materials on the floodplains. These fresh or recent deposits, commonly topsoil, comprise the parent materials for the soils developed on these floodplains. The type of parent material found on flood plains is referred to as recent alluvium. As a stream flows down a gentle slope, it tends to wander across the landscape in a series of “s” curves. This curving is called meandering. Over a long period of time, this meandering will cause them to have broad, flat 6 bottomlands on both sides. These bottomlands are called flood plains. Every time the stream floods, the rushing water will carry sediments (created by erosion on small feeder streams, bottom scouring, and stream bank erosion) downstream and will overflow the banks and flood the bottomlands. As floodwaters leave the rushing stream channel and spread over floodplains, they immediately lose their velocity and began to drop the sediments they are carrying. The decrease in energy of the water means that the larger soil particles will drop out first and will layer. With successive flooding the old stream channels caused by meanderings are filled with sediments and a broad flat flood plain with alluvial parent materials is formed. Sometimes oxbows are formed when the stream channel becomes blocked by sediment. Oxbows are dead end river channels that have their downstream end blocked. If the upper end also becomes blocked by sediment it is known as an oxbow lake. Since there is new material added almost annually, these soils never have time to form well-developed horizons. These young soils have poorly developed profiles, and most of their character is inherited from the parent material. These soils commonly are stratified (layered) in their lower portions. These distinct layers are a result of sediment deposition from different flood events. Flood plains may contain poorly drained soils and are a common location for wetlands. They provide good habitat for wildlife, help to recharge groundwater, and help filter out pollutants. The composition of flood plain sediments depends on the materials that were eroded. They may contain high amounts of organic matter deposited when the floodwaters recede, soil, and other materials from adjacent woodlands. Flood plain sediments may also be high in nutrients (nitrogen, phosphorus) because of erosion from highly fertilized cropland and from over fertilized urban areas. They can also contain pesticides or other contaminants washed in from farms, streets, parking lots, and other urban areas. 7  River or Stream Terraces Over time, rivers and streams cut downward into the underlying material. Formerly low-lying areas that once flooded frequently are left in higher positions on the landscape that may flood only rarely or not at all. These areas are called river or stream terraces. Soils located on terrace positions have parent materials referred to as old alluvium. These soils were originally deposited by water and commonly have had time to form well-developed horizons. In North Carolina, the largest river terraces are found adjacent to major rivers such as the Roanoke, Tar, Neuse, Cape Fear, and Pee Dee (Yadkin) rivers. Alluvial fans As a stream moves down a mountain it attains a high water velocity and is capable of moving large amounts of sediment. These sediments come from erosion of adjoining soils in the upper watershed, bottom scouring during storm events, and stream bank erosion. When a stream reaches a flat valley, it suddenly loses its velocity or energy and many of the sediments it is carrying settle out at the foot of the slope, the larger particles dropping out first. The areas where these are dropped are called alluvial fans. These alluvial fans are quite common at the foot of mountain slopes in both the Appalachian and Rocky Mountains. Generally, the soils within alluvial fans are well drained and their composition depends on the type of rocks and minerals found on the mountain slopes above the alluvial area. Deltas Whenever a river empties into a large body of water, the wave action is not sufficient to keep the river sediments suspended in the water column and the sediments drop, with the largest particles dropping out first, creating a delta. Deltas are usually swampy and dissected by many small stream channels. They are subject to frequent flooding and some flood control practices may be needed in order to farm them. Because most of the larger, coarse material carried downstream by the stream is generally 8 deposited upstream on the flood plains, deltas usually contain large amounts of clays, silt, and other fine sediments. The deltas of the Mississippi and Nile rivers are the best-known examples of this type of alluvial deposits. Lacustrine Deposits Whenever a river or stream flows into a still body of water such as a lake, any sediment in suspension settles out and is deposited on the lake bottom. Over time these lacustrine deposits can become quite thick, eventually filling up the volume of the lake. Water levels of lakes also fluctuate over time. When water levels drop, these lacustrine deposits can rise above the level of the lake allowing soils to form. Some of the largest lacustrine deposits in the United States are located on the shores of the Great Lakes in areas the lakes once covered. Marine Deposits If stream sediments are not deposited in flood plains or deltas, they eventually find their way into the ocean. Once in the ocean they will be sorted by soil particle size, with the larger particles being deposited either close to the shore or in high energy areas like beaches, and the clays farther out or in low energy areas The Coastal Plain like tidal marshes. Because of wave action and storms, these in NC marine sediments seldom become thick enough to rise above sea level. When they do, they often form barrier islands and tidal marshes. The Outer Banks of North Carolina are an example of this. When continental uplift occurs and or ocean levels fall, these marine sediments are often exposed. Along the Atlantic Coast and the Gulf of Mexico, a strip of those marine sediments has formed a flat broad coastal plain as much as 50 – 150 miles wide in many places. In eastern North Carolina, this large area of marine deposits is known as the Coastal Plain. These marine deposits are very old in relation to the alluvial deposits of fans, floodplains, and deltas. Because of the long amount of time they have been there, these marine deposits often have thick, well developed horizons. Colluvial Deposits In areas with long steep slopes, such as the mountains, soil material and rock fragments may move downhill under the influence of gravity and water. This disorganized mass of material that generally accumulates on the lower portion of slopes and in depressions is called colluvium. Unlike the rounded water worn rock fragments found in alluvium, rock fragments found in colluvium are generally angular in shape. In the North Carolina mountains, colluvial soils are found in areas locally called coves. 9 Eolian deposits Materials that were deposited due to the actions of the wind are known as eolian deposits. Two major types of eolian deposits are dune sands and loess. A third type of deposit carried by the wind, volcanic ash, will be discussed later. Dune sandsare areas where strong winds have picked up sand grains and piled them into hills of sand called dunes.. Unless they are well vegetated, these dunes are constantly shifting in size and shape in response to prevailing winds. In North Carolina, the sand dunes seen at the beach are a good example of an eolian soil. Jockey’s Ridge (photo), located in Nag’s Head, North Carolina is the tallest sand dune system in the eastern United States. The Sandhills area of North Carolina also contains soils that may have once been coastal sand dunes millions of years ago. Loess is another type of eolian deposit found in the midwestern area of the United States. Loess deposits consist of windblown silts that originated in the broad flood plains of the Mississippi and other rivers and were carried by the wind into the valleys. Loess derived soils are highly productive agricultural soils, but they present problems for engineers since they will shift and slide under stress and flow when wet. Organic deposits Organic deposits form in swamps and marshy areas. As plants die and shed their leaves, the remains that are submerged in water decompose very slowly. Over the years they will accumulate and gradually fill in the low-lying area. Organic deposits are grouped into peat and muck. Peat contains identifiable portions of organic matter. In muck, the organic matter is decomposed to the point it cannot be identified. In North Carolina, organic soils can be found in very poorly drained areas of the Coastal Plain. Glacial deposits In the last million years there have been four major glacial periods. During this time massive sheets of ice, often as much as 2-3 miles thick spread over the northern parts of North America, Asia and Central Europe, sweeping existing soils and rocks ahead of them. When the glaciers retreated, the transported rocks and soil were left behind and became parent material for some of today’s soils. This is called glacial drift. There are two types of glacial drift, glacial outwash and glacial till. 10 Glacial outwash is the material washed away from the glacier by water from the melting ice. Soil made from this material is coarse and contains a lot of gravel. Glacial till is the material deposited when the glacier receded. It is finer and contains a lot of clay. Volcanic Deposits During volcanic eruptions large amounts of material can be ejected from a volcano. Depending upon the type of volcano, this material usually takes the form of volcanic ash or lava. Volcanic ash consists of cinders that are carried by the wind. Coarse to medium size particles fall in the immediate vicinity of the volcano while fine ash particles may travel hundreds of miles. Large to small size rocks can also be ejected during explosive eruptions. The Cascade Range found in the northwestern part of the United States is an example of this type of volcanic deposit. Some volcanoes eject large amounts of lava rather than ash. In these volcanoes, lava flows down the flanks of the volcano onto the nearby land. The volcanoes which formed the Hawaiian Islands are examples of these. Climate The term climate refers to both temperature and rainfall. The effects of climate result in weathering. Weathering is the breakdown or disintegration of rock at or near the earth's surface, by the actions of nature. Temperature and water are major climatic forces that influence weathering. The forces of weathering may be mechanical (physical) or chemical processes. 