composition of average soil minerals pore space · considering all these factors, soil is a...

“The nation that destroys its soil, destroys itself.” Franklin D. Roosevelt

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What is soil?There are several definitions of soil, depending on the discipline that supplies thedefinition. The traditional definition is –material which nourishes and supportsgrowing plants. The component definition is–a mixture of mineral matter, organicmatter, water and air. Soil is actually an ecosystem. It has physical, chemical andbiological properties that continually change. It supports a variety of life frommicroscopic bacteria to higher forms of life including invertebrates, such as spiders,mites, ants, nematodes and earthworms, and vertebrates, such as moles, voles, andshrews.

Considering all these factors, soil is a naturally occurring mixture of mineral andorganic matter with a definite form, structure, and composition. Soil is composedprimarily of minerals that are produced from rock (the parent material) that isweathered - broken down into small pieces. The organic matter of the soil includesboth living organisms and dead organisms. Soils have four major components:mineral matter, organic matter, air, and water. Generally speaking, the average soilsample is 45% minerals, 25% water, 25% air, and 5% organic matter. The amountof air and water filling the pore space in the soil varies depending on the amount ofprecipitation, location, and water holding capability of the soil. The air and watercontent of the soil is essential for most living organisms.

Composition of Average SoilMineralsThe mineral portion of the soil is formed from bedrock that has been weatheredinto smaller particles–sand, silt, clay, gravel, or stones. Soils range in depth fromonly a few inches to tens of meters. They have often been transported far from thesite where they were initially formed and lie atop materials that are very differentfrom their original parent material.

Pore SpaceBetween the mineral particles of soil there are voids called pores. These smallspaces are filled with air and water. These pore spaces are essential to the soilorganisms and plants that receive oxygen, water, and nutrients from the soil.

There are two types of pore spaces that generally occur in soils. Macropores arelarger and allow air and water to move through the soil more rapidly. Sandy soilsare an example. In sandy soils, macropores dominate and air and water movement

It is often statedthat 50% of the soilis pore space sincethis portion of thesoil is filled withair and water invarying amounts.

Sand, silt, and claymake up from 40 –60% of most soils.The percentage ofeach will determinethe soil texture.


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is very rapid. Micropores are smaller. The movement of air and water in thesesoils is slowed greatly.

Although fine textured, clay soils have large amounts of pore space and themovement of air and water is relatively slow. Clay is the smallest of the three mainmineral particles, and these soils have unusually large amounts of total pore space.However, the pore spaces between the particles are very small, restricting themovement of air and water. Aeration, especially in the subsoil, is not adequate forgood root development and desirable microbial activity. These soils demonstratethat the size of the individual pore space is more important than a large number ofsmaller sized pores. Activities, such as continuous cropping and plowing of soilswith very high organic matter, can result in a reduction of the larger macroporespaces and, over time, cause these soils to become more infertile.

Organic MatterThe final ingredient of soil is organic matter comprised of dead plant and animalmaterial and the billions of living organisms that inhabit the soil.

The Importance of Soil

Soil is often referred to as “the wealth beneath our feet.” The soil performs manyfunctions which make this a true statement. Soil provides the home on which manbuilds structures and lives. It provides the medium for much of the plant growththat supplies the base of the food chain for most other organisms, including man’s crops. The soil also provides us a snapshot of our geologic history. A summaryof soil’s physical, chemical, and biological functions follows:

▪ Controlling the distribution of rainfall or irrigation water to runoff, infiltration, storage, or deep drainage which affects the movement ofsoluble materials, such as nitrates, nitrites, phosphorus, or pesticides.▪ Regulating the biological activity and changes among solid, liquid and

gaseous phases of the nutrient cycle, which affect plant growth and thedecomposition of organic matter.▪ Filtering and buffering impurities from air and water.

The organiccomponent of thesoil is composed ofliving organisms,humus, residue,and decomposingorganic matter, inthe percentagesshown in this chart.

Humus –darkcolored, stickyresidue from thebodies of deadorganisms, that canbe broken down nofurther; gives soil itsstructure, coatingmineral particles andholding themtogether; serves as amajor source of plantnutrients.


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▪ Providing the mechanical support for plants and animals (growth medium for vegetation that provides food and habitat).▪ Storing vast amounts of groundwater.▪ Providing a place where plants can conduct photosynthesis, converting

the sun’s radiant energy into food energy for the plant and for animals.▪ Providing a site for waste decomposition.▪ Providing us with food, fiber, building materials, and other raw materials.▪ Providing a record of geological, climatic, and ecological history.▪ Providing the foundation on which we build .

The Soil EcosystemThe soil is a part of all ecosystems and, in most, more life and a greater diversity oflife lives underground than on top. The soil is also an ecosystem with biotic(living) and abiotic (non-living) factors. As with any ecosystem, there is dynamicinterplay between the living and non-living components, which are often limitingfactors to the biological community. In the soil, limiting factors can include:temperature, moisture, pore size, pH level, aeration, and food supply available.These factors will determine the diversity, number of each, and activity of speciespresent. Weather determines daily and seasonal variations in biological activity.Fungi, bacteria, protozoa, nematodes, earthworms, and arthropods are typicallyfound in soil and many other organisms are full or part-time residents. The site ofmost of the biological activity is the rhizosphere, the interface between the soiland plant roots. Here, as in any ecosystem, the interactions among the organismsinclude symbiotic relationships, parasitism, infectious disease, predator-prey, andcompetition. For example, fungi can cause diseases of plants and animals, but theyalso form symbiotic relationships, called mycorrhizae, with the roots of manyplants, allowing the plants to more readily take in water and nutrients.

Organisms within the soil provide many services:- Decompose organic matter–saprophytic fungi are fungi that decompose

dead organic matter- Break-down or degrade pollutants before they reach surface

water or groundwater- Storage and release of nutrients- The cycling of mineral nutrients

- Nitrogen cycle ( nitrogen fixation, nitrification, and denitrification)- Carbon, nitrogen, phosphorus, sulfur, potassium cycling -breakdown organic matter, releasing nutrients for use)

- Creation of macropores by burrowing invertebrates and vertebratesallowing flow of air and water through the soil

- Addition of organic matter to the soil–feces and decomposing plant andanimal matter

- Production of substances by fungi and bacteria that help bind soilparticles together and stabilize soil aggregates

- Some soil organisms compete with or prey upon plant pest or disease-causing organisms.

- Some bacteria release plant growth factors that increase plant growth- Mycorrhizal fungi form symbiotic relationships with plant roots helping

plants acquire nutrients from the soil, and help stabilize soilaggregates.

Rhisosphere – theinterface between thesoil and plant roots.


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- The biological community (mix of organisms in the soil) partiallydetermines a soil’s resilience, or ability to recover its functions

after a disturbance, such as tillage, compaction, or fire.- Organisms release CO2, which is converted to carbonic acid and dissolves

rocks.

A Soil Ecosystem

The management of soil can impact the soil community. Some organisms aresensitive to trampling. Soil compaction, lack of vegetation, or lack of plant littercovering the soil surface tends to reduce the number of soil arthropods. Reducingtillage, or plowing of the soil, tends to result in increased growth of fungi, includingthe beneficial mycorrhizal fungi. The soil community also impacts the soil content.It takes about 4,000 to 6,000 pounds of crop residue per year to maintain thecontent of organic matter in a soil. The management of croplands, rangelands,forestlands, and gardens benefits from and affects the soil food web.

Energy flows from the sun, through the plants, and through the other livingcomponents of the soil in a complex food web that has different compositions indifferent ecosystems. Plants are producers which create organic compounds byusing the energy of sunlight, in the presence of chlorophyll, to combine carbondioxide from the atmosphere and water from the soil to produce sugar by theprocess of photosynthesis. By-products of photosynthesis are oxygen, essential formost other organisms, and water.

6CO2 + 12H2O + energy = C6H12O6 + 6O2 + 6H2O

Photosynthesis provides the food supply for plants and most all other livingorganisms, the consumers. As the plants, animals, and other organisms excretewaste or die, the material is broken down by decomposers, and organic material isadded to the soil. Mineral nutrients are also returned to the soil where they willagain be available for plant growth. Plants and animals break down the sugars

Respiration:C6H12O6 + 6O2

=6CO2 +12H2O


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produced during photosynthesis to obtain the energy necessary for life processesthrough the process of respiration, which produces carbon dioxide and water asby-products. These processes, photosynthesis and respiration, are important to thecycling of carbon and oxygen.

Oxygen produced during photosynthesis can permeate through pore spaces in thesoil where it is used by organisms for aerobic respiration. A second type of energyrelease, anaerobic respiration, occurs in the absence of oxygen. This form ofrespiration is used by some bacteria. The excess energy and oxygen producedduring photosynthesis maintain the soil community.

Soil microorganisms, such as bacteria, also use soil organic matter for energy tosupport life. They incorporate some of the energy, but much is released throughrespiration as carbon dioxide, water, and some gaseous nitrogen. However, somenitrogen along with most phosphorus and sulfur are retained. These decomposersplay an important role in the cycling of these and other essential mineral nutrients.

Energy TransferThe only significant source of energy for life on Earth is sunlight. The organismsthat can use the sun’s energy to produce food by photosynthesis are called autotrophs. They are the producers, and the most well known producers areplants. Autotrophs use the energy from the sugars produced for growth,maintenance, and reproduction, but more energy is produced than is needed.

Most other organisms cannot produce their own food and must obtain the energythey need by feeding on plants or on other organisms. These organisms are calledheterotrophs. They are the consumers. Almost all soil organisms are


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heterotrophs. A simple sequence of what eats what is called a food chain. Innature, this rarely occurs. Organisms consume many different types of food ororganisms. A more realistic representation of feeding regimes is a food web,which depicts the interactions among multiple food chains.

As energy moves from one organism to another, or one trophic level to the next,much is lost as heat. The decreasing amount of energy passed from one trophiclevel to the next is known as the energy pyramid.

Energy Flow in the SoilEnergy flows to and from the soil ecosystem and is distributed at various feeding ortrophic levels. Some organisms, like parasitic nematodes, fed directly on livingautotrophs (plants), eating their roots. Other organisms feed on heterotrophs thatate the plants. The energy received is used by the heterotrophs for life processes,such as growth, maintenance, movement, and reproduction. What is not used inthese processes is excreted or stored. Excreted material is used as an energy sourceby other microorganisms, such as bacteria, protozoa, and fungi.

Since most organisms in the soil are heterotrophs, the soil ecosystem cannot becompletely self-sufficient. Energy produced and stored by autotrophs outside theecosystem must be brought into the soil. One of the largest sources of energy in thesoil comes from fallen dead leaves, twigs, and other dead plant and animal material,called detritus, that make up the litter on the soil surface. This material can betaken into the soil ecosystem by various methods. Litter feeders, detritivores, candrag or incorporate surface material into the soil or it may enter through input fromplant roots through die back or consumption by root eaters.

Niches, Roles Within the EcosystemProducers, consumers, or decomposers are the main roles played by organisms inan ecosystem. Plants and other photosynthetic organisms are producers. Theheterotrophs that consume plants or other organisms are the consumers anddecomposers. Consumers usually eat living or recently live tissues, digesting foodinternally, and can be divided into several groups. Herbivores eat plants.Carnivores eat meat, or organisms other than plants. Omnivores eat both plant andanimal matter. Detritivores eat dead and decaying plant or animal matter includingexcrement. Decomposers break down and feed on dead organic material. Theydecompose litter and create available nitrogen, phosphorus, potassium, carbon, andother nutrients for plant growth. They release enzymes that decay and transformplant and animal material into smaller organic compounds that they can absorb.Bacteria and fungi are the two main types of decomposers, and they occur in hugequantities in the soil ecosystem. Transformers are a special group of decomposerscapable of breaking down large organic molecules into molecules small enough tobe absorbed by plant roots. This final step in the breakdown of detritus is critical tothe survival of terrestrial (land) ecosystems because it is essential to the cycling ofnutrients. It releases macro and micro nutrient elements required by plants and isnecessary to the recovery and recycling of essential nutrients that maintain naturalecosystems.


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Nutrient CyclesNitrogen, phosphorus, and potassium are called macronutrients because plants arecomposed mainly of these elements. Minor quantities of a dozen other elements(micronutrients) are also required for certain plant processes and plant health.Magnesium is an example of a necessary micronutrient. Magnesium is acomponent of chlorophyll, the green pigment in plants required for photosynthesis.The cycling of macronutrients and micronutrients is critical to the maintenance ofall aspects of ecosystems. The nutrient that is often the most limiting factor forcrop and other plant growth is potassium.