11 Mechanical weathering consists of natural forces physically breaking rocks into smaller pieces. Examples of these forces are temperature, wind, ice, water, and plant roots. Rapid temperature changes can cause expansion and cracking of rocks. Wind may literally sandblast the rock surfaces away. Ice causes rocks to break up by expanding in crevices and cracks, and ice in large amounts (glaciers) can grind rocks into dust if it is forced to slide over them. Running water is quite abrasive as it either moves sediments over rocks or as it carries rocks downstream. As plant roots grow, they may extend into cracks in rocks and cause breaking and further cracking. Although these mechanical forces operate quite slowly, they are very effective over long periods of time. Mechanical weathering occurs fastest in cool dry regions. Chemical weathering is the process by which rocks and minerals decompose due to chemical reactions. The rate of chemical weathering is influenced by water, oxygen, and the presence of organic and inorganic acids resulting from biochemical activity. The rate of chemical weathering increases in hot moist climates, while it progresses slowly in cool dry climates. The climate in North Carolina is favorable to a high amount of chemical weathering in our soils. Examples of chemical weathering include carbonation, dissolution, hydration, hydrolysis, and oxidation- reduction reactions. Relief or Topography Topography relates to the shape and of slope of the land, as well as the elevation and landscape position. Soils vary with topography because of its influence on soil moisture, erosion rate, and soil temperature. Topography can delay or hasten weathering caused by climatic forces. Soils on steep slopes tend to be drier than in flatter areas due to increased amounts of runoff. Soils in low-lying areas tend to be wetter due to little runoff and an influx of water from adjacent higher areas. On steep slopes, erosion tends to prevent topsoil from accumulating because soil particles are washed away almost as quickly as they are formed in these areas, the topsoil layer may be thin. On gentle slopes there is a lower erosion hazard, resulting in thicker amounts of topsoil. In footslope and toeslope positions, topsoil tends to be thicker due to accumulations from higher lying areas. 12 In steep areas such as the mountains, the direction of the slope can influence soil temperature. Steep, north-facing slopes are cooler, moister, and have higher amounts of organic matter in the topsoil than slopes facing south or west. Biological Factors Both plants and animals help to create a soil. As they die, plants and animals add organic matter to weathered parent material to help form subsoil and topsoil. Plant roots also alter the soil. When discussing soil formation, there are two important categories of vegetation – forest and grasses. Areas that have been in forests for a considerable time often have thin O horizons as a result of falling leaves and limbs. These leaves and limbs decompose slowly and allow this organic layer to remain on the soil surface. In contrast, areas that have been in grassland for a considerable time usually lack an O horizon. Dead grasses decompose too rapidly to form an O horizon. Instead they usually have rich, dark A horizons due to the thick surface root system, which is constantly dying and growing resulting in the constant recycling of plant nutrients. Animals also play a role in soil formation. As animals dig through the soil, they break it up, permitting more air and water to enter. They also mix the organic matter throughout the soil. Microorganisms such as bacteria and fungi play an important role in the decomposition of organic matter. Small soil inhabiting organisms such as earthworms and nematodes enrich soil by breaking down organic matter into simpler nutrients. When nutrients are returned to the soil through decomposition, they once again become available to plants, and through plants, to animals. The actions of plants and animals are vital to the biological recycling of nutrients. Organic Matter Plants produce organic compounds through photosynthesis using the energy of sunlight to combine carbon dioxide from the atmosphere and water from the soil. Soil organic matter is created by the cycling of these organic compounds in plants, animals and microorganisms into the soil. The organic matter of the soil is largely derived from the decomposing bodies of plants and animals, as well as animal wastes. Well-decomposed organic matter forms humus, a dark brown, porous spongy material that has a pleasant, earthy smell. In most soils, the organic matter accounts for about five percent or less of the total volume. Maintaining humus is an important aspect of good soil management. Humus is important because it: Improves soil structure Increases pore space making it easier for air and water to penetrate the soil Reduces the soil’s susceptibility to erosion Increases the workability of the soil Minimizes the leaching of nutrients Provides a suitable medium for valuable soil organisms, like bacteria, fungi, and earthworms 13 The amount of soil organic matter is controlled by a balance between additions of plant and animal materials and losses by decomposition or erosion. Both additions and losses are very strongly controlled by management activities. The portions of total plant biomass that reach the soil as a source of organic matter depends largely on the amounts consumed by mammal and insects, destroyed by fire, washed away by water, or harvested for human use. When soils are tilled, organic matter decomposes faster because of the changes in water, aeration, and temperature conditions. The amount of organic matter lost after clearing a wooded area or tilling native grassland varies according to the kind of soil, but most organic matter is lost within the first 10 years. Rates of decomposition are very low at temperatures below 38 degrees F (4 degrees C) but rise steadily with increasing temperature to at least 102 degrees F (40 degrees C). Losses are higher with aerobic decomposition (with oxygen) than with anaerobic decomposition (without oxygen). Available nitrogen also promotes organic matter decomposition. Soil microorganisms also use soil organic matter for the energy they need to support their own life. Some of the energy they produce is actually incorporated into the microbes, but most is released through respiration as carbon dioxide and water. Some nitrogen is released in gaseous form, but some is retained along with most of the phosphorus and sulfur. Some of the energy is lost as heat. Organic matter is an essential component of soil because it: Provides a carbon and energy source for microbes Stabilizes and holds soil particles together, reducing the chances of erosion Aids the growth of crops by improving the soil’s ability to store and transmit air and water Stores and supplies such nutrients as nitrogen, phosphorus, and sulfur, that are needed for plant and animals’ growth Retains nutrients Helps the soil resist compaction Makes soil more friable, less sticky, and easier to work Retains and stores carbon from the atmosphere and other sources Reduces the negative environmental effects of pesticides, heavy metals, and many other pollutants by storing them. Provides a food source for valuable soil organisms that help to maintain a healthy soil. Soil organic matter also improves tilth in the surface horizons, reduces crusting, increases the rate of water infiltration, reduces runoff, and facilitates penetration of plant roots. 14 Time The length of time that a soil’s parent materials have been exposed to the forces of mechanical and chemical weathering and the other soil forming factors will greatly influence the kinds of soils present today. A typical soil’s age must be measured in thousands of years. It may actually take hundreds of years for these soil-creating factors to form one inch of soil from parent material. Over time, soils exhibit features that reflect the other forming factors. Soil formation processes are continuous. Recently deposited materials, such as deposition from a flood, exhibit no features from soil development activities. The previous soil surface and underlying horizons become buried. The time clock resets for these soils. Terraces above the active floodplain, while genetically similar to the floodplain, are older land surfaces and exhibit more development features. Older soils usually have deeper well-developed soil profiles with thicker A and B horizons, while young soils still retain many characteristics of their parent material. Older soils are often less fertile than young soils, having experienced a loss of nutrients due to leaching. The soil forming factors continue to affect soils even on stable landscapes. Materials are deposited on and blown or washed away from the surface. Additions, removals, and alterations are slow or rapid, depending on climate, landscape position, and biological activity. Soil Forming Processes The four major processes that change parent material into soil are additions, losses, translocations, and transformations. Additions are anything that is added to the soil profile from outside sources. These additions may include organic matter, atmospheric dust, and soluble nutrients in rainfall and groundwater. As soon as plants begin to grow in soil, organic matter begins to accumulate. Organic matter gives a black or dark brown color to the surface layer. Most organic matter additions to the soil increase the cation-exchange capacity (CEC) and nutrients, which also increase plant nutrient availability. Other additions may come with rainfall or deposition by wind, such as the windblown or eolian material. On average, rainfall adds about 5 pounds of nitrogen per acre per year. By causing rivers to flood, rainfall is indirectly responsible for the addition of new sediment to the soil on a flood plain. Nutrients may also be added to the soil from the lateral flow of groundwater. Losses occur from leaching as well as wind and water erosion. In moist climates leaching causes the greatest losses. Water moving through the soil can dissolve minerals and transport them into deeper layers. Some materials, especially sodium salts, gypsum, and calcium carbonate, are relatively soluble. They are removed early in the soil's formation. As a result, soils in humid regions generally do not have carbonates in the upper horizons. Quartz, aluminum, iron oxide, and kaolinitic clay weather slowly. They remain in the soil and become 15 the main components of highly weathered soil. Nitrogen, a common ingredient in fertilizers is readily lost by leaching, either by natural rainfall or by irrigation water. Long-term use of fertilizers based on ammonium may cause acidity in the soil and contribute to the loss of carbonates in some areas. Oxygen, a gas, is released into the atmosphere by growing plants. Carbon dioxide is consumed by growing plants but lost to the soil as fresh organic matter decays. When soil is wet, nitrogen can be changed to a gas and lost to the atmosphere. Soil particles and organic matter can be lost by wind and water erosion. Such losses can be serious because the material lost is usually from the topsoil, which is the most productive part of the soil profile. Translocations are the movement of organic and inorganic matter within a soil profile. As water moves in the soil, material can be transported by it. Organic matter can move downward through cracks and root channels. Clay particles also migrate downward in a soil. The A and E horizons in a soil are zones of clay losses while the B horizon is a zone of clay accumulation. In low rainfall areas, leaching often is incomplete. As water moves down through the soil it begins to dissolve soluble minerals. However, when there is not enough water to move throughout the soil profile, the water stops moving and evaporates, leaving salts behind. Soil layers with calcium carbonate or other salt accumulations form this way. If this cycle occurs frequently enough, a calcareous hardpan can form. Upward and lateral translocation is also possible. In arid climates, evaporation at the surface causes water to move upward. Salts that are dissolved in solution will move upward with water to be deposited on the surface as the water evaporates. However, arid environments may have soils with high water tables if the area is in a low-lying location. Transformations are changes that take place in the soil such as mineral weathering and organic matter breakdown. Microorganisms that live in the soil feed on fresh organic matter and change it into humus. Chemical weathering changes parent material by destroying or changing existing minerals. Many of the clay-sized particles in soil are actually new minerals that form during soil development. Key Concept 2: Soil Ecology Nutrient Cycles Nitrogen, phosphorus, and potassium along with other essential elements are called macronutrients because plants need large amounts of these elements. Minor quantities of more than a dozen other elements (micronutrients) are also required for certain plant processes. An example of a micronutrient is manganese. Manganese (Mn) is an essential plant mineral nutrient that is particularly important in the process of photosynthesis. Ecosystems maintain themselves by cycling macronutrients and micronutrients. These nutrient cycles are critical to the maintenance of all aspects of ecosystems. 16 Energy Transfer (see Appendix: General Scientific Concepts for High School; Energy in Ecosystems p. 4) Plants (producers/autotrophs) fix much more energy from light than they require. As a by-product, they release much more oxygen than required for respiration. The excess oxygen produced during photosynthesis permeates pore space in soils and is used by consumers (heterotrophs) for aerobic respiration to break down energy-rich molecules and release energy. The excess energy and oxygen from plants maintain the soil community. One of the largest sources of energy in the soil comes from fallen leaves, twig, and other dead plant and animal material added as litter to the soil surface. Some organisms, like parasitic nematodes, feed directly on the autotrophs and eat their roots. Other organisms do not eat the green plants directly but eat the heterotrophs that eat the plants. Microorganisms decompose the litter and create available nitrogen, phosphorous, potassium, and carbon for plants and other fauna. Almost all soil organisms are heterotrophs. Energy flow (see Appendix: General Scientific Concepts for High School; Energy in Ecosystems p. 4) Energy flows to and from the biosphere to the soil ecosystem and is distributed at several feeding (trophic) levels. Of the energy required by the root-eating nematode, some is used for growth, body maintenance, heat, reproduction, and mobility. What is not used is excreted or stored. Excreted material is used as an energy source by other microorganisms, such as fungi and bacteria. Some of the energy is stored in the body of nematodes as part of their body weight and can be transferred to the next trophic level when the nematode is eaten. Since most of the organisms in the soil are heterotrophs, the soil ecosystem can be completely self- sufficient. Energy fixed by autotrophs outside the soil ecosystem must be brought into it by various routes, either litter feeders that drag or incorporate surface material into the soil or through input from plant roots through die back, or by consumption from root eaters. 17 Soil Ecosystem The soil ecosystem is a group of interrelated biotic (biological factors, living or dead) interacting with abiotic (non-living factors). These components are related so that a change in any one factor results in changes in all the other factors. The biotic component is composed of living plants and animals represented by soil flora and fauna (bacteria, fungi, springtails, nematodes, earthworms, and arthropods) and also includes organic material from dead plants, animals, fungi, etc. The non-living components are the physical and chemical properties of soil (for example, minerals, water, gases, pH, temperature, and nutrients). Soils are dynamic natural resources, teeming with life. One teaspoon of soil can contain billions of organisms from simple bacteria to fungus to more advanced forms of life like earthworms, insects, and spiders. Roles within the Ecosystem (see Appendix: General Scientific Concepts for High School; Energy in Ecosystems p. 5) Bacteria and fungi are the two main kinds of decomposers and occur in enormous quantities in soil ecosystems. Transformers are a special group of decomposers that break down large organic molecules into molecules small enough to be absorbed by plant roots. This final breakdown step in the decomposition of detritus is critical to the survival of terrestrial ecosystems because it releases macro and micronutrient elements required by plants. To maintain natural ecosystems, it is necessary to recover essential nutrients, stored in the detritus so that plants may recycle them. The most important activity of soil organisms is acquiring energy. As a result, the chief interactions between species are eating and being eaten. This is called a food chain. Leaf— springtail—centipede is a common food chain in soil ecosystems. Key Concept 3: Soil Properties Soil Properties Soils have both physical and chemical properties. When analyzing a soil’s suitability for a specific use, soil scientists must first look at the properties or physical makeup of a given soil sample. There are over 20,000 different types of soils on earth. There are many characteristics that differentiate one soil from another. For example, soil fertility, soil texture, soil color, and permeability are all used to describe different types of soil. The important physical properties of soil include color, texture, structure, consistence, shrink- swell, compaction, soil depth, permeability, internal drainage, and available water capacity. Physical characteristics are the easiest to observe. By looking at a few physical characteristics several generalizations can be made about a soil. Two important physical traits are color and texture. The important chemical properties of soil include reaction and cation-exchange capacity. 