The Water or Hydrologic CycleWater is essential for life. It cycles through the atmosphere, soil, and biota (livingorganisms). Water falls as precipitation – rain, snow, sleet, or hail onto Earth’s surface where it can do several things. It may infiltrate the soil, flow over the landas surface runoff, or evaporate back to the atmosphere.

Water that infiltrates the soil moves downward through the soil into holes and porespaces becoming groundwater. Here, within the root zone, it can be absorbed byplant roots, moved through the stems and leaves, and transpired back to theatmosphere as water vapor. In the atmosphere, water vapor can cool and condenseto form clouds and water droplets that fall back to the surface as precipitation.Water in the soil, not utilized by plants, continues to move through the soil. Someof the water flows laterally, eventually surfacing as a spring or seep or entersanother body of water. Some water continues downward, eventually recharging thegroundwater and being stored in aquifers.

The water that falls on land but does not infiltrate the soil, the surface runoff, flowsoverland and eventually enters a stream or river. Much water is evaporated beforeever reaching the land, returning to the atmosphere as water vapor.

The Carbon CycleThe soil is only partially involved in the carbon cycle, as with the water cycle.Plants use carbon dioxide from the atmosphere for the production of carbohydrates(sugars) by photosynthesis. Plants and animals release carbon dioxide to the


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atmosphere as a by-product of respiration. When the sugars in organic matter areconsumed by soil organisms, the carbon is incorporated into their tissues, respired,or excreted as waste. Decomposers consume organic waste and dead organisms,releasing carbon dioxide and using oxygen. Oxygen is a by-product ofphotosynthesis, and is used by plants and animals for respiration. Some carbondioxide combines with atmospheric moisture to form a weak carbonic acid solutionthat falls to the soil as precipitation and can play a role in the leaching of soils.

The Nitrogen CycleNitrogen, like water and carbon, cycles through a gaseous phase. The Earth’s atmosphere is 78% nitrogen gas (N2) and is the primary reservoir of nitrogen.Nitrogen-fixing bacteria, volcanic action and lightning can convert nitrogen gas,fixing it into usable forms for plants as nitrate (NO3) and ammonium (NH4). Thehigh temperature and pressures of internal combustion engines, as those in cars, canalso create usable forms of nitrogen that enter the atmosphere.

Nitrogen fixation occurs in legumes, plants that have a symbiotic relationship withthe nitrogen-fixing bacteria (Rhizobium), such as clover, alfalfa, and soybean.Cyanobacteria (blue-green algae) and other bacteria, free living or actinomycetes inassociation with non-leguminous plants, can fix atmospheric nitrogen, as can somealgae-fungus and plant-algae associations. Plants are the only source of nitrogenfor animals and other organisms that cannot fix nitrogen.

Biological nitrogen fixation by bacteria is the major source of nitrogen input tosoils. Other sources of nitrogen include atmospheric deposition of nitrates andammonium in precipitation or dry particulate matter, the addition of nitrogen-basedfertilizers, and decomposition of organic matter, which contains nitrogen.

Nitrogen losses from soil are great in agricultural areas, where nitrogen is lost whencrop plants are harvested. Nitrogen can also be lost due to leaching and soilerosion, which can transport the nitrogen into the groundwater or aquaticenvironments, such as streams, rivers, lakes, or ponds. Excess nitrogen in aquaticenvironments can cause eutrophication and the associated increased plant growth.The plants later die, and their decomposition by bacteria can decrease the dissolvedoxygen content of the water resulting in fish kills.

Carbon dioxide –Oxygen Cycle

Actinomycete –filamentousbacteria, whichform associationswithnonleguminousplants and fixatmosphericnitrogen.

d. coulterMining phosphatefor use in fertilizers

N.C. Coop. Ext. ServiceA soil particle(colloid) issurrounded bynutrients.


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Soil FormationThe study of soil formation is called soil genesis, which means explaining theorigin of soils. Soil is a natural body, and forms as a result of physical, chemical,and biological processes of nature involved in complex interaction with the parentmaterial, topography, climate, and living organisms. Natural geologic processes ofthe rock dictate which soil originates.

Climate and biological factors break down the rock by the process of weathering,impacted by topography and a function of time and exposure of the rock to theseagents. Organic materials can produce soils, such as peat. The soil’s properties are a result of these agents, which work together to create a unique soil profile, thusproducing the great variety of soils that exist today.

The soil forming factors are:♦ Parent material♦ Climate♦ Relief or Topography♦ Biota (biological factors –organisms)♦ Time/Exposure

There are four basic processes that act on parent material, alter its properties, anddifferentiate one soil from another. These processes are:

~ Translocations~ Transformations~ Additions~ Losses

The interaction of these soil forming factors, in various combinations, give us thegreat variety of soils we see today. The effects of these soil-forming factors resultsin layers within the soil from the surface down to varying depths, depending on theintensity of the weathering. These layers are called horizons. The combination ofthese layers in a sequence, from the surface of the soil down, represents a soilprofile. The horizons recognized in a soil profile are identified by letters A, E, B,C, O and R. Horizons and soil profiles will be covered in detail later in this sectionbut are referred to in the following description of soil forming factors.

Soil genesis –thestudy of the factorsand processes of soilformation.

For graphic onsoil formation,see appendix A.

☻A way toremember the fivefactors of soilformation is tomemorize this:the type of soildeveloped dependson the amount oftime the parentmaterial on a specifictopography isexposed to the effectsof climate andbiology.


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Parent Material

The parent material is the material underlying the soil from which, in most cases,the soil develops. It influences both the physical and chemical properties of thesoil. Parent material can come from many different materials. Most of the mineralmatter that makes up soil comes form weathered bedrock or from other inorganicmaterials transported and deposited by wind, water, or glaciers, but soil can alsoform from sedimentation of organic material, such as peat, in bogs and marshes.These sources result in the two broad groups of soils –mineral soils and organicsoils.

Moderately developed and well developed soils are formed when a parent materialis changed chemically and physically over time. When the parent material containshigh proportions of less resistant minerals, this process proceeds more rapidly. Theavailability of nutrients in the soil depends on the amount of decomposed materialthat remains in the soil or that is removed in drainage water.

Minerals and RocksMinerals and the rocks they compose are important to soil because they are mostoften the materials that weather or break down to form soil. A mineral is anaturally occurring, inorganic crystalline material with definite physical propertiesand a unique chemical composition. Rocks are complex mineral aggregates. Mostrocks are made up of mineral crystals, or broken pieces of crystals or broken piecesof rocks. Some are composed of shells from animals or compressed pieces ofplants. Most soils are formed from rocks, not from pure mineral deposits.Rocks are classified into three groups based upon their formation. The threeclassifications of rocks are igneous, sedimentary, and metamorphic.

Igneous rocks are rocks said to be “born of fire.” These rocks are of volcanicorigin and form as molten material pushed up from the mantle cools either beneaththe surface or on the surface. The molten material is called lava if it flows onto thesurface or magma if it remains beneath the surface. Rocks formed from lava arecalled extrusive igneous rocks. The lava cools quickly, forming the fine-grainedextrusive rocks that weather to produce fine-textured soils. Rocks formed frommagma, the more dominant of the two rock materials, are called intrusive igneousrocks. Magma which remains beneath the surface cools very slowly producinglarger-grained rocks that weather to produce coarser textured soils.

Sedimentary rocks are formed from deposition of sediments or small rocks (theweathered products of preexisting rock) or remains of organisms, such as shells,that are compacted and cemented into solid rock. Sedimentary rocks comprise onlyfive percent of the rock volume of the outer ten miles of the earth’s crust, but they cover 75% of the surface.

The deposition of sediments is called sedimentation. This deposition is the resultof the process of erosion, which is the process of moving the products ofweathering from one place to another. The agents of erosion are wind, runningwater, gravity, glaciers, and waves. Strong winds blowing over bare soil may pickup sand and soil particles, transporting them until the winds dissipate or their

Mineral groups ofprime importance insoil development:FeldsparsAmphiboles and

pyroxenesMicasSilicatesIron OxideCarbonates

d. coulterFlows of basalt(extrusive igneousrock) in Idaho.

d.coulterDomes of granite(intrusive igneous rock)Yosemite NationalPark, California

d. coulterLayers of SedimentaryRock resulting fromdeposition of differentmaterials


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velocity decreases. As the winds slow, larger particles settle out first, andgradually smaller and smaller particles settle out until the winds stop. Thesedeposits can accumulate to become quite thick and eventually be cemented intosedimentary rocks.

Running water is a powerful agent of erosion, capable of transporting largervolumes of sediment and larger particles than wind. As the water slows, thesediment carried will settle out much the same as when carried by the wind–largerparticles first. As deposits become thicker, these materials will gradually becompacted or cemented into sedimentary rocks.

The type of sedimentary rock formed is based on its composition, which dependson the type of material transported and/or deposited. Sandstone forms fromdeposits of sand. Limestone forms from deposits of the shells of marine organismsor precipitates from seawater. Though a wide variety of minerals and rockfragments are found in sedimentary rocks formed from weathered rock, clayminerals and quartz dominate.

A few sedimentary deposits can result from leaching. Leaching occurs as watermoves downward, or percolates through the soil, dissolving minerals, such ascalcite, and gypsum. The minerals continue to dissolve as long as water is movingdownward from the surface. If the water stops moving, the minerals can becomeconcentrated and be redeposited or may precipitate out as the water evaporatesforming sedimentary deposits. Chemical precipitation forms the stalactites andstalagmites seen in caves.

Metamorphic rocks are rocks that have been changed. They are igneous orsedimentary rock that have gone through the process of metamorphism.Metamorphism can be caused by exposure to high heat; high pressure; hot, mineral-rich fluids; or some combination of these factors. It results in rock with radicallydifferent characteristic than the original rock from which it formed. The conditionsthat cause metamorphism are found deep within the Earth where tectonic platesmeet, or are associated with volcanic activity or with the compression of mountainbuilding. The soils formed from metamorphic rock are very similar to thoseformed from the original igneous or sedimentary parent rock.

The Seven General Categories of Parent Material are:● Residual material● Glacial deposits● Eolian deposits● Alluvial and marine deposits● Colluvial deposits● Volcanic deposits● Organic deposits

Residual MaterialThe weathering of bedrock, such as granite, gneiss, limestone, sandstone, schist,and slate among others, results in residuum. The residuum becomes the parentmaterial of the soil which will have some of the characteristics of the parent

Leaching –thedissolving andtransport of mineralsas water percolates,or moves downward,through the soil.

For figure ofrock cycle, seeappendix A.

Residuum -weathered, brokendown bedrock.

Residual soils –soilsformed fromresiduum that formon the underlyingparent bedrock.


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material. Soils that form in this manner, over underlying bedrock, are calledresidual soils. These soils are common in the Piedmont and Mountain regions ofNorth Carolina.

Glacial DepositsThere have been four major glacial periods in the last million years during whichmassive ice sheets covered the northern parts of North America. The ice movedsoils, rocks, gravel, and boulders. As the ice sheets melted, these materials weredeposited and became part of the parent material for some of today’s soils. This type of transport of material is called glacial drift. The material may be depositedas lateral, terminal, or medial moraine. There are no glacial deposits in N.C.

Eolian DepositsWind also transports weathered parent materials. The deposits due to the action ofwind are eolian deposits. The two major types of eolian deposits are dune sandsand loess. A third type of eolian deposit is volcanic ash.

Dune sands are formed as strong winds transport sand and pile it into hills calleddunes. Dunes are very unstable, constantly shifting in size and shape, unless wellcovered with vegetation. Sand dunes of North Carolina’s beaches, including Jockey’s Ridge, the tallest sand dune system in the eastern U.S., are examples of eolian deposits. The soils of the Sandhills area of North Carolina may have formedfrom ancient coastal sand dunes.

Loess deposits are windblown silt deposited in valleys that originated from broadfloodplains associated with rivers, such as the Mississippi. Loess is extremelyfertile, productive agricultural soils. They present problems for engineers becausethey shift and slide under stress and flow when wet.

Volcanic ash usually forms fine-textured, very fertile soils. These soils usuallytake less time to develop than soils that develop primarily from rock, but they areeasily eroded.

Alluvial and Marine DepositsWater also plays an important role in the transport of parent material. Materialeroded from the surrounding watershed is carried into streams. The stream scoursthe streambed during storm events and erodes the banks. These sediments arecarried until the water loses velocity. When the stream reaches a flat valley, thesediments settle out at the foot of the slope, the larger particles settling out first.These deposits are called alluvial fans. These formations are common at the foot ofmountains in the Appalachian and Rocky Mountains. The soils of alluvial fans aregenerally well-drained, and their composition depends on the rock and mineralmaterial of the mountain slopes above.

FloodplainsStreams and rivers on gentle slopes or low lying areas typically flow in a series ofs-shaped curves called meanders. Over a long period of time, these meanders andflooding will cause broad, flat bottomlands on both sides of the stream. Thesebottomlands are called floodplains.

d. coulterU-shaped valleyand morainedepositscharacteristic ofglaciers. (GlacierNational Park,Montana)

Typical landscape inthe Sandhills–withEolian sand dunes

d.coulterSand dune withnative vegetation

d. coulterAlluvial Fan

Cape Fear Riverfloodplain withmeanders in theriver


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The most common type of alluvial deposit is in floodplains. As streams and riversoverflow their banks, they deposit new material on the floodplain. These depositscomprise the parent material for soils developed in these areas. Since new materialis added almost annually, these soils never have time to form well-developedhorizons, and these young soils have poorly developed profiles. This type of parentmaterial is referred to as recent alluvium.

The composition of floodplain sediments depends on the materials that wereeroded. They may contain high amounts of organic matter deposited fromwoodland or vegetated areas as floodwaters receded. Floodplain sediments mayalso contain high levels of nutrients (nitrogen and phosphorus) due to erosion fromhighly fertilized cropland or urban areas. They may also contain pesticides or othercontaminants carried by runoff from farms, or the streets, parking lots, and yards ofurban areas. Floodplains may contain poorly drained soils and are often thelocations of wetlands. These areas provide good habitat for wildlife, help rechargegroundwater, and help filter out pollutants.

River or Stream TerracesOver time, rivers and streams cut down into the underlying material, and formerlylow lying areas that once flooded frequently will be left in higher positions thatmay flood only rarely or not at all. These areas are called river or stream terraces.The soils on terrace positions have parent materials called old alluvium. Thesesoils, originally deposited by water, have had time to form well-developedhorizons. North Carolina’s largest river terraces are adjacent to major rivers, such as the Roanoke, Tar, Neuse, Cape Fear, and Pee Dee (Yadkin) rivers.

DeltasWhen a river enters a large body of relatively still waters (oceans or lakes), itsvelocity drops rapidly and is not sufficient to keep sediments suspended. Thesediments drop out and the deposits form a delta. Deltas usually contain largeamounts of clays, silt, and other fine sediments. They are typically swampy anddissected by many small stream channels.

Lacustrine DepositsAs a river or stream enters a still body of water, such as a lake, sediment settles outand is deposited on the lake bottom, forming lacustrine deposits. These depositsbuild up over time, filling in the lake, or the lake water level may fluctuate. Whenthe water level drops, these deposits can be above the level of the lake allowingsoils to form. Some of the largest lacustrine deposits in the U.S. are located alongthe shores of the Great Lakes.

Marine DepositsSediments not deposited on floodplains or deltas eventually make their way into theoceans. Upon entering the ocean, they settle out by particle size –largest particlebeing deposited closest to shore and finer materials, such as clay, farther out. Ifcontinental uplift occurs or sea level falls, these sediments are exposed. Along theAtlantic Coast and the Gulf of Mexico, a strip of marine sediments as much as 50–150 miles wide have formed a broad coastal plain. In North Carolina, this area isknown as the Coastal Plain region of the state. These marine deposits are very old

d. coulterFloodplain withmeandering river

d. coulter

Delta –anaccumulation ofsediment formedwhere a streamenters a lake orocean.

The Mississippidelta is the mostwell known exampleof this type ofalluvial deposit inthe U.S.

Marine Deposits onCoastal Plain


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in comparison to the alluvial deposits of fans, floodplains, and deltas. Fresh waterstreams often dissect them leaving new alluvial deposits on top of the old marinesediments.

Colluvial DepositsIn areas with long steep slopes, such as mountains, soil material and rock fragmentsmay move downhill under the influence of gravity and water. The disorganizedmass of material that accumulates on the lower portion of the slope or indepressions is called colluvium. The rock fragments in colluvium are generallyangular rather that the rounded shape of the water worn fragments of alluvium.Colluvial soils are found in North Carolina mountain coves.

Volcanic DepositsLarge amounts of volcanic ash or lava can be ejected during eruptions of volcanoes.The material ejected depends on the type of volcano. Lava flows down the sides ofthe volcano onto nearby land forming new extrusive igneous rock, such as thevolcanic islands of Hawaii. Explosive volcanic eruptions eject material of manysizes including bombs, cinders, ash, and dust. The coarser material of bombs andcinders fall on or near the volcano, while the fine ash and dust may be carriedhundreds of miles by the wind. Depending on the type of volcanic material, thesoils formed from these deposits may be fertile, as with basalt, or very infertile,such as the pumice desert near Crater Lake.

Organic DepositsOrganic deposits form in swamps, bogs, and marshy areas. As plants die or shedleaves, the organic materials are submerged in water and cannot decompose(oxidize). Over long periods of time, these deposits fill in the wetland area.Organic deposits are divided into two categories, peat and muck. Peat containsidentifiable portions of organic matter. Muck is organic matter that is decomposedto the point it cannot be identified. Organic soils can be found in poorly drainedareas of North Carolina’s Coastal Plain.

ClimateClimate, the temperature, and rainfall affect the rate of weathering. Weathering isthe breakdown or disintegration of rock at or near earth’s surface, by natural processes. Temperature and water are major climatic forces that influenceweathering. Weathering may be by physical forces (mechanical) or by chemicalprocesses.

Mechanical weathering consists of natural forces that physically break rocks intosmaller pieces –temperature, wind, ice, water, and plant roots. Rapid temperaturechange can cause expansion and contraction resulting in cracking of rocks, and thefreezing and expansion of ice in cracks further breaks rock. Unloading of overlyinglayers also causes expansion, breaking off layers of rock. Glaciers grind andabrade rock as they move over it. Running water carrying sediment abrades therock it flows over. Wind carrying sand particles abrades rock surfaces, literallysandblasting rock away. As plant roots extending into cracks in rock grow, theymay cause further cracking or breaking of the rock. Mechanical weathering occursslowly but has a significant impact over long periods of time.

d. coulterColluvial depositsat base of mountain

d. coulterMount Saint Helensand volcanic ashdeposits

d. coulterMechanical weatheringby expansion andcontraction ofrock and freezing and

expansion of ice in thecracks formed.


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Mechanical weathering will occur most rapidly in cool, dry regions.

Chemical weathering is the process by which rocks and minerals decompose due tochemical reactions. Examples of these reactions include carbonation, dissolution,hydration, hydrolysis, and oxidation-reduction reactions. The rate of chemicalweathering is influenced by water, oxygen, and the presence of organic or inorganicacids. Chemical weathering occurs more rapidly in hot, moist climates. Climaticconditions in North Carolina are favorable to a high amount of chemicalweathering.

Relief or Topography

Topography, or relief, relates to the shape and slope of the land, as well as theelevation and landscape position. Topography can slow or hasten weathering byclimatic forces. Soils vary with topography because of its influence on soilmoisture, rate of weathering and erosion, and soil temperature.

Soils on steep slopes tend to be well-drained and drier that those in flatter areas dueto increased runoff. Soils in low-lying areas tend to be wetter due to less runoffand the influx of water from surrounding higher areas.

On steep slopes, erosion tends to prevent much topsoil from accumulating, thusthese areas may have only a thin layer of topsoil. On gentle slopes there istypically less erosion, resulting in a thicker layer of topsoil. Farther down slope infootslope and toeslope positions, topsoil tends to be thicker due to accumulations ofmaterial from higher areas.

On steep slopes, the direction of the slope can also influence soil temperature andcontent. Steep north-facing slopes have soils that are cooler, moister, and havehigher amounts of organic matter in the topsoil than slopes that face south or west.

Biota (Biological Factors)

Plants, animals and other organisms help create and enrich soil. The excrement(waste) from animals adds nutrients and matter to the soil. Burrowinginvertebrates, such as earthworms, beetles, ants, and termites, create macroporesthat allow easier entry and flow of water and air through the soil. They also mixorganic matter throughout the soil. Fungi and bacteria produce substances that bindsoil particles together and stabilize soil aggregates. Lichens and mosses play a rolein first colonizing rock and produce acids that help break down rock into soilparticles. When animals or plants die or plants shed leaves, they are broken downby the decomposers, fungi and bacteria, and add organic matter and nutrients to theweathered parent material. The recycling of nutrients is essential to plant growth,

Cross section of apocosin illustratingthe relations amongsoils, topography,and vegetation(J.A.Gagnon, Jr. l996

Soil Systems in NorthCarolina


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which is essential to supplying food and nutrients to other organisms. Plant rootsalter the soil - adding nutrients and sometimes accelerating weathering of rock.

With regard to soil formation, there are two categories of vegetation –forests andgrasses. Forest areas result in a thick organic layer (O horizon) as a result of thebuild-up of forest litter, the leaves and limbs of trees. The thicker horizons do notrecycle nutrients as quickly but the nutrients go deeper into the soil. Grasslandsproduce a rich, dark topsoil (A horizon) due to the thick surface root system, whichis constantly dying and growing resulting in the continuous recycling of plantnutrients. The native vegetation depends on climate, topography, biologicalfactors, and soil factors, such as soil depth, density, chemistry, temperature, andmoisture.

Temperatures impact the decomposition rates of organic materials. Decompositionrates are low below 380 F (40 C) but rise steadily with increasing temperature to atleast 1020 F (400 C). Available nitrogen also promotes organic matterdecomposition.

Organic MatterThe organic matter enriches the soil. The organic matter of the soil is largelyderived from decomposed plant and animal material (dead bodies, litter, andwastes). Well-decomposed organic matter forms humus, a dark brown, porous,spongy material that has an earthy smell. Organic matter also includes the billionsof organisms that inhabit the soil. While organic matter accounts for only fivepercent or less of the total volume of soil, it is an important component andimportant for good soil management. The soil organic matter is also used as a foodsource by soil organisms from which they receive the energy needed for life.

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 suitable medium for valuable soil organisms, like bacteria,

fungi, and earthworms

Organic matter can be lost through erosion. When soils are tilled, organic matterdecomposes more rapidly because of the changes in water, aeration, andtemperature conditions. The amount of organic matter lost after clearing a woodedarea or tilling native grassland varies based on the kind of soil, but most organicmatter is lost within the first 10 years. Losses are higher in areas with the fasteraerobic decomposition than in areas with the slower aerobic decomposition, as inwetland soils.

The amount of organic matter is determined by a balance between additions ofplant and animal materials and losses by decomposition and erosion. Bothadditions and losses are strongly controlled by management of agricultural lands.

Organic soils of apocosin


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The portion of the plant biomass that reaches the soil’s organic matter depends largely on the amount consumed by organisms, such as mammals and insects;destroyed by fire; washed away by water; or harvested for human use.

Organic matter is an essential component of soil because it:● Provides food, carbon, and energy source for microbes and valuable

soil organisms that contribute to aggregate stability and help maintain ahealthy soil

● Stabilizes and holds soil particles together, reducing the chances of erosion

● Improves the soil’s ability to store and transmit air and water which aids the growth of crops (improves aeration and water-holding capacity)

● Stores, retains, and supplies nutrients, such as nitrogen, phosphorus, and sulfur, needed for plant and animal growth

● Helps the soil resist compaction● Retains and stores carbon from the atmosphere and other sources.● Reduces the negative environmental effects of pesticides, heavy metals,

and other pollutants by storing them● Reduces crusting● Facilitates easier penetration of plant roots● Reduces runoff andincreases the rate of water infiltration

Time

The length of time that a soil’s parent material is exposed to mechanical and chemical weathering and the other soil forming factors is also a factor and greatlyinfluences 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 factor toform one inch of soil from parent material. It is estimated that it takes over athousand years to produce one inch of topsoil from parent material that is primarilygranite. Soils, whose parent material is volcanic ash, may develop and be able tosupport life after only 40–50 years.

The soil formation processes are continuous, and over time, soils exhibit featuresthat reflect the other soil forming factors. Recently deposited material, such as thedeposition from a flood, exhibits no features from soil development activities. Theprevious soil surface and underlying horizons become buried. The time clockresets for these soils. Terraces above the active floodplain, while geneticallysimilar to the floodplain, are older land surfaces and exhibit more developmentfeatures. Older soils usually have deeper well-developed soil profiles with thick Aand B horizons (topsoil and subsoil), while young soils still retain manycharacteristics of their parent material. Older soils are often less fertile than youngsoils, due to a loss of nutrients by leaching.