18 Color Soil color is a useful tool for providing information about other soil properties. The relative amounts of organic matter, the presence and abundance of certain elements and minerals, and the depth and duration of water tables can all be inferred by soil color.. When judging the color of a soil, most soil scientists use the Munsell Soil Color Chart to identify the soil color. This allows soil scientists to uniformly describe soil color using a standardized chart. In the field, soil scientists compare the colors found in the soil with the color chips found on the chart. On this chart, color chips are categorized by hue, value, and chroma. Names are also assigned each color. The photo on this page shows one page of the Munsell Soil Color Chart (10YR). Most color books have between 7 and 9 pages ranging from red (10R) to yellow (5Y). Soils in very wet areas like wetlands may require additional pages (Gley 1, Gley 2) to color correctly. Soil scientists use the Munsell Soil Color Chart to determine soil color. It is best to use a Munsell Color Chart using natural lighting. Colors can appear differently under artificial lighting, especially florescent lights. Organic matter and humus are dark in color and can give soil a blackish or brownish color when present in sufficient quantities. It is this higher organic matter and humus content that gives most topsoil a darker color than the underlying subsoil. Soils that are black or very dark brown usually have high amounts of organic matter. Lighter colored topsoil contains lesser amounts of organic matter. The element iron is also a powerful coloring agent. The presence of significant amounts of iron oxides (ferric form) usually gives soils a yellowish, brownish, or reddish color, depending upon the type of iron minerals that are present. Bright red soils are usually high in iron and usually do not have any significant drainage problems. Other elements such as manganese can also alter soil color. When found in sufficient amounts, manganese can give soil a brown or black color. Often manganese can be seen as a coating in cracks or in discrete masses such as nodules. In soils with drainage problems, some horizons may be saturated with water for significant 19 periods during the year. In these soils, the iron oxides lose some of their oxygen and are referred to as being reduced. The reduced form of iron (ferrous) is mobile and can be removed from the soil by leaching. After the iron is gone, the leached area generally has a grayish color. Repeated cycles of saturation and drying create a mottled look to the soil (several different colors intermixed). Part of the soil is gray because of the loss of iron (iron depletion), and part is a redder, yellower or browner color where the iron oxide has accumulated (iron concentration). When this mottled pattern of gray along with red, yellow, or brown colors is seen in a soil, it is referred to as redoximorphic features. Soil scientists use the depth to redoximorphic features to make determinations about the depth and duration of soil water tables. Soils saturated for only short periods may only have a few iron depletions. Soils that are saturated for very long periods frequently have had most of their iron leached from them and may have a gray color throughout. This is commonly seen in soils found in wetlands. During long periods of saturation, gray lined root channels can also develop. This may indicate a possible loss of iron or an addition of humus from decayed roots. Redoximorphic Features Iron Iron depletion depletion Iron Iron concentration concentration Gray Gray lined lined root root channel channel 20 Soil Texture Texture is the size and shape of the individual soil particles, as well as the proportions in which they occur. The USDA classifies soil particles in categories of diminishing size as sand, silt, and clay. Despite the dynamic nature of soils and the continuously physical, chemical, and biological activity, gravel will not turn to sand or silt to clay within our lifetime. Particle Size Soil is composed of three primary particle sizes. They are sand, silt, and clay. Sand particles are the largest and can easily be seen. Sand feels gritty when rubbed between the fingers. Silt is the medium sized particle and is so small that an individual particle is barely visible. Silt feels silky when wet and like flour when dry. Clay is the smallest sized particle and is microscopic. Clay feels sticky when wet and gives a soil its plasticity. Particle Size Sand 2.0-0.05 mm Silt 0.05-0.002 mm Clay 60" 28 Water Movement in Soils Understanding the movement of water through soils is important for determining the suitability of a soil for all types of land uses whether it is agriculture, forestry, or urban development. Soil properties such as texture, structure, and the type and amount of pores have a great influence on water movement. Capillary water – Unlike gravitational water, capillary water does not flow through the soil. It occurs as water flows around individual soil particles and moves to the highest points of tension. Plant roots will remove capillary water until the attraction between the water molecules and the soil particles becomes stronger than the attractive forces of the roots/root hairs. When this happens, water is not available for the plant to use and the permanent wilting point is reached. Temporary wilting may occur when the water film gets thin and the capillary movement of water slows down. Gravitational water – Gravitational water is free water. This means it simply flows through or off the soil. Gravitational water is responsible for nutrient leaching and soil erosion. To avoid problems with leaching, it is essential that nutrients be applied at the right time when plants need them. Otherwise, they could be leached below the plant root zone. This is very important with sandy soils. When applying irrigation water, you should regulate flows so that leaching, and erosion do not occur. Capillary rise of water – Capillary rise occurs when water from a water table moves upward through the soil against the force of gravity. This occurs because the attraction between the soil particles and water molecules is greater than the force of gravity. The water or hydrologic cycle begins with rain striking the soil surface where it can infiltrate the soil, run off the soil surface, or evaporate. (see Appendix: General Scientific Concepts for High School p. 2-4) Water that is not utilized by plants continues to move through the soil. Some of this water flows laterally, eventually surfacing as a spring or seep. Some water continues downward, eventually recharging the groundwater. Not all precipitation enters the soil; some of it is lost as surface runoff. This water flows overland, eventually entering streams and rivers. 29 Permeability Permeability refers to the movement of air and water within the soil. Permeability rate is the rate at which a saturated soil transmits water, usually expressed in inches per hour. Texture, structure, bulk density, and the type and connectivity of macropores influence permeability. Hydraulic conductivity is a measurement of the amount of water that can move downward through a unit area of unsaturated soil in a unit of time. Saturated hydraulic conductivity (KSAT) is a measurement of the amount of water that can move downward through a unit area of saturated soil in a unit of time. Both hydraulic conductivity and saturated hydraulic conductivity are usually measured in terms of inches per hour. In sandy soils, the macropores are large and continuous, allowing water to rapidly move downward through the soil. In soils with high clay contents, the macropores are often small and discontinuous, resulting in slower permeability. The swelling of clay also decreases permeability. As clay gets wet and begins to swell, the size of macropores decreases. Soils high in expansive clays may swell enough to close these pores altogether. Structure also affects permeability by influencing the path by which water can flow through the soil. Granular structure with its large number of interconnected macropores readily EFFECT OF SOIL STRUCTURE ON WATER FLOW permits downward movement of water. In platy structure, water is required to flow over a longer and slower path. Soils with platy structure usually have slow to very slow permeability. Horizons with prismatic structure tend to have high amounts of clay, which swells upon wetting to close most macropores. In a soil survey, the “Physical and Chemical Properties” table gives information about the permeability of soils. Granular Prismatic Blocky Platy Infiltration Infiltration is the downward entry of water into the immediate surface of the soil. Infiltration rate is the rate at which water penetrates the surface of the soil at any given instant, usually expressed in inches per hour. Like permeability, infiltration is influenced by texture, structure, bulk density, and the type and connectivity of macropores. 30 Soils with a high infiltration rate are resistant to erosion because there is little runoff. In areas where drought is a problem, a high infiltration rate maximizes the amount of rainfall or irrigation that enters the soil. The infiltration rate of a soil can be maintained or enhanced by adding organic matter and keeping the soil surface sheltered from the force of raindrops by the use of cover crops, crop residue, or mulch. Soils left bare often form a crust on their surface which inhibits the ready infiltration of water. Soil Crust Drainage Drainage refers to the frequency and duration of periods of saturation or partial saturation. Internal soil drainage is important because of its effect on land use and management decisions. The internal drainage of soils is related to the depth of any water tables. A water table is a zone in the soil that is saturated with water for significant periods during the year. The depth and duration of these water tables is influenced by many factors including landscape position and permeability. The presence of redoximorphic features is an indication that a soil has a high water table. There are two types of water tables, apparent and perched. An apparent water table is a type of saturation in which all horizons between the upper boundary of saturation and a depth of 6 feet are saturated. A perched water table is a water table in a soil in which saturated layers are underlain by one or more unsaturated layers within 6 feet of the surface. Apparent water tables are frequently encountered in low-lying or flat areas where the slope of the land is insufficient to provide good drainage. Flood plains, depressions, and flat areas of the Coastal Plain of North Carolina are common areas for apparent water tables. Perched water tables are often caused by a soil horizon that has slow or very slow permeability. Water moves downward in these soils until it encounters the slowly permeable layer. When water enters the soil faster than it can move though this restrictive layer, the zone immediately above becomes saturated, while the zone below the restrictive layer is still unsaturated. Perched water tables are common in soils that have clayey subsoils and poor structure. The depth to soil water tables tends to fluctuate throughout the year. Most of this is due to changes in the amount of rainfall and the rate of evapotranspiration. In North Carolina, the highest water tables are normally encountered in the winter and early spring. In a soil survey, the “Soil and Water Features” table gives information about the kind, depth, and duration of soil water tables. 31 Drainage Classes The seven classes of natural soil drainage are: excessively drained, somewhat excessively drained, well drained, moderately well drained, somewhat poorly drained, poorly drained, and very poorly drained. The exact definition of what constitutes each drainage class differs for each state. A general definition of each follows: Excessively drained – Water is removed very rapidly. The occurrence of internal free water commonly is very rare or very deep. The soils are commonly coarse-textured and have very high hydraulic conductivity or are very shallow. Somewhat excessively drained – Water is removed from the soil rapidly. Internal free water occurrence commonly is very rare or very Well Drained Soil deep. The soils are commonly coarse-textured and have high saturated hydraulic conductivity or are very shallow. Well drained – Water is removed from the soil readily but not rapidly. Internal free water occurrence commonly is deep or very deep; annual duration is not specified. Water is available to plants throughout most of the growing season in humid regions. Wetness does not inhibit the growth of roots for significant periods during most growing seasons. Moderately well drained – Water is removed from the soil somewhat slowly during some periods of the year. Internal free water occurrence commonly is moderately deep and may persist for short to long periods. The soils are wet for only a short time within the rooting depth during the growing season, but long enough that most plants intolerant of wetness are affected. They commonly have a Moderately Well Drained Soil moderately low or lower saturated hydraulic conductivity in a layer within the upper 6 feet. Somewhat poorly drained – Water is removed slowly so that the soil is wet at a shallow depth for significant periods during the growing season. The occurrence of free water commonly is shallow to moderately deep and may persist for short to long periods of time. Wetness markedly restricts the growth of plants intolerant of wetness, unless artificial drainage is provided. The soils commonly have one or more of the following characteristics: low or very low saturated hydraulic conductivity, a high water table, or additional water from seepage. Poorly drained – Water is removed so slowly that the soil is wet at shallow depths periodically during the growing season or remains wet for long periods. The occurrence of free water is shallow or very Poorly Drained Soil 32 shallow and may persist for moderate to very long periods. Free water is commonly at or near the surface long enough during the growing season so that most plants intolerant of wetness, cannot be grown unless the soil is artificially drained. The soil, however, is not continuously wet directly below plow-depth. Free water at shallow depth is usually present. This water table is commonly the result of low or very low saturated hydraulic conductivity. Very poorly drained – Water is removed from the soil so slowly that free water remains at or very near the ground surface during much of the growing season. The occurrence of internal free water is very shallow and may persist for very long periods or be permanent. Unless the soil is artificially drained, most plants intolerant of wetness cannot be grown. The soils are commonly level or depressed and frequently ponded. In a soil survey, information about a soil’s drainage class can be found in the map unit descriptions and the series descriptions. Flooding Flooding is the temporary covering of the soil surface by flowing water and is caused by overflow from streams or by runoff from adjacent slopes. Shallow water standing or flowing for short periods after rainfall or snowmelt is not considered flooding. Standing water in marshes and swamps or in closed depressions is considered to be ponding. In a soil survey, the "Water Features" table gives the frequency and duration of flooding and the time of year when flooding is most likely to occur. Flood Frequency None: Flooding is not probable Rare: Flooding is unlikely but is possible under unusual weather conditions (the chance of flooding is nearly 0 percent to 5 percent in any year) Occasional: Flooding occurs infrequently under normal weather conditions (the chance of flooding is 5 to 50 percent in any year) Frequent: Flooding occurs often under normal weather conditions (the chance of flooding is 50 percent in any year). Flood Duration Very brief: less than 2 days Brief: 2 to 7 days Long: 7 to 30 days Very Long: more than 30 days 33 Soil surveys list the time of year that flooding is most likely to occur. About two-thirds to three- fourths of all floods occur during the stated period. The information on flooding is based on evidence in the soil profile, namely thin strata of gravel, sand, silt, or clay deposited by floodwater; irregular decrease in organic matter content with increasing depth; and little or no horizon development. Also considered is local information about the extent and level of flooding and the relation of each soil on the landscape to historic floods. Information on the extent of flooding based on soil data is less specific than that provided by detailed engineering surveys that delineate flood-prone areas at specific flood frequency levels. In a soil survey, the “Soil and Water Features” table provides information about the frequency, duration, and typical months of flooding. Available Water Capacity An estimate on the amount of water a soil can hold and release for use by plants is the available water capacity. Available water capacity is measured in inches of water per inch of soil. It is influenced by soil texture, content of rock fragments, depth to a root- restrictive layer, organic matter content, and compaction. The size and grade of soil structure can influence the availability and the rate of water released to plant roots. Available water capacity is estimated by calculating the difference between the amount of soil water at field capacity and the amount at wilting point. Field capacity is the amount of water that soil can hold after the gravitational water has drained away. Wilting point is the amount of water held by a soil that is beyond the ability of most plants to extract. The available water capacity in soils is affected by the soil texture and the amount of rock fragments. Sandy soils tend to have low available water capacities. Clayey soils tend to have medium available water capacities. Silty soils tend to have high available water capacities. Since rocks to not contain any available water, the greater the volume of rock fragments in the soil, the lower the available water capacity. In a soil survey, the “Physical and Chemical Properties” table provides information about available water capacity. 34 Reaction Reaction is a measure of acidity or alkalinity of a soil. Acidity or alkalinity is determined by the amount of hydrogen and hydroxyl ions in the soil. When hydrogen ions outnumber hydroxyl ions the soil is acidic. In the reverse condition the soil is basic. A pH scale is used to measure the level of acidity or alkalinity. A pH of 7 is neutral. In North Carolina, the natural soil pH will range from 4.0 to 8.0 with most soils tending to be acidic with pH values of 4.5 to 6.0. In the western part of the United States, where the climate is much drier, pH values tend to be much higher. Plants such as azaleas, blueberries, camellias, ferns, pines, rhododendron, spruce, and firs prefer a soil with a pH of 4.0 to 5.0. Most agricultural and ornamental plants prefer a pH of 5.5 to 7.0. Asparagus and sagebrush tolerate soils with a pH 7.0 to 8.0. Above 8.5 the soil is too alkaline for most plants and soil with a pH less than 3.5 is too acid. The pH in a soil will vary by layer. In order to optimize plant growth, farmers and homeowners commonly adjust the pH of a soil by adding soil amendments. Lime, made from ground limestone, is the most common ingredient used to raise the pH of the soil. The amount of lime that needs to be added should be determined by a soil test. The availability of essential plant elements changes as the pH rises or falls. In a soil survey, the “Physical and Chemical Properties” table provides information on soil reaction. 