The soil forming factors continue to affect soils even on “stable” landscapes.Materials are deposited on their surface and materials are blown or washed awayfrom the surface. Additions, removals, and alterations are slow or rapid, dependingon climate, landscape position, and biological activity.


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Soil Forming Processes

There are four major processes that act on parent material to alter its properties anddifferentiate one soil from another. These processes are:

- Additions- Losses- Translocations- Transformations

Additions are anything that is added to the soil profile from outside sources.Additions may include organic matter, atmospheric deposits, and soluble nutrientsin rainfall and groundwater.

Plants and other organisms add organic matter to surface layers, giving them ablack or dark brown colored organic layer. Most organic matter additions increasethe cation-exchange capacity (CEC) and nutrient levels which increase plantnutrient availability.

Rainfall brings other additions. On average, rainfall adds about five pounds ofnitrogen per acre per year. Rainfall is indirectly responsible for the additions ofnew sediments to the soils of floodplains, deposited during flooding. Nutrientsmay also be added to the soil from flooding or lateral flow of groundwater. Windscarry and add new material, such as the wind blown or eolian material.

Losses occur from leaching and erosion by wind and water. In moist climates,leaching causes the greatest losses. Some minerals, especially sodium salts,gypsum, and calcium carbonate are relatively soluble and are removed from upperlayers of the soil early in the soil’s formation. Thus, soils in humid regions generally do not have carbonates in the upper horizons. Nitrogen, a commoningredient in fertilizers, is readily lost by leaching due to natural rainfall orirrigation. Long-term use of fertilizers based on ammonium may cause increasedacidity of the soil and contribute to a loss of carbonates. Quartz, aluminum, ironoxide, and kaolinitic clay weather slowly and remain in the soil, becoming the maincomponent of highly weathered soil.

Oxygen is released to the atmosphere by plants, which have taken in some oxygenfrom the soil via their roots. Carbon dioxide is consumed by plants but added tothe soil as fresh organic matter decays. In wet soil, nitrogen can be changed to agas and be lost to the atmosphere, but it is also deposited by the atmosphere andfixed by lightning and nitrogen-fixing bacteria.

Soil particle losses can result due to all agents of erosion, especially wind andwater. Such losses can be serious since most material lost is usually topsoil, themost productive part of the soil profile.

Translocations are the movement of organic and inorganic matter within a soilprofile. Water moving through the soil can transport material. Organic mattermoves downward through cracks and root channels. Clay particles migratedownward through the soil. The topsoil (A horizon) and zone of leaching (E


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horizon) in a soil are zones of clay losses while the subsoil (B horizon) is a zone ofclay accumulation.

In areas of low rainfall, leaching is often incomplete. Water moving down throughthe soil dissolves soluble minerals, but when there is not enough water to movethroughout the soil profile, the water stops, evaporates, and leaves mineral saltsbehind. Soil layers with calcium carbonate or other salt accumulations form in thismanner. If this cycle occurs frequently enough, a calcareous hardpan can form.

Upward and lateral translocation is also possible. In arid climates, evaporation atthe surface causes water to move upward. Salts dissolved into solution will moveupward with the water and be deposited on the surface as the water evaporates.

Transformations are changes that take place in the soil, such as mineral weatheringand organic matter decomposition. Decomposers breakdown organic matter intohumus and transform organic matter into smaller organic compounds andmolecules small enough to be absorbed by plant roots. Chemical weathering altersparent material by destroying or changing existing minerals. Many of the clay-sized particles in soil are actually new minerals that form during soil development.

Studying the SoilSoil science is an interdisciplinary field that incorporates physical, chemical,geological, and biological sciences. Soil Scientists gather data about soil propertiesand interpret the information to solve land use and land management problems.Agronomy is the application of scientific principles to the raising of field crops. Inaddition to farming and agricultural issues, soil scientists look at urban growthissues and engineering issues, including building placement, placement of septictanks, and land appraisal.

The Soil ProfileSoil is a three-dimensional natural body, in the same sense that a hill, valley, ormountain has three dimensions. To study a soil as a natural body you must go tothe field. Soils occur on landscapes and are delineated on aerial photographs bytrained soil scientists. These delineations are called polygons or polypedons andhave many pedons (soil profiles) within their boundaries.

Relationship ofLandscapes toSoil Polygonsand a SoilProfile


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A Soil Profile and Soil HorizonsThe soil forming factors result in the formation of layers within the soil from thesurface down to varying depths. These layers are called horizons. Thecombination of these layers in a sequence from the surface down represent a soilprofile, or a vertical cross section of all the soil horizons at a particular location.The soil horizons, or natural layers of the soil, vary in thickness, depending uponlocation, and have somewhat irregular boundaries. However, all the boundariesgenerally parallel the earth’s surface. The uppermost layers have been changed the most, while the deepest layers are more similar to the original parent material.Exceptions to this vertical aging process occur when soil material is transportedand deposited on the surface of previously formed soil profiles. Soil horizons cansometimes be easily identified and at other times be very gradual and faint. Inolder, well-developed soils, as many as five or six master horizons may be found inthe soil profile. Younger, less-developed soils may have only two master horizons.Master horizons that may be found in soils include O, A, E, B, C, and R.

Not all horizons willbe present in youngsoils or underdisturbed conditions,such as urban areas,intensive agriculture,or where erosion issevere.

The soil seriesdescription, in asoil survey, showswhich horizonsare typicallyfound in aparticular soil.


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O horizon –When found, it is generally the uppermost layer of the soil and ispredominantly made up of organic material. It consists of leaves, needles, twigs,mosses, lichens, and other accumulations of organic matter in various stages ofdecay. This horizon is typically found in wetlands and forested area, but is notpresent in cultivated fields due to soil mixing during cultivation.

A horizon –Commonly called topsoil, this horizon is darker colored than lowerhorizons because it contains more organic matter, and it is often the most fertilelayer of the soil. This is the layer that is plowed in cultivated fields and is wheremost root activity occurs. As rainwater percolates downward through the Ahorizon it dissolves minerals, and they are leached from the topsoil.

E horizon–This horizon is characterized by its light color or bleached appearance.This is a zone which has been strongly leached. The main feature of this horizon isthe loss of clay, iron, aluminum, humus, or some combination of these, whichmigrated downward as water passed through and a concentration of sand and siltparticles. This horizon is commonly found in older, well-developed soils ofwoodlands. It is rarely found in cultivated areas, since plowing usually mixes thishorizon with the A horizon.

B horizon –Commonly called subsoil, this layer is usually lighter in color than theA horizon due to its lower content of organic matter. It is the zone of accumulationfor 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. Itis less weathered than the upper horizons and contains partially disintegrated orweathered parent material from the underlying bedrock or transported to the area byglaciers, wind, or water.

R horizon –This horizon is the bedrock. As the bedrock weathers, it contributesparent material to the C horizon above. Bedrock can be within a few inches of thesurface or many feet below the surface. In areas of eolian and alluvial soils, wherethe bedrock is very deep and below normal depth of observation, an R horizon isnot described.

Looking at Soil Properties

Soils have physical and chemical properties. When analyzing a soil’s suitability fora specific use, soil scientists must first look at the chemical properties and physicalmakeup of a given soil sample. There are over 20,000 different soil types. In truth,no two soils are expected to be just alike, but soils which are similar are groupedtogether for purposes of classification for cultural practices and uses. There aremany characteristics that differentiate one soil from another. For example, amountof available nutrients, erosion potential, and permeability are all used to describedifferent types of soil.


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Physical Properties of SoilsThe important physical properties of soil include: texture, structure, internaldrainage, compaction, consistency, color, and shrink-swell among others.

ColorSoil color is an important feature in recognizing different soil types as color is anindicator of certain physical and chemical characteristics. Color in soils is dueprimarily to two factors:

■ Humus content –organic matter■ The abundance and chemical nature of the iron compounds and certain

elements and mineralsThe depth and duration of water tables can also be inferred by soil color.Organic matter and humus are dark colored and can give soil a dark brown orblackish color when present in sufficient quantities. It is the higher levels oforganic matter and humus content that give most topsoils a darker color than theunderlying subsoil. Soils with a black or dark brown coloring usually containhigh amounts of organic matter. Lighter colored topsoil contains less organicmatter.

The element iron is a powerful coloring agent. When iron is unweathered, it haslittle or no influence on color. As iron is exposed to oxygen in the soil, it oxidizes,forming iron oxides. The presence of significant amounts of iron oxides (ferricform) usually gives soils a yellowish, brownish, or reddish color –depending onthe type of iron minerals that are present. Bright red soils are usually high in iron,well aerated, and usually do not have any significant drainage problems. Yellowcolor is produced in the presence of oxygen and moist conditions, or hydration.

In soils with drainage problems, some horizons may be saturated with water forsignificant periods during the year. When too much water is present and there is anabsence of oxygen, anaerobic microorganisms reduce the ferric iron to the ferrousform, and a gray color will result. The iron is referred to as being reduced. Thisform of iron is mobile and can be removed from the soil by leaching. After the ironis gone, the leached area generally has the grayish color.

Other elements, such as manganese, can also alter soil color. When found insufficient amounts, manganese can give soil a brown or black color. Oftenmanganese can be seen as a coating in cracks or in discrete masses, such asnodules.

Repeated cycles of saturation with water and drying create a mottled (severaldifferent colors intermixed) look to the soil. Part of the soil is gray due to the lossof iron (iron depletion) and other parts appear more red, yellow, or brown–where

Soil with oxidizediron and humus ontop

Poorly drained soilwith reduced iron


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iron oxide has accumulated (iron concentration). When the mottled pattern exhibitsgray along with red, yellow, or brown colors in a soil, it is referred to asredoximorphic features.

Soil scientists use the depth of redoximorphic features to determine the depth andduration of soil water tables. Soils saturated for short periods may only have a fewiron depletions. Most of the iron will have leached from soils that are saturated forvery long periods, and they may have a gray color throughout. This is common inwetland soils. Gray lined root channels can also develop during long periods ofsaturation. This color may indicate a loss of iron or an addition of humus fromdecayed roots.

When judging the color of a soil, most soil scientists use the Munsell Soil ColorChart to identify the soil color. This is a standardized chart that allows soilscientists to uniformly describe soil color. On the chart, color chips are categorizedby hue, value, and chroma. Names are also assigned to each color. In the field,soil scientists compare the colors found in the soil with the color chips of the chart.When using the chart, it is best to do so in natural lighting. Artificial lighting,especially florescent lights, makes colors appear different.

The photo below shows one page of the Munsell Soil Color Chart (5YR), and thehue, value, and chroma of one color chip. The hue is the color page. The value ofthe soil color is read along the right side of the page, and the chroma is read alongthe bottom of the page. The Munsell Chart books have between seven and ninepages ranging from red (10R) to yellow (5Y). Soils in very wet areas like wetlandsneed additional pages of gley soils to color correctly.

RedoximorphicFeatures


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Soil TextureSoil texture is determined by the relative proportions, the size, and the shape of theindividual soil particles –sand, silt, and clay –in the soil. Despite the dynamicnature of soils and the continuous physical, chemical, and biological activity,gravel will not turn into sand or silt into clay within our lifetime.

Particle sizeSoil is composed of three primary particles –sand, silt, and clay. Sand is thelargest and can easily be seen. Sand feels gritty when rubbed between the fingers.Silt is a medium sized particle, and is so small that individual particles are barelyvisible. Silt feels silky like dry flour. Clay is the smallest particle and cannot beseen with the unaided eye. It is microscopic. Clay feels sticky when wet and givesa soil its plasticity.

Sample MunsellChart page of GleySoils (found inwetland area)


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Sand –Soils containing large amounts of sand exhibit little plasticity, are veryporous, and cannot retain large amounts of water or nutrients. They have largevoids between the particles and can readily transport water and air. These soils arenot considered good for agriculture and can be prone to erosion.

Silt–The properties of silt are intermediate between sand and clay. Soils primarilymade of silt have moderate ability to retain water and nutrients. Mineralogically,silt particles are more like sand because they are composed largely of primaryminerals. Soils high in silt content are generally considered favorable foragriculture, due to their water-holding capacity. They can cause engineeringproblems because they will shift under stress and slide and flow when wet.

Clay –By far the smallest, clay particles are flat, many-sided, wafer-shapedparticles. Soils with a high clay content retain water very well, and do not allowwater to move through the soil quickly. The plasticity and cohesiveness (stickynature) allow it to be molded into many shapes, such as pottery bowls andsculpture. The clay particles control most of the important properties of a soil.When wet, soils high in clay are worked only with difficulty because of theirstickiness.

The total amount of pore space is greater in clays due to the higher number ofspaces than in sand with its large pore spaces. The pore spaces in clays areextremely small resulting in a many times slower rate of water and air movementthrough clay than through sand.

A portion of the water held by clay is bound so tightly that most root systemscannot absorb it, and young plant rootlets cannot readily penetrate poorlyoxygenated clay soils. Therefore, soils composed of very high clay content are notwell suited for growing crops.

Clay is an important reservoir of plant food. This function depends largely on twoimportant characteristics of clay particles: their large surface area and theirnegative electrical charge. Clay particles do not retain the negatively chargednitrate (NO3) or nitrite (NO2) ions very well. As a result, nitrates are easily leachedfrom the soil by rainfall or washed away by runoff. Clay attracts positively chargedions of nutrients like calcium, potassium, magnesium, phosphorus, zinc, and iron.

Nitrates washed off the land can cause problems for aquatic ecosystems. Thenitrates fertilize the waters promoting excessive plant growth. This can change alake or pond into a weed-choked body with undesirable, trash fish. This process iscalled eutrophication and if caused by the actions of man, such as fertilizers, it isreferred to as cultural eutrophication. It can also result in fish kills. As the plantsdie when nutrients are depleted and are decomposed, the higher rate of respirationrobs the water of dissolved oxygen essential for fish, invertebrates, and otherorganisms within the ecosystem. The reduction in application of commercialfertilizers containing nitrogen should help solve this problem.

Loamy soils –Very few soils are dominated exclusively by one texture class.Usually there is a mixture of all three particles–sand, silt, and clay–in all soils.


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The most productive soil textures for agriculture are sandy loam, loam, and siltloam, which are mixtures of the three particles. These three textures are the mostcommon topsoil textures in areas of productive farmland.

USDA Textural Triangle

The proportions of sand, silt, and clay in a soil are placed into various soil classes.Each class name has maximum and minimum percentages of each fraction(particle). A soil textural triangle shows the range and limits for each fraction andthe various class names. The triangle can be used to determine the class, or type, ofsoil if percentages of particles are known.

The following procedure is recommended in using the textural triangle:(Soil sample example –assume the soil sample contains 15% clay, 70% silt,

and 15% sand)1. First, consider the clay. The baseline at the bottom of the triangle is 0%

clay. Read up the left side of the triangle to 15% clay. Draw a pencilline parallel to the baseline for clay and through the 15% point for clay.

2. Next, consider the silt. The zero line for silt is along the left edge of thetriangle. Read down the right side of the triangle to just past 70% silt.Draw another line parallel to the zero line for silt. The lines should crossin the area designated Silt loam. This is the class name of the soil.

3. To check your accuracy, it is a good idea to draw a line for the sand. Thezero line for sand is along the right side of the triangle. Read to the leftalong the bottom of the triangle to 15%. Draw a line parallel to the zeroline for sand. You should see that all three lines cross at the same point.

The texture triangleshows thepercentages of sand,silt, and clay in thetextural classes. Theintersection of thedotted lines showsthat a soil with 55%clay, 32% silt, and13% sand has a claytexture.

To use the texturaltriangle, thepercentages of eachparticle must bedetermined. This canbe done by soilfractionation:- Put 25-30 ml of soil

in a graduated,cylinder or beaker,record the amount

- Add water to 100ml- Cover top and shake

well- After 24 hours the

particles will havesettled by size:top–claymiddle–siltbottom–sand

- measure the amountof each andcalculatepercentages


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How to Texture Soils

Texturing soils byhand.


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Soil StructureSoil structure refers to the arrangement of soil particles. It is defined as thenaturally occurring arrangement or grouping of a soil’s primary particles (sand, silt, and clay) into aggregates. These aggregates may have a variety of shapes includinggranular, blocky, prismatic, and platy. Soil scientists describe structure by type,grade, and size.

Structure is of importance in:- absorption of water (water infiltration)- aeration (circulation of air)- moisture content and drainage- fertility- erosion resistance- seedbed preparation- overall soil health

Factors affecting soil structure are:~ The kind of clay and chemical elements associated with clay~ The nature of the organic matter~ The microbial population and mycelial growth of fungi~ Freezing and thawing~ Wetting and drying~ Action of burrowing animals~ Growth of root systems of plants~ Slimy secretions by animals~ Compaction, such as by farm equipment and vehicles

Good structure enhances crop production and growth, permits a greater rate ofwater absorption, and increases resistance to erosion. Soil with good structure hasan abundance of pores, allowing water and air to move readily to plant rootsystems. Pores also allow infiltration of water after rainfall, helping to preventrunoff carrying away sediments. Soil structure can be improved by plowing undercover crops and crop residue, using no-till planting practices, cultivating, liming,or manuring.

Poor structure has a minimum of pore spaces for air and water due to closelypacked soil aggregates. Individual soil aggregates tend to fall apart, which canresult in crusting of soils. In areas of abundant rainfall, the lack of pore spaces cancause drainage problems on low-lying sites, and result in severe runoff and erosionof upland sites. Because of the limited water infiltration of these soils, water maynot penetrate deep enough to sustain crops in arid regions.

Types of Soil StructureThere are seven structural types commonly recognized in soil profiles: granular,single grain, blocky, prismatic, columnar, platy, and massive.

Granular –small, rounded aggregates usually less than ¼ inch in diameter. Theseaggregates lie loosely on the surface and are readily shaken apart and easily movedby forces of nature. The aggregates are called granules, and the pattern is calledgranular. This is the most common type of structure found in topsoil.


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Single grain –a lack of any particle aggregation. Each soil particle is separate, andthere is essentially no structure. This structure is only found in very sandy soilsand is commonly seen in sand dunes at beaches.

Blocky –cube-shaped aggregates. The original aggregates have been reduced toblocks, irregularly faced, and basically equal in height, width, and depth. If theedges of the cubes are sharp and the rectangular faces distinct, it is called angularblocky. If edges and corners are rounded, the aggregate is called subangularblocky. This is the most common type of structure in the subsoil (B horizon) inNorth Carolina.

Prismatic –vertical oriented aggregates or pillars with flat tops. These elongatedcolumns vary in length in different soils. This structure is commonly seen in soilswith high clay content and in horizons dominated by high shrink-swell clays.

Columnar –vertical oriented aggregates or pillars with rounded tops. Theseaggregates vary in length in different soils. This structure is commonly seen insoils with high salt content.

Platy –arranged in thin, horizontal layers of plates or sheets. This structure iscommonly found in soil layers that have been compacted. These soils have a slowor very slow permeability.

Massive –complete absence of structure. This condition is the opposite of singlegrain because all the soil particles cling together. This type of structure is commonfor the parent material.

Grade of Soil StructureGrade describes the distinctness of the soil structure. It is rated strong, moderate,weak, or structureless depending on how strongly it is expressed (seen in the soil).Strong structure is easy to see and separates cleanly when the soil is disturbed.Strong structure promotes the movement of water and air. Moderate structure isnot as well formed and is not as apparent in the undisturbed soil. Weak structure isdifficult to see. Structureless horizons have no recognizable structure.


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Soil ConsistenceSoil consistence is the degree and kind of cohesion and adhesion that soil exhibitsand /or the resistance of soil to deformation or rupture under stress. Fieldevaluations of consistence usually include rupture, resistance, stickiness, andplasticity.

Rupture resistance is a measure of the soil’s ability to withstand applied stress. To test rupture resistance, a naturally occurring soil aggregate from moist soil is placedbetween the thumb and index finger. Pressure is slowly applied to estimate theamount of force required to rupture the soil aggregate.

A soil that will not hold together at all is called loose. This is typically associatedwith soil that has single grain structure. If only very slight force is required, thesoil is classified as very friable. Soils requiring a slight amount of force arefriable. The aggregates requiring moderate force are considered firm, and thoserequiring very strong force are classified as very firm. Extremely firm is used if theamount of force required is beyond the strength of your fingers and requires the useof both hands.

Stickiness is the capacity of a soil to adhere to other objects. It is estimated at themoisture content that displays the greatest adherence when pressed between thethumb and forefinger, which is normally when the soil is quite wet.

Non-sticky–little or no soil adheres to fingers after release of pressure.Slightly sticky –soil adheres to both fingers after release but stretches little on

the separation of fingers.Moderately sticky–soil adheres to both fingers after release and stretches some on

the separation of fingers.Very sticky –soil adheres firmly to both fingers after release and stretches greatly

on the separation of fingers

Plasticity is the degree to which a reworked soil can be permanently deformedwithout rupturing. It is evaluated by forming a roll (wire) of soil 4 cm long. Thisis done by placing a small amount of soil in your hands and adding sufficientmoisture. This will probably require some testing with varying moisture content.The soil usually needs to be quite wet. If a roll can be made, it is then tested to seeif it will support its weight when held at one end.

Non-plastic–cannot form a 6 mm wide role (4 cm long) or cannot supportitself if held by one end

Slightly plastic–a 6 mm wide roll supports itself, but a 4 mm wide rolldoes not

Moderately plastic–a 4 mm wide roll supports itself, but a 2 mm wide rolldoes not

Very plastic–a 2 mm wide roll supports itself

Shrink-swell PotentialShrink-swell potential is a measurement of the amount of volume change that canoccur when a soil wets and dries. Most of this volume change is due to the claycontent of the soil. Clay swells when wet and shrinks when dry.


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The amount of shrink-swell is dependant on the amount and type of clay present.Soils low in clay content usually have a low shrink-swell potential. The shrink-swell potential increases as clay content increases.

The type of clay minerals in the soil greatly influences shrink-swell potential.Clays, such as kaolinite, normally shrink and swell a small amount, and soils withkaolinite usually have low shrink-swell potential. Other clay minerals, such asmontmorillonite, are expansive and shrink and swell large amounts. Soilscontaining this type of clay have moderate, high, or very high shrink-swellpotentials.

Shrink-swell is an important soil property because of potential building andengineering problems unless proper precautions are taken. A very high shrink-swell can cause severe problems when used for urban development unlessengineering precautions are taken. It can buckle roads, crack dams and buildingfoundations, and damage plant roots. Soil surveys provide information on shrink-swell potential of soils in the “Physical and Chemical Properties” of the soils.

Soil CompactionSoil compaction occurs when soil particles are pressed together, reducing the porespace between them. This increases the weight of soils per unit volume (bulkdensity). Compaction is caused by field machinery used for tillage or harvesting orby grazing animals when the soils are wet. The soil water content influencescompaction. Dry soil is much more resistant to compaction. The length of time thesoil will remain compacted is determined by how deep the compaction goes, theshrink-swell potential of the soil, and the climate.

Compaction restricts root depth, which reduces the plant’s uptake of water and nutrients. It also decreases pore space and soil temperature, which affect theactivity of soil organisms, thus decreasing the rate of decomposition of organicmatter and subsequent release of nutrients. Compaction also decreases infiltrationof water, increasing runoff and possible erosion.

Methods to reduce compaction include:- addition of organic matter- reduce travelling across the same area- till or harvest when soils are dry- use of the lightest weight equipment that can perform the job effectively- harvest timber on frozen soil or snow or use a harvest system that does notcause soil compaction

Soil DepthThe usable depth of the soil is an important consideration when evaluating soils forparticular uses. For agriculture, the soil must be deep enough to provide a suitablerooting zone.

Root restrictive layers are any permanent zones in the soil that restrict plant rootgrowth. Bedrock is the most common root restrictive layer. A less common root


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restrictive layer is fragipan, a dense, brittle horizon usually found in soils formed intransported parent material. Fragipans are not as hard as bedrock, but they willlimit the movement of water and growth of plant roots.

Shallow soils do not have sufficient water holding capacity to enable crops tosurvive droughts. In agricultural areas, soils that are root limited are generally usedfor pasture, wildlife habitat, or forestland instead of cropland. Plants with deeproots, such as trees, alfalfa, and corn require deep soils for best growth.

Soil depth is also an important consideration for suitability of a soil for urban uses.Houses with basements or septic systems should be built on deep, well-drainedsoils. Shallow soils necessitate building houses with a slab or shallow foundation,and septic systems may not be permitted. Shallow soils also increase the time andcost of installing sewer and water lines. Blasting can be used to overcome theproblem of shallow soils, but it is expensive and time consuming. The “Soil and Water Features” table of a soil survey provides information about the depth and hardness of bedrock.