35 Acid deposition (acid rain) Acid rain is a serious environmental problem. Although referred to as rain, the term actually means any kind of precipitation with a low pH. The severity of the problem often depends on the soil pH. If soil is alkaline, acid “rain” is neutralized. If a soil is acid, then acid “rain” can severely increase soil acidity. This lowering of soil pH can damage plants and other living organisms. In the United States, acid rain is mainly a problem in the east where many of the soils are naturally acidic. Cation-Exchange Capacity (CEC) Cation-exchange capacity is a measure of the ability of a soil to hold and exchange cations. It is one of the most important chemical properties of soil and is usually closely related to soil fertility. A few of the plant nutrient cations that are part of CEC include calcium, magnesium, potassium, iron, and ammonium. Generally, as CEC levels decrease, more frequent and smaller applications of fertilizers are desirable. Smaller applications of fertilizer applied to soils that have low CEC levels may reduce fertilizer loss to surface and ground waters, lessening the impact on water quality. The ion exchange between clay and plant roots – The positive ions of nutrient elements like potassium (K), calcium (Ca), and magnesium (Mg) are attracted to the negatively charged surface of the clay particle. These nutrient ions are replaced by hydrogen ions from the root systems of plants and then are absorbed by the plants. It is the role of the clay particle that makes it so valuable to the farmer in crop production. 36 Soil Horizons A soil profile is a vertical cross section of all the soil horizons at a particular location. Soil horizons are natural layers of soil that usually run approximately parallel to the surface. Layers of different kinds of soil can commonly be seen on road cuts and A horizon other excavated areas. In older well-developed soils, as many as five or six master horizons may be found in the soil profile. Younger less developed soils may have only two E horizon master horizons. Master horizons that may be found in soils include O, A, E, B, C, and R. The thickness of each layer varies with location. Under disturbed conditions, such as urban areas, intensive B horizon agriculture, or where erosion is severe, not all horizons will be present. O horizon – When found, it is generally the uppermost layer of the soil and is predominantly made up of organic material. It consists of leaves, needles, twigs, moss, lichens, and other accumulations of organic matter in various stages of decay. This horizon is often found in wetlands and in forested areas. Due to soil mixing during cultivation it is not present in cultivated fields. C horizon A horizon – Commonly called topsoil, this horizon frequently has friable granular structure. Because of its higher organic matter content, it is usually darker than the lower layers and is often the most fertile layer in the soil. This is the layer that is plowed in cultivated fields and is where most root activity occurs. As water seeps through this horizon dissolved materials leach from the topsoil. E horizon – This horizon is characterized by its light color or bleached appearance and is a zone of leaching. Dissolved minerals, nutrients and clay migrate downward as water passes through this horizon. The main feature of this horizon is the loss of clay, iron, aluminum, humus, or some combination of these, leaving a concentration of sand and silt particles. This horizon is commonly found in older well-developed soils in woodlands. It is rarely found in cultivated areas since the act of plowing usually mixes this horizon with the A horizon. B horizon – Commonly called subsoil, this layer is usually lighter in color than the A horizon due to its lower content of organic matter. It is the zone of accumulation for materials leached from the A and E horizons. In older well-developed soils this horizon commonly has the highest clay content. C horizon - This horizon is the transition layer between soil and parent material. This layer is less weathered than the upper horizons and contains partially disintegrated or weathered parent material brought to the area by glaciers, wind, water or from the underlying bedrock. 37 R horizon – This horizon is bedrock. As the bedrock weathers, it contributes parent material to the C horizon above. Bedrock can be within a few inches of the surface or many feet below the surface. In areas of eolian and alluvial soils where the bedrock is very deep and below normal depths of observation, an R horizon is not described. In a soil survey, the series description shows which horizons are typically found in a particular soil. Key Concept 4: Soil Classification Soil Classification The National Cooperative Soil Survey identifies and maps over 20,000 different kinds of soils in the United States. Most soils are given a name, which generally comes from the locale where the soil was first mapped. Named soils are referred to as soil series. Soils are named and Soil Order Formative Terms Pronunciation classified on the basis of Alfisols Alf, meaningless syllable Pedalfer physical and chemical Andisols Modified from ando Ando properties in their Aridisols Latin, aridies, dry Arid horizons. The official book Entisols Ent, meaningless Recent used for classifying soils is Gelisols Latin gelare, to freeze Jell Soil Taxonomy. It uses Histosols Greek, histos, tissue Histology color, texture, structure, and other properties of the Inceptisols Latin, incepum, beginning Inception soil from the surface to two Mollisols Latin, mollis, soft Mollify meters deep to key the soil Oxisols French oxide Oxide into a classification Spodosols Greek spodos, wood ash Odd system. This system also Ultisols Latin ultimus, last Ultimate provides a common Vertisols Latin verto, turn Invert language for scientists. Soil taxonomy at the highest hierarchical level identifies 12 soil orders. The names for the orders and taxonomic soil properties relate to Greek, Latin, or other root words that reveal something about the soil. Sixty-four suborders are recognized at the next level of classification. There are about 300 great groups and more than 2,400 subgroups. Soils within a subgroup that have similar physical and chemical properties that affect their responses to management and manipulation are families. The soil series is the lowest category in the soil classification system. Ultisols (orange color on soil order map on page 40) are the most dominant soil order in North Carolina. These soils are characterized by a low amount of plant nutrients and a clay increase in the B horizon. They are often developed under a mixture of coniferous and deciduous cover and are usually highly weathered because of high average rainfall. With the additions of lime and fertilizer, many of these soils can be productive farmland and produce a wide variety of crops. 38 Soil Surveys A soil survey is a systematic examination, description, classification, and mapping of soils in an area. Soil surveys are classified according to the kind and intensity of field examination. The program of the National Cooperative Soil Survey includes developing and implementing standards for describing, classifying, mapping, writing, and publishing information about soils of a specific area. Soil survey reports include detailed and general soil maps, general and detailed soil descriptions, information on the use and management of soils, technical information on physical and chemical properties of soils, and tables for soil interpretations. These soil survey reports are published by the National Cooperative Soil Survey and are available to everyone. In North Carolina, most counties either have a published soil survey or have one in progress. To obtain a copy of a soil survey, check with the local Soil and Water Conservation District office. At the beginning of the soil survey report are sections that describe the general nature of the county, the relief and drainage of the county, the climate of the county, and how the survey was made. General soils maps are good for broad planning purposes and can be used to compare the suitability of large areas for general land uses. Because of the small scale used, these maps are not suitable for planning the management of a farm or field or for selecting a site for a building or other structure. The general soil map can be found at the beginning of the maps section near the back section of the soil survey. General soils map unit descriptions give information about the dominant soils, and their properties and limitations. These are located near the front of the survey. 