Movement of Water in SoilsUnderstanding the movement of water through soils is important for determiningthe suitability of a soil for all types of uses. Soil properties, such as texture,structure, and the type and amount of pores, have a great influence on watermovement.

Gravitational water –Gravitational water is free water, which means it simplyflows through or off the soil. Gravitational water is responsible for nutrientleaching and soil erosion. It is essential to apply nutrients at the right time, whenplants need them, to avoid problems with leaching. Otherwise, they could beleached below the plant root zone. This is particularly important with sandy soils.The rate of flow of irrigation water should be regulated to prevent leaching anderosion.

Capillary water –Unlike gravitational water, capillary water does not flowthrough the soil. It is water flow around individual soil particles, and it moves tothe highest point of tension. Plant roots remove capillary water until the attractionbetween the water molecule and soil particles becomes stronger than the attractiveforces of the roots/root hairs. At this point, water is not available for the plant’s use and the permanent wilting point is reached. Temporary wilting can occur when thewater film gets thin and the capillary movement of water slows.

Capillary rise of water –Capillary rise occurs when water from a water tablemoves upward through the soil against the force of gravity. This is due to theattraction between the water molecule and soil particles being stronger than theforce of gravity.


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PermeabilityPermeability refers to the movement of air and water within the soil. Permeabilityrate is the rate at which a saturated soil transmits water, usually expressed in inchesper hour. Texture, structure, bulk density, and the type of connectivity ofmacropores influence permeability.

In sandy soils, the macropores are large and continuous, allowing water to moverapidly downward through the soil. In soils with high clay content, the macroporesare often small and discontinuous, resulting in slower permeability. As clay getswet and begins to swell, the size of macropores decreases which also decreasespermeability. Soils high in expansive clays may swell enough to close these porescompletely.

Structure affects permeability by influencing the path by which water can flowthrough the soil. Granular structure has a large number of interconnectedmacropores and permits downward movement of water readily. Blocky structureallows moderate permeability. Platy structure requires water to flow over a longerand slower path, usually resulting in slow to very slow permeability in soils withthis structure. Prismatic structure tend to have high amounts of clay which swellswhen wet, closing most macropores. The “Physical and Chemical Properties” table of a soil survey provides information about permeability of soils.

InfiltrationInfiltration 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 anygiven instant, usually expressed in inches per hour. Like permeability, infiltrationis influenced by texture, structure, bulk density, and the type and connectivity ofmacropores.

DrainageDrainage is the removal of excess water from soil. The term refers to thefrequency and duration of periods of saturation or partial saturation. Soil drainageis important because it effects land use and management decisions. The internaldrainage of soils is related to the depth of any water tables.

HydraulicConductivity –is ameasurement of theamount of water thatcan move downwardthrough a unit areaof unsaturated soilin a unit of time.

Saturated hydraulicconductivity –is ameasurement of theamount of water thatcan move downwardthrough a unit areaof saturated soil in aunit of time.


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A water table is the ground water level –a zone in the soil that is saturated withwater for significant periods during the year. There are two types of water tables,apparent and perched. An apparent water table is one in which all horizonsbetween the upper boundary of saturation and a depth of six feet is saturated. Aperched water table is a water table in which saturated layers are underlain by oneor more unsaturated layers within six feet of the surface. The depth and duration ofthese water tables is influenced by many factors including landscape position,permeability, and rainfall. The presence of redoximorphic features is an indicatorof a high water table.

Apparent water tables are frequently encountered in low-lying or flat areas wherethe slope of the land is insufficient to provide good drainage. Apparent watertables are commonly found in floodplains, depressions, and flat areas of the CoastalPlain of North Carolina.

Perched water tables are often found in soil horizons that have slow or very slowpermeability. Water moves downward until it encounters the slowly permeablelayer. When water enters the soil faster than it can move through this restrictivelayer, the zone immediately above the layer becomes saturated, while the zonebelow the restrictive layer remains unsaturated. These water tables are common insoils with clayey subsoils and poor structure.

The depth of soil water tables tends to fluctuate throughout the year due largely tochanges in the amount of rainfall and the rate of transpiration. In North Carolina,the highest water tables are normally encountered in the winter and early spring.The “Soil and Water Features” table of a soil survey provides information about thekind, depth, and duration of water tables.

Drainage ClassesThere are seven classes of natural soil drainage: excessively drained, somewhatexcessively drained, well drained, moderately well drained, somewhat poorlydrained, poorly drained, and very poorly drained. Exact definitions of whatconstitutes each drainage class differ for each state. A general definition of eachfollows.

Excessively drained –Water is removed very rapidly, and the occurrence ofinternal free water is very rare or very deep. The soils are commonly coarse-textured and have very high hydraulic conductivity or are extremely shallow.

Somewhat excessively drained –Water is removed from the soil rapidly, andinternal free water is very rare or very deep. The soils are commonly coarse-textured with high-saturated hydraulic conductivity or are very shallow.

Well drained - Water is removed readily but not rapidly. Internal free water isdeep or very deep. Water is available to plants throughout most of the growingseason in humid regions, and wetness does not inhibit the growth of roots forsignificant periods during most growing seasons.

N.C. CooperativeExtension Service

Sandy soils drainmore rapidly thanclayey soils

Somewhat excessivelydrained soil

Well drained soil


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Moderately well drained–Water is removed from the soil somewhat slowly duringsome periods of the year. Internal free water commonly is moderately deep andmay persist from a short time to long periods. Although soils are wet for only ashort time within the rooting depth during the growing season, it is long enoughthat most plants intolerant of wetness are affected. The soils commonly havemoderately low or lower saturated hydraulic conductivity in a layer within theupper six feet.

Somewhat poorly drained–Water is removed slowly causing the soil to be wet at ashallow depth for significant periods during the growing season. The occurrence offree water commonly is shallow to moderately deep and may persist for short tolong periods. Wetness restricts growth of plants intolerant of wetness, unlessartificial drainage is provided. The soils commonly have one or more of thefollowing characteristics: low or very low saturated conductivity, a high watertable, or additional water from seepage.

Poorly drained–Water is removed so slowly that the soil is wet at shallow depthsperiodically during the growing season or remains wet for long periods. Free wateris shallow or very shallow and usually present - persisting for moderate to longperiods. Free water is at or near the surface long enough during the growing seasonto prevent the growth of most plants intolerant of wetness unless the soil isartificially drained. However, the soil is not continuously wet directly below plow-depth. The water table is commonly the result of low or very low saturatedhydraulic conductivity.

Very Poorly drained - Water is removed so slowly that free water remains at orvery near the ground surface for much of the growing season. Internal free water isvery shallow and may persist for long periods or permanently. Unless the soil isartificially drained, most plants intolerant of wetness cannot be grown. These soilsare commonly level or depressed and frequently ponded (or wetland soils).

Drainage class information for soils can be found in the map unit descriptions andseries descriptions of a soil survey.

FloodingFlooding is the temporary covering of the soil surface by flowing water. It can becaused by overflow from streams or rivers or by runoff from adjacent slopes.Shallow water standing or flowing for short periods after rainfall or snowmelt is notconsidered flooding. Neither is the standing water of marshes, swamps, or closeddepressions, which is considered ponding.

Flood Frequency Classes● None: Flooding is not probable● Rare: Flooding is unlikely but possible under unusual weather

conditions (chance of flooding nearly 0% to 5% in any year)● Occasional: Flooding occurs infrequently under normal weather

conditions (chance of flooding 5%–50% in any year)● Frequent: Flooding occurs often under normal weather conditions

(chance of flooding is 50% in any year)

Moderately well drainedsoil

Somewhat poorlydrained soil

Poorly drained soil

Surface of very poorlydrained soil


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Flood Duration Classes◊ Very brief: less than 2 days◊ Brief: 2 - 7 days◊ Long: 7 - 30 days◊ Very Long: more than 30 days

The time of year that flooding is most likely to occur is expressed in months, andtwo-thirds to three-fourths of all flooding occurs during the stated period.Inundation by overflowing streams or runoff from nearby slopes may damage cropsor delay their planting or harvesting. Scouring of the land by floodwaters canremove favorable soil material. Deposition of soil material can be beneficial ordetrimental. Long periods of flooding reduce crop yields.

Information on flooding is based on evidence in the soil profile:- thin strata of gravel, sand, silt, or clay deposited by floodwater- irregular decrease in organic matter content with increasing depth- little or no horizon development- soil stratification is an indication of deposition by flooding

Local information about the extent and level of flooding and the relation of eachsoil on the landscape to historic floods are also considered. Information on theextent of flooding that is based on soil data is less specific than that provided bydetailed engineering surveys that delineate flood-prone areas at specific floodfrequency levels. The “Soil and Water Features” table of a soil survey provides information about the frequency, duration, and typical months of flooding.

Available Water CapacityThe available water capacity is an estimate of how much water a soil can hold andrelease for use by plants. It is measured in inches of water per inch of soil, and isestimated by calculating the difference between the amount of soil water at fieldcapacity and the amount at wilting point. Field capacity is the amount of waterthat soil can hold after the gravitational water has drained away. Wilting point isthe amount of water held by a soil that is beyond the ability of most plants toextract.

Available water capacity is influenced by soil texture, content of rock fragments,depth to a root-restrictive layer, organic matter content, and compaction. The sizeand strength of soil structure can influence the availability and rate of waterreleased to plant roots. Sandy soils tend to have low available water capacities.Silty soils tend to have high available water capacities. Rocks do not contain anyavailable water, therefore the greater the volume of rock fragments in the soil, thelower the available water capacity. The “Physical and Chemical Properties” table of a soil survey provides information about available water capacity.

ReactionReaction (soil pH) is a measure of acidity and alkalinity of a soil. Acidity oralkalinity is determined by the amount of hydrogen or hydroxyl ions in the soil.When the numbers of hydrogen ions outnumber the hydroxyl ions, the soil isacidic. In the reverse condition, the soil is alkaline (basic). A pH scale is used to


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measure the level of acidity or alkalinity. The pH scale ranges from 0–14m with 7being neutral. From 0 –7 is acidic, and the lower the number the more acidic thesoil. From 7 –14 is basic, and the higher the number the more alkaline, or basic,the soil.

In North Carolina, the natural soil pH ranges from 4.0 to 8.0 with most soilstending to be acidic–having pH values of 4.5 to 6.0. In the western United States,where the climate is much drier, pH values tend to be much higher.

Most agricultural and ornamental plants prefer a pH of 5.5 to 7.0. Some plants,such as azaleas, blueberries, camellias, ferns, pines, rhododendron, spruce, and firs,prefer a soil with a pH of 4.0 to 5.0. Others, such as asparagus and sagebrush,tolerate soils with a pH of 7.0 to 8.0. Above 8.5, the soil is too alkaline for mostplants, and soil with a pH less than 3.5 is too acidic. The presence of certain plants,such as broom sedge in pastures, is indicative of acidic soil. The pH in a soil willvary by layer. To optimize plant growth, farmers and homeowners commonlyapply products to adjust the pH. Lime, made from ground limestone, is the mostcommon ingredient used to raise the pH of the soil. The amount of lime neededshould be determined by a soil test. The “Physical and Chemical Properties” table of a soil survey provides information on soil reaction.

Acid Deposition (acid precipitation or acid rain)Acid deposition is a serious environmental problem. Although usually thought ofas acid precipitation and referred to as acid rain, the term actually means any kindof precipitation or cloud vapor with a low pH and the deposit of dry acidicparticles. Natural precipitation is slightly acidic, with a pH of 5.0 –5.6. Theaddition of nitrogen oxides and sulfur dioxide to the atmosphere from the burningof fossil fuels, such as coal by power plants and industry and gasoline by motorvehicles, has increased the acidity. These chemicals are dissolved into a solutionproducing sulfuric and nitric acids, when exposed to atmospheric moisture.

The severity of the problem often depends on the soil pH. If the soil is alkaline,acid “rain” is neutralized. If soil is acid, then acid “rain” can severely increase soil acidity. The lowering of soil pH can damage plants and other living organisms andlower crop yields. In the United States, acid rain is mainly a problem in the East,where many of the soils are naturally acidic.

pH Scale0 → 7 → 14

Acidic ▲ BasicNeutral

Soil samples can betested through thelocal CooperativeExtension Office forpH and nutrientlevels.

Flooding along ariver


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Cation-Exchange Capacity (CEC)Cation-exchange capacity is a measure of the ability of a soil to hold and exchangecations (positively charged ions). It is one of the most important chemicalproperties in soil and is usually closely related to soil fertility. A few plant nutrientcations that are a part of CEC include calcium, magnesium, potassium, iron, andammonium.

Generally, as CEC levels decrease, more frequent and smaller applications offertilizers are desirable. Smaller applications of fertilizer applied to soils that havelow CEC levels may reduce fertilizer loss to surface and ground waters, lesseningthe impact on water quality.

Soil ClassificationThe National Cooperative Soil Survey identifies and maps over 20,000 differentkinds of soil in the United States. Most soils are given a name, which generallycomes from the locale where the soil was first mapped. Named soils are referred toas soil series.

Soils are named and classified on the basis of physical and chemical properties intheir horizon. The official book used for classifying soils, Soil Taxonomy, usescolor, texture, structure, and other properties of the soil from the surface to a depthof two meters to key the soil into a classification system. This system provides acommon language for scientists.

Ultisols are the dominant soil order in North Carolina. These soils arecharacterized by a low amount of plant nutrients and a clay increase in the Bhorizon. They are often developed under a mixed forest (conifers and deciduoustrees) and are usually highly weathered due to high average rainfall. These soils


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require the addition of lime and fertilizer to be productive farmland and produce awide variety of crops.

Soil SurveysA soil survey is a systematic examination, description, classification, and mappingof soils in an area. Most counties in North Carolina have a published soil survey orhave one in progress. A soil survey can be obtained from the Soil and WaterConservation office. A soil survey includes: general information on the county(relief, drainage, and climate), detailed and general soil maps, general and detailedsoil descriptions, definitions, information on physical and chemical properties ofsoils, tables for soil interpretations, and information on the use and management ofsoils –appropriate uses of the specific soils –recreational, commercial, andresidential, and limitations–shrink-swell, poor drainage, and erodibility.

General soil maps are good for broad planning purposes and can be used tocompare the suitability of large areas for general land uses. These maps are foundat the beginning of the map section near the back of the soil survey. General soilmap unit descriptions providing information about the dominant soils, theirproperties and limitations are located near the front of the survey.

Detailed soil maps should be consulted for specific areas and their suitability forplanning the management of a farm, field, or selection of a building or otherstructure. These maps are located near the back or are loose in the survey, with anindex map for use in locating specific map sheets and a legend of map symbolslocated at the beginning of the section. Map unit descriptions of the detailed soilmaps provide information on the general nature of the map units, brief non-technical soil descriptions, and the properties and limitations.

The Classification of the Soils section provides information about specific soilseries including a detailed description and characteristics of the soil and thematerial from which it formed.

The Use and Management of the Soils section provides information on primefarmland, cropland and yields per acre, pasture and hayland, orchards, woodlandmanagement and productivity, recreation, and wildlife habitat. It also includes anengineering section covering 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.

Interpretive tables may include: temperature and precipitation, freeze dates forspring and fall, growing season, acreage and proportionate extent of the soils, landcapability and yields per acre of crops and pasture, soil and water features, andclassification of the soils, in addition to the types of information already listedunder the use and management section above.

Envirothon participants should be familiar with all parts of a soil survey and beable to answer questions using maps, tables, and written sections of the survey.

General Soil Map

Detailed Soil Map


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Soil ApplicationsSoil scientists evaluate sites in terms of particular projects to determine land usecapabilities and guidelines. The projects may include: building sites, agriculturaluses, septic systems, landfill locations, road placement, and recreationaldevelopment. Acceptable activities depend on soil quality and landscape.Economic and environmental objectives must also be examined as well as howlaws and regulations affect these decisions and how the decisions affect laws andregulations.

Land Capability ClassificationLand capability classes, and in most cases subclasses, are assigned to each soil.The class suggests the suitability of the soil for field crops or pasture and provide ageneral indication of the need for conservation treatment and management.

There are eight capability classes, which are designated by Arabic or Romannumerals (I through VIII) representing progressively greater limitations andnarrower choices for practical land use. Capability subclasses are noted as an e, w,s, or c following the capability class numeral (i.e. ІІs). Capability subclass indications are as defined in the right margin.

Of the eight capability classes, only the first four are considered usable forcropland. Class I land is the best agricultural land. Classes II, III, and IV needprogressively more care and protection when cultivated crops are grown. Soils inclasses V, VI, and VII are suited for native plants, such as forests, although somesoils in classes V and VI can produce specialized crops, such as fruit trees orornamentals. Soils in class VIII include the very steep and rocky areas of themountain region and the very wet tidal marshes. They do not respond tomanagement without major reclamation.

Hydric soils are wet soils defined as a group for the purpose of implementation oflegislation for preserving wetlands and for assessing the potential habitat forwildlife. The soils were designated on the basis of flooding, water table, anddrainage class criteria. These soils develop under wet conditions, meaning they areanaerobic (without oxygen) within 12 inches, and can support the growth andregeneration of hydrophytic vegetation.

Land CapabilityClassification:

Class I–fewlimitations thatrestrict useClass II–somelimitation thatreduce choices ofplants or requiremoderateconservationpracticesClass III–severelimitationsaffecting plantchoices orrequiring specialconservationpractices or bothClassIV–verysevere limitationsrestricting plantchoices orrequiring verycarefulmanagement orbothClass V–notsuited forcultivationClass VI–unsuitedto cultivation,limited to pasture,woodland, orwildlife habitatClass VII - userestricted largely tograzing, woodland,or wildlifeClass VIII - userestricted torecreation, watersupply, wildlife, oraesthetics

Subclasses:“e”–indicates thesoil is erosive“w”–indicates awetland limitation“s”–indicatesshallow, droughty,or stony soil“c” –indicates aclimatic limitation


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Prime FarmlandPrime farmland, as defined by the U.S. Department of Agriculture, is land that hasthe best combination of physical and chemical characteristics for producing food,feed, forage, fiber, and oilseed crops. It can be cultivated land, pastureland,forestland, or other land available for these uses, but it is not urban or built-up landor water areas. Unique farmland is land, other than prime farmland, that is used forproduction of specific high value crops, such as citrus, tree nuts, olives, cranberries,fruit, and vegetables. Public land in National Parks, National Forests, militaryreservations, and State Parks is not available for farming.

The acreage of prime farmland is limited, and is of major importance in meetingthe nation’s short- and long-range needs for food and fiber. Government at alllevels - local, State, and Federal, as well as individuals must encourage andfacilitate the management and wise use of this valuable resource. A recent trend inland use has been the loss of some prime farmland to industrial and urban uses.The loss of this land puts pressure on marginal lands, which generally are moreerodible, droughty, less productive, and cannot be easily cultivated.

Soil RecyclingThe weathering of bedrock releases nutrients, such as phosphorus and sulfur,needed for plant growth. Nitrogen fixing bacteria (nitrifying bacteria) that live inthe soil change atmospheric nitrogen to a form plants can use. Lightning can alsofix atmospheric nitrogen. Plants absorb these nutrients, storing them in theirtissues. As plants or the organisms that consumed them emit waste or die, the plantresidue and tissue of organisms is decomposed by fungi and bacteria, thusreleasing the nutrients to enrich the soil and once again be used by plants.Nutrients can be leached from the upper layers of soil and no longer available forplant uptake. The fungi and bacteria, along with some soil invertebrates, secretesubstances and help stabilize soil aggregates. Nutrients, such as nitrogen,phosphorus, and potassium, are added by the application of fertilizers. Nutrientscan be lost to runoff of surface water. Soil is lost to various forms of erosion, andnew soil is added by weathering of bedrock and addition of organic material.When crops are harvested, much of the organic material and nutrients within arelost and unavailable to replenish the soil. The cycle is a never-ending interplaybetween the atmosphere, biosphere, lithosphere, and the actions of man.

Lithosphere –Therigid outer layer ofthe earth.

Soil formationand the gainand loss ofnutrients is acontinuousinterplaybetweenatmospheric,biological, andlithosphericfactors.


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The biotic, or “organism,” factor of soil formation include, the plants that grow in the soil, the array of organisms that live and die in the soil, and the organisms thatimpact soil through their burrowing and mixing of soil. Organisms, through theirlitter and death, contribute organic matter to the soil. Organic matter helps keep thesoil from compacting, helps retain moisture, adds mineral nutrients, provides thefood for microorganisms, and promotes the formation of soil structure throughaggregation of the soil particles. Well structured soils are porous and permeable,allowing the movement of water and gases needed by plants and other organisms.The application of lime reduces soil acidity and increases the availability ofcalcium. This improves conditions for microbiotic activity, increasing the numbersand diversity of soil organisms, which in turn increases organic matterdecomposition and the release of plant nutrients. Lime, in combination withorganic matter, is necessary for well-developed soil structure.

The recycling of nutrients, from the subsoil to the vegetation and back again to thesoil surface as litter, counters the nutrient loss from leaching. The litter is alsoimportant in providing soil cover that reduces evaporation, moderating soiltemperature fluctuations, and protecting soils from the erosive forces of wind,water runoff, and rainfall.

The figure above displays a soil food web and organisms beneficial to the soil andto its recycling.

Organic Matter From Tree LitterConiferous trees produce needle litter that contains fewer nutrients and is moreresistant to decomposition than the broadleaf litter of deciduous trees. Freezing,drought, low summer temperatures, acidity, and the resistant nature of conifer litterslow the decomposition of organic matter. The by-products of the decompositionacidify the soil, creating an adverse environment for earthworms and other soilorganisms, which prefer less acidic conditions. This results in little of the humusbeing mixed with the mineral soil. Infiltrating water is also made more acidic andintensifies chemical weathering and leaching of plant nutrients and other minerals.

Burrowing animals,such as beetles, ants,spiders, earthworms,pseudoscorpions,wireworms, andmites, help aerate thesoil, allow forimproved infiltrationof water, and mixorganic matter intothe soil.

Fungi, bacteria,protozoa, and manyinvertebrates excretea material that helpsstabilize the soilaggregates.

Nodule bacteria,Rhizobium ,andactinomycetes fixnitrogen to the soil.

Fungi and bacteriaare decomposers thatbreak down organicmatter and returnessential nutrients tothe soil.

The wastes of all theorganisms, litter fromplants, and thedecomposed tissue ofdead plants andorganisms provideorganic matter andnutrients to the soil.


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The litter from hardwood, deciduous trees is more adaptable to soil organisms andhigher in nutrients. As a result, decomposition occurs more rapidly and humus isincorporated into the soil surface layer. The mixing of organic matter into the soilsurface produces a finely structured, well-humidified, highly fertile soil.

ErosionErosion is the process by which soil is transported from one place to another bywind, running water, gravity, glaciers, and waves. It is a natural process that isoften accelerated by man’s activities. Erosion has many harmful effects: poorersoils, smaller crop harvests, higher food prices, silt buildup in waterways, increasesflooding, contributes to costly dredging and navigation problems, degrades aquaticplant and animal habitat, and is detrimental to aquatic life - decreasing the bioticpotential (survival and reproduction) of fish, oysters, clams, and other aquaticspecies.

Running water, which includes rivers, streams, and runoff, is the major cause oferosion. The three types of erosion by surface water runoff are sheet, rill, andgully.

Sheet erosion is the most difficult to see. It is the gradual eroding away of a verythin, uniform layer (sheet) of soil. Sheet erosion occurs where there is enoughvegetative cover to prevent rill erosion but not enough to stop erosion completely.It is seen as muddy runoff.

Rill erosion occurs when runoff water flows in threads of current, typicallydeveloping into tiny channels called rills. Many channels can be seen and mayrange from extremely small to a few inches in depth. This erosion occurs on gentleslopes with little protective vegetation.

Gully erosion occurs as rills become larger, forming deeper and wider channels. Itis the most dramatic form of soil erosion. Gullies may become too large or deepfor farm equipment to cross. Gully erosion develops on steeper slopes with little orno vegetation. Although gully erosion is the most visible, sheet and rill erosion area greater national concern. Sheet and rill erosion remove an average of five tons ofsoil from every acre of cultivated cropland each year. This is about the same as theamount of soil that can build up, or accumulate, in one year.

Stream bank erosion is loss of soil along the banks of streams or rivers, usuallycaused by an increased volume of water after a heavy rainfall. Most of the time theincreased stream flow is due to changes in the watershed, which cause increasedamounts of runoff. Human activities, such as logging of forests and urbanizationwith increased impervious surfaces, as well as natural events, such as forest fires,can result in the increased runoff.

Most wind erosion occurs in areas of high prevailing winds and low annual rainfallor very sandy soils. The soils have smooth surfaces and are composed of particlesthat move easily. The planting of windbreaks or maintaining a cover crop are goodmethods to control wind erosion.