39 For specific areas, the detailed soil maps need to be consulted. These maps are located at either the back of the soil survey or are loose in the folder. At the beginning of this section are an index map used to locate the specific map sheet that you need and a soil legend that tells the user what the map symbols mean. Map unit descriptions of the detailed soil map units give information about the general nature of the map units, brief non-technical soil descriptions, and their properties and limitations. For information about a specific soil series the Classification of the Soils section should be consulted. In this section, each soil series is described in detail and the characteristics of the soil and the material from which it formed are identified. The Use and Management of the Soils section gives information on cropland, pasture and hay land, orchards, yields per acre, land capability classification, prime farmland, woodland management and productivity, recreation, and wildlife habitat. Also included is a section on engineering that covers building site development, sanitary facilities, construction materials, and water management. The Soil Properties section provides information on engineering index properties, physical and chemical properties, and soil and water features. Also included is a glossary that defines the terms used in the soil survey. The final section of the soil survey includes the interpretive tables. The tables will vary depending upon the age of the survey but typically include: temperature and precipitation, freeze dates in spring and fall, growing season, acreage and proportionate extent of the soils, land capability and yields per acre of crops and pasture, prime farmland, woodland management and productivity, recreational development, wildlife habitat, building site development, sanitary facilities, construction materials, water management, engineering index properties, physical and chemical properties of the soil, soil and water features, and classification of the soils. Envirothon teams are encouraged to obtain a copy of a local soil survey and to familiarize themselves with all parts of the survey. Participants should be able to answer specific questions using the maps, tables, and written sections of the survey. Soil surveys can be obtained at the office of your local Soil and Water Conservation District. Applications Soil scientists evaluate sites in terms of particular projects in order to determine land use capabilities and guidelines. The projects include things like building site determinations, agriculture uses, septic systems, landfill locations, road placement, and recreation development. Acceptable activities depend on soil quality and landscape. The tradeoff between economic and environmental objectives must also be examined as well as how laws and regulations affect these decisions and how these decisions affect laws and regulations. 40 Land Capability Classification Land capability classes and in most cases, subclasses are assigned to each soil. They suggest the suitability of the soil for field crops or pasture and provide a general indication of the need for conservation treatment and management. There are 8 capability classes. Capability classes are designated by Arabic or Roman numerals (I through VIII), which represent progressively greater limitations and narrower choices for practical land use. Capability subclasses are noted with an e, w, s, or c following the capability class. For example, in class IIe, the "e" indicates that the soil is erosive. In class IIw, the "w" signifies a wetness limitation. In class IIs, the "s" denotes a shallow, droughty, or stony soil. In class IIc, the "c" indicates a climatic limitation. No subclasses are shown for capability class I because these soils have few limitations. Of the eight capability classes, only the first four are considered usable for cropland. Class I land has little or no hazard for crop production and is the best agricultural land. Classes II, III and IV need progressively more care and protection when cultivated crops are grown. Soils in classes V, VI and VII are suited for adapted native plants (such as forests), although some soils in classes V and VI are capable of producing specialized crops such as fruit trees and ornamentals. Soils in class VIII do not respond to management without major reclamation since they include the very steep and rocky areas of the mountain regions and the very wet tidal marshes. 41 Capability Classes Suitable for Cultivation of Row Crops Class I Soils In this class have few limitations that restrict their use. These soils are the best in nearly all respects for both agricultural production and nonagricultural uses. They are deep (40 inches or more), well drained and medium textured with medium to high available water capacities, moderate permeability, and none to moderate erosion. These soils are easily worked and are among the most productive in the state. Slopes should not exceed 2 percent. Management should include maintenance of proper plant nutrient balance and tilth. Class II Soils in this class have some limitations that reduce the choice of plants or require moderate conservation practices. Although these soils are rated good and usually are productive, some physical conditions render them less desirable than class I land. Likewise, the drainage class, soil depth, permeability or available water capacity may be less desirable than class I soils. In general, slopes ranging between 2 and 6 percent place this soil in class II. Drainage may be the limiting factor with redoximorphic features within 20 to 40 inches of the surface. A slow or rapid permeability, low available water capacity or moderate soil depth (20 to 40 inches) also could eliminate this soil from class I. Although several limitations may exist, only one is necessary to place this soil in class II. Management practices, in addition to those for class I, should include moderate erosion control (including rotations with sod or cover crops), contour farming, moisture retention methods or drainage depending on the type of limitation. Class III Soils in this class have severe limitations that reduce the choice of plants or require special conservation practices, or both. Limitations similar to class II soils may be present in these soils, but these limitations are more severe, restricting the use of these soils. Large acreages of class III land are strongly sloping and subject to moderate to severe erosion. Slope limits for this class usually range from 6 to 10 percent. If drainage is the limiting factor, redoximorphic features should occur within 20 inches of the surface, indicating that saturated conditions or high water tables are present at some time during the year. Shallow soils (less than 20 inches), coarse-textured surface layers, fine-textured subsoils with slow permeability or very low available water capacity also can limit the use of soils to the extent that they are placed in this class. The very coarse soils with very low available water capacities also fit into this class and require irrigation to realize production. The soils in this class require more intense management than the previous classes. Management practices should include intensive erosion control measures such as terracing and strip-cropping. Where excessive water is limiting, drainage practices are necessary to make these soils productive. 42 Class IV Soils in this class have very severe limitations that restrict the choice of plants or require very careful management, or both. Where erosion is limiting, this land is good for only occasional cultivation under careful management. Sod crops should occupy a large portion of the rotation because of the severe erosion hazard. Slope limits for this class usually range from 10 to 15 percent. Very poorly drained soils in depressions have such high water tables, or are saturated for such long periods, that only very intensive drainage management can make these soils productive. Soils that are severely eroded or gullied with little or no surface soil must be placed in this capability class, even though these soils may occur on slopes similar to those required for class III soils. Very intensive management practices are required for production on these soils. Where erosion is the hazard, cultivated crops may be grown only once in several seasons. Sod crops such as hay, pasture or cover crops are necessary to minimize the erosion loss. Even under excellent management, crop failures or severe yield reductions can be expected occasionally. Capability Classes Unsuited for Cultivation Class V Soils in this class are nearly level and not subject to erosion, but because of excessive wetness resulting from frequent flooding or some permanent obstruction like rock outcrops, they are not suited for cultivation. Streams that overflow frequently, excessive seepage, very stony soils or numerous outcroppings of bedrock make these soils unsuited for cultivation. Many of these soils are deep, however, and they have few limitations for pasture or forestry. These soils respond to good management, which is necessary for satisfactory production. Class VI Soils in this class have severe limitations that make them generally unsuited for cultivation, and that limit their use largely to pasture, woodland, or wildlife food and cover. These soils have continuing limitations that cannot be corrected economically such as steep slopes (15 to 25 percent), a severe erosion hazard, effects of past erosion, or stoniness. These factors produce some limitation for pasture and forestry. It should be pointed out that even for most of these uses, the better classes are preferred for maximum protection. Class VII Soils in this class have very severe limitations that make them unsuited for cultivation and that restrict their use largely to grazing, woodland or wildlife. 43 Although not suited for cultivation, intensive management can make productive pasture and woodland possible. Even in rough, timbered areas, special care is required to prevent excessive erosion. Soils on very steep slopes, very shallow soils and very stony soils that occur on slopes greater than 25 percent are the most common members of this class. This class includes the least capable soils with regard to pasture and woodland. Class VIII Soils and landforms in this class have limitations that preclude their use for commercial production of plants and restrict their use to recreation, water supply, wildlife, or aesthetic purposes. Tidal marshes that are flooded daily, continuously ponded areas (areas containing water for more than 6 months of a year), and areas with greater than 90 percent rock outcrop, stones or boulders are included in this class as well as borrow pits, barren mine dumps and sandy beaches. These land areas have few or none of the physical soil features (found in class I soils) necessary to support any type of agriculture. Hydric Soils Hydric soils are wet soils defined as a group for the purpose of implementation of legislation for preserving wetlands and for assessing the potential habitat for wild l if e. The soils considered to be hydric were selected on the basis of flooding, water table, and drainage class criteria. Hydric soils developed under wet conditions (anaerobic within 12 inches) and can support the growth and regeneration of hydrophytic vegetation. Indicators we look for in the field to identify hydric soils include organic soils (16 inches of organic soil material in the upper 32 inches); histic epipedon (8 inches of organic soil material in the upper 16 inches); gleyed or low chroma colors (a predominance of gray colors due to wetness); high organic matter in sandy soils; and organic streaking in sandy soils. By using a soil survey, you can also identify the soil series and look to see if it is listed on the county hydric soils list. The Hydric Soils List, developed for the 1982 Farm Bill, is included in the Natural Resources Conservation Service Field Office Technical Guide, Section II. Some map units that have inclusions of soils that meet the hydric are added to the field office listing. Prime Farmland Prime farmland is one of several kinds of important farmland defined by the U.S. Department of Agriculture. Prime farmland is of major importance in meeting the Nation's short- and long- range needs for food and fiber. The acreage of high-quality farmland is limited, and the U.S. Department of Agriculture recognizes that government at local, State, and Federal levels, as well as individuals, must encourage and facilitate the wise use of our Nation's prime farmland. 