Rill Erosion


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Land slippage refers to blocks of water-saturated soils moving down slopes inresponse to gravity. Examples would be the cave-in of a cliff or bluff thatoverhangs a river or a steeply cut road embankment.

Sediment, soil particles that enter the water as a result of erosion, is the numberone pollutant (by volume) of North Carolina’s waterways. Sedimentationisdetrimental to aquatic life, but it can either improve or degrade the soil where it isdeposited. The impact of deposited sediment depends on the characteristics of theoriginal soil that was eroded, the rate of deposition, the type of material, and thedepth of deposition.

No one benefits from erosion. Nutrient losses from erosion and possible crop yieldreductions are only part of the costs involved. Erosion may also result in the loss oforganic matter and deterioration of soil physical characteristics. Coarser materialgenerally results in degraded soil structure and decreased fertility. Sedimentcontributes to costly problems including: removal of sediment from drinking watersupplies; the filling in of harbors, waterways, and canals; and increased wear andtear on equipment.

Soil, as a natural resource, is not renewable over the short term. Depending on howsoil is defined, it may take nature 50–500 years or longer to form one inch of soil.While a loss of five tons of soil per acre is acceptable because this is generally thereplacement rate, it also represents an average loss of 15 pounds of nitrogen peracre.

Factors Affecting the Rate of Soil ErosionRainfall, runoff, slope of the land, soil erodibility, and vegetative cover affect therate of erosion.

Rainfall and RunoffAnnual precipitation in the United States ranges from almost nothing in parts ofDeath Valley, California, to 140 inches per year in Washington State. The amountof precipitation an area receives greatly affects the erosion rate. Even moreimportant, is the seasonal rainfall pattern, which may be large volumes of rain in ashort period of time rather than spread throughout the year.

Soil ErodibilityThe soil erodibility factor is a relative index of the susceptibility of bare, cultivatedsoil to particle detachment and removal and transport by rainfall. It is computedfrom soil composition, saturated hydraulic conductivity, and structure. Soils withmore silt and very fine sand are generally more erosive because of weaker bonding.Soil structure greatly influences soil erodibility. If the rate of water infiltration isslow, as in platy or massive structure, there is more water runoff, thus morepotential for erosion. Soil structure can be improved and erosion decreased by theaddition of organic materials and a good cover crop.

Soils are also assigned a soil loss tolerance, which is defined as an estimatedmaximum rate of annual soil erosion that will permit crop productivity to be


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sustained economically and indefinitely. Deeper soils can lose larger amounts ofsoil per acre per year and sustain productivity. The five classes range from one tonper acre per year for very shallow soils to five tons per acre per year for very deepsoils.

Vegetative CoverThe type and amount of vegetative cover affects the rate of erosion. If the cover isa grass, the erosion will be considerably less than if the field is planted inconventional row crops, such as corn or cotton. The amount of cover per surfacearea plus the root system that helps bind soil aggregates are important factors.Soils that are heavily vegetated seldom erode at a high rate.

Topography (Steepness and Length of Slope)The slope of the land greatly affects the intensity of surface runoff and soil erosion.The steepness of the slope is indicated in terms of percentages. A 10 percent slopeis one that drops 10 feet in elevation over a distance of 100 feet. The higher thepercentage, the steeper the slope, and therefore the amount and velocity of runoffwill be greater. The length of the slope also effects erosion. The longer the slope,the greater the velocity of runoff water will be.

Problems Facing FarmersUrbanization, soil erosion, flood and sediment damage, limited water supplies,salinization, harmful effects of pesticides, atmospheric pollution, soil compaction,high fuel costs, and high fertilizer costs are a few of the problems facing farmers.In response to these problems, governmental agencies and farmers have borrowed aterm from the corporate world and adopted the use of a Best Management Practicessystem.

Best Management Practices (BMPs) Best Management Practices (BMPs)are conservation practices that farmers, as well as suburban and urban landowners,can employ to help conserve soil and water resources. BMPs are very effectivewhen implemented according to design parameters and used and maintainedproperly.

Crop rotation refers to planting different crops in the same field in different years,or simply stated, rotating the crop planted in a field from year to year. Differentcrops use different nutrients, and if the same crop is planted in a field year afteryear, it depletes the soil and crop yield will be lower. This practice works best ifone of the crops is a legume that can add nitrogen to the soil. Using grass or a haycrop, which reduces erosion, as one of the crops is also helpful.

Sod-based rotation is the planting of a grain or other close growing crop, whichestablishes sod, in rotation with a different crop as a crop rotation. Very littleerosion occurs in fields with a thick cover crop forming a sod base. Planting asod-based crop every other year effectively cuts the erosion rate by one-half.

Grasses and legumes in rotation is the practice of using grasses and


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legumes in rotation. Grasses are sod-building crops that help hold the soil in placeand break the erosive force of raindrops and runoff. The legumes, with theirsymbiotic nitrogen-fixing bacteria in nodules on their roots, add nitrogen to thesoil.

Conservation tillage Conservation tillage refers to no-till, strip-till, minimum-till,ridge till, and sod planting. For maximum benefits from conservation tillage, somekind of crop residue must be left as surface mulch. This can be dead residue fromthe prior year’s crop or residue of grasses and/or legumes that are killed with herbicides. The amount of mulch should be sufficient to cover at least 30 percent ofthe soil surface and should be evenly distributed. This mulch reduces soil waterevaporation and erosion, and adds organic matter. The next planting season, thecrop is planted through the mulch without plowing the soil.

Grass waterway is created by planting grass in a constructed channel that carriesrunoff from a field. This allows surplus water to leave the field without causingsevere erosion problems. The grass slows the velocity of the water, allowing mostof the sediment (soil in solution) to be deposited before the water exits the field.

Field borders are created by planting a 10–30 foot strip of grass around the edgeof a field. This allows for travel and turnaround of equipment while slowingerosion. It is most effective when water leaves the field crossing the grassedborder. The field border slows the velocity of the water allowing soil particles andthe attached phosphorus to be deposited from suspension. Field borders can alsoprovide beneficial wildlife areas for quail, turkey, rabbits, deer, and predators.

Contour farming is the practice of plowing around the contour of the terrain(across the changing slope) rather than in straight lines. It creates small dams thatdivert and retard the flow of water downhill, sending it along the contour with therow. This practice helps retain water and prevent soil erosion.

Strip cropping is the practice of growing at least two different crops in the samefield in alternating strips. One crop is usually a row crop with a higher rate oferosion, and the other crop is usually a grass or other low growing crop with alower rate of erosion. Sediment-filled water leaving the row crop strip deposits thesoil particles when slowed by the close growing crop strip.

Terraces involve constructing several embankment ridges of soil along the contourto control runoff by breaking up the slope into smaller sections. Terraces collectrunoff, forcing it to flow along the contour where it can soak into the soil or alongchannels into a grassed waterway or other means to leave the field, thus reducingerosion. Sometimes, terraces are constructed parallel to one another (parallelterraces).

Cover crop is the practice of planting a crop in the fall, after harvest, to have plantcover on the bare field. The cover crop protects the soil during the winter. Thesecrops are usually a small grain that may be harvested the next year, turned under toprovide organic material, or used as a cover when planting a no-till crop. Thesecrops can be an important winter food source for some wildlife.

d.coulter

Contour farmingshown on the hill inthe background

Strip croppingshown inforeground and oncontours

d.coulter


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Crop residue management is the practice of, during harvest, cutting up andleaving unharvested remains on top of the soil to provide ground cover. The cut upplant material is called residue. The residue helps reduce the impact of rain hittingthe soil and slows runoff, allowing more of the rainwater to soak into the soil.

Filter strips are the practice of planting strips of permanent vegetation to filter outpollutants including: sediment, nutrients, pesticides, and debris from runoff. Theyare usually planted around bodies of water, such as ponds, lakes, and streams, oraround sensitive areas. Sometimes, wildlife plantings are used in conjunction witha grass in filter strips.

Windbreaks are the practice of planting trees in rows perpendicular to the flow ofthe wind for the purpose of breaking up the wind. They help protect the soil fromerosion and are used in areas with sandy soils and high prevailing winds. Twotrees often used in windbreaks in North Carolina are Eastern red cedar and loblollypine.

Diversion is the practice of placing an individually designed channel across ahillside to divert runoff out of the field and into an area where the water can befiltered before contaminating surface water.

Water control structure is the practice of placing wooden or metal doors indrainage canals to manage water. Planks can be removed to allow more water flowout during times of high rainfall or added to store more water during drier periods.These structures are also effective in retaining nutrients and pesticides, allowingaquatic plants and algae to use the nutrients, and allowing the break down ofpesticides into harmless components.

Integrated Pest Management (IPM) is the practice of using various or multiplemethods to control pest populations. A knowledgeable professional scouts thecrops for insects, determines the level of pest infestation, and if and when achemical pesticide is necessary. When needed, the chemical pesticide is appliedonly to that particular area. If the infestation never reaches the level requiringpesticide use, the chemicals are not applied, or other methods may be employed,such as natural predators or mixing crops with others that are not as prone toinfestation. The practice reduces the amount of pesticides added to theenvironment and reduces soil compaction due to equipment use.

Soil test is the practice of testing soils for nutrient content to determine thenutrients needed for the crop and the application rate. This BMP is effective inlimiting the amount of unnecessary nutrients added to the soil and reduces theamount of nutrients that could be carried by runoff into near-by waters, pollutingthe water or causing eutrophication (excessive plant growth).

Best Management Practices are particularly effective when used together in aconservation system, for example: contour farming with strip cropping, grassedwaterways, filter strips, diversion, field borders, no-till, sod-based rotation, andIPM.

USDAConservation tillagefor crop residuemanagement (top),60% corn residue(bottom)

USDAFilter strip (top)Riparian buffer(bottom)

Windbreak


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Human Actions and Soil

Human actions can reduce the quality of soils. Soil degradation usually occursbecause people do not understand soils and how they act under different conditions.Soil degradation occurs in both rural and urban areas.

The removal of agricultural crops can remove some of the soil’s nutrients and degrade the soil. If the entire plant is removed, valuable organic matter is lost.These actions also reduce the ability of the soil to hold water and air. In addition, aplowed agricultural field, left without cover, erodes more easily and chemicalpesticides can accumulate in the soil. The tires of heavy equipment used inagriculture may compact the soil reducing its ability to take in water and air.

Highly mechanized, intensive agriculture operations, without proper soilconservation practices, tend to reduce soil quality. Ironically, farmers must thenspend more time and money to raise crops on the poorer soil.

Human actions in urban areas are also detrimental to the soil. On constructionsites, all trees and other vegetation are usually removed, exposing the soil toerosion. Although they may cover much smaller areas, these sites generally havetremendous erosion rates per acre. Sometimes as much as 150 tons per acre peryear, versus the 16 tons per acre per year for agricultural land.

www.blm.gov/nstc/soil/Kids/images


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Appendix A

U.S. Dept. of Agriculture

Soil Development

Landscapeposition, climate,time, organisms,and parent materialinfluence soildevelopment.

Rock Cycle


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Resources

North Carolina Division of Soil and Water Conservationwww.enr.state.nc.us/EHNR/DSWC

USDA Natural Resource Conservation Servicewww.nc.nrcs.usda.gov or www.soils.usda.gov/education/resources/k_12.htm

- From the Surface Down –An Introduction to Soil Surveys forAgronomic Use (www.soils.usda.gov –follow links to Education–Resources–K-12–From the Surface Down

- Helping People Understand Soils

Local Soil and Water Conservation OfficePublications available

- Water Quality Field Guide- Soil Erosion by Water- Soil Erosion by Wind- What is a Watershed- Conservation and the Water Cycle- Soil Erosion –The Work of Uncontrolled Water

Other pamphlets on various topics from the USDA and NRCSVery Important Resources:

- Any Modern Soil Survey- FFA Land Judging Manual www.soil.ncsu.edu (handbook notonline)

- Soil Textural Triangle

North Carolina Cooperative Extension Service www.ces.ncsu.edu- Soil Facts(pamphlets on various topics, such as “Soils and Water Quality” - Landscaping to Protect Water Quality; Fertilizer Recommendations and

Techniques to Maintain Landscapes and Protect Water Quality

Investigating Your Environment, www.ncfs.umn.edu/consed/iye/contents.htm

Soil Science Education, www.gsfc.nasa.gov/globe/appsoil/hmsoil.htm

Conserving Soil - U.S. Department of Agriculture

Teaching Soil and Water Conservation; A Classroom Field Guide - U.S.Department of Agriculture SCS #3

Soil and Water Conservation Society www.swcs.org


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composition of average soil minerals pore space · considering all these factors, soil is a...

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