44 The soils in a survey area that are considered prime farmland are listed in the prime farmland table. Prime farmland, as defined by the U.S. Department of Agriculture, is land that has the best combination of physical and chemical characteristics for producing food, feed, forage, fiber, and oilseed crops and is available for these uses. It could be cultivated land, pastureland, forestland, or other land, but it is not urban or built-up land or water areas. The soil qualities, growing season, and moisture supply are those needed for the soil to economically produce sustained high yields of crops when proper management, including water management, and acceptable farming methods are applied. In general, prime farmland has an adequate and dependable supply of moisture from precipitation or irrigation, a favorable temperature and growing season, acceptable acidity or alkalinity, an acceptable salt and sodium content, and few or no rocks. It is permeable to water and air. It is not excessively erodible or saturated with water for long periods, and it either is not frequently flooded during the growing season or is protected from flooding. Slope ranges mainly from 0 to 6 percent. More detailed information about the criteria for prime farmland is available at the local office of the Natural Resources Conservation Service. Prime farmland soils may presently be used as cropland, pasture, or forestland or for other purposes. They either are used for food and fiber or are available for these uses. Urban or built-up land, public land, and water areas cannot be considered prime farmland. Urban or built-up land is any contiguous unit of land 10 acres or more in size that is used for such purposes as housing, industrial, and commercial sites, sites for institutions or public buildings, small parks, golf courses, cemeteries, railroad yards, airports, sanitary landfills, sewage treatment plants, and water-control structures. Public land is land not available for farming in National forests, National parks, military reservations, and State parks. Prime farmland soils commonly receive an adequate and dependable supply of moisture from precipitation or irrigation. The temperature and growing season are favorable, and the level of acidity or alkalinity and the content of salts and sodium are acceptable. The soils have few, if any, rocks and are permeable to water and air. They are not excessively erodible or saturated with water for long periods, and they are not frequently flooded during the growing season or are protected from flooding. Slopes range mainly from 0 to 6 percent. Soils that have a high water table, are subject to flooding, or are droughty may qualify as prime farmland where these limitations are overcome by drainage measures, flood control, or irrigation. Onsite evaluation is necessary to determine the effectiveness of corrective measures. More information about the criteria for prime farmland can be obtained at the local office of the Natural Resources Conservation Service. 45 The map units in a survey area that are considered prime farmland are listed in the prime farmland table. This list does not constitute a recommendation for a particular land use. The location of each map unit is shown on the detailed soil maps. On some soils included in the table, measures that overcome a hazard or limitation, such as flooding, wetness, and droughtiness, are needed. The need for such measures is indicated in parentheses after the map unit name. Onsite evaluation is needed to determine whether or not the hazard or limitation has been overcome by corrective measures. The amount of prime farmland in the United States is currently in decline. As the population grows, more and more prime farmland is lost to industrial and urban uses. This loss of prime farmland to other uses puts greater pressure on marginal lands, which generally are more erodible, droughty, and less productive and cannot be easily cultivated. Soil Recycling The “organism” factors of soil formation include the plants that grow in soil, the array of organisms that live in and die in soil, and the organisms that impact soil through their burrowing and mixing. Farming also plays a role in soil formation. Organisms, through their litter and death, contribute organic matter to the soil. Organic matter is a food source for microorganisms, contains nutrients, and promotes the formation of soil structure or the aggregation of the primary soil particles – sand, silt, and clay. Well-structured soils are porous and permeable and allow the movement of gases and water. Optimal Growing Conditions Soils with good structure provide optional conditions for nutrients and moisture storage, drainage of surplus water, and gas exchange from respiration in the root zone. The incorporation of organic matter into the soil is significant under grasslands, but less so under forest. Organic matter provides a source of nutrients, especially nitrogen, for soil organisms. Microorganisms play a vital role in decomposing organic matter and releasing plant nutrients for uptake by plants. The application of lime increases the availability of calcium, reduces soil acidity, and raises the soil pH. This improves conditions for microbiotic activity increasing numbers and diversity of macroorganisms, which in turn increases organic matter decomposition and the release of plaint nutrients. Lime, in combination with organic matter, is necessary for well-developed soil structure. Organic Matter from Tree Litter Coniferous trees produce needle litter that contains fewer nutrients and is more resistant to decomposition than the broadleaf litter found in hardwood forests. Organic surface horizons under conifers contain an abundance of fungi, which permeate the organic material during the moist growing season. Freezing, drought, low summer temperatures, acidity, and the resistant 46 nature of conifer forest litter slows the decomposition of the organic material. Needle litter accumulates on the soil surface as raw humus. The by-products of decomposition acidify the soil creating an adverse environment for earthworms, and other soil organisms that prefer less acidic conditions. Little of the humus is mixed with the mineral soil because of the small numbers of soil fauna like earthworms. Infiltrating water is made more acidic by the by- products of decomposition. This acidic soil solution intensifies chemical weathering and leaches plant nutrients and other bases from the soil minerals. The litter from hardwood trees is more adaptable to soil organisms and higher in nutrients than conifer litter. As a result, hardwood litter is more easily decomposed and incorporated into the surface mineral soil forming an A horizon and resulting in the formation of humus. Under less acidic conditions and more nutrient-rich soil conditions, hardwood litter is consumed and readily incorporated into the surface mineral soil by soil fauna such as earthworms. This mixing of organic matter into the mineral soil surface produces finely structured, well-humidified, highly fertile mull humus, which is a mixture of evenly, divided organic matter and mineral material. Plant nutrients, absorbed by roots deep within the soil, enrich the soil surface through litter fall. This cycling of nutrients, from the subsoil to the vegetation and back again to the soil surface through litter fall, counters nutrient loss from leaching. Leaching potential, which is governed by litter types and its decomposition by-products, is highest under coniferous and moss litter, somewhat less, under hardwoods, and least under grasses in freely drained soils. Besides its nutrient content, litter is important in providing soil cover that reduces evaporation, moderates soil temperature fluctuations, and protects soils from the erosive forces of wind, water run-off, and rainfall. Key Concept 5: Conservation and Management of Soil Erosion Erosion is the process where soil is moved from one location to another by wind, water, or other natural action. It is a natural process until accelerated by human activities. Erosion has several harmful effects: poorer soils, smaller harvested crops, silt buildups in waterways, degrading aquatic plant and animal habitat, contributing to frequent flooding, contributing to costly dredging and navigation problems. It is very detrimental to aquatic life in water. Types of erosion It is easy to find evidence of soil erosion from moving water. Scientists have identifie