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Soil science for teachers (massive-the real dirt on soil)

Soil science for teachers (massive-the real dirt on soil)



A very large Slide Show on soil. Useful for anyone without formal earth science training who might need to bone up or teach any aspect of soil. It's huge, 159 slides. It could also be split up ...

A very large Slide Show on soil. Useful for anyone without formal earth science training who might need to bone up or teach any aspect of soil. It's huge, 159 slides. It could also be split up into several lessons for students.



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    Soil science for teachers (massive-the real dirt on soil) Soil science for teachers (massive-the real dirt on soil) Presentation Transcript

    • The Complete SoilBy Moira Whitehouse PhD
    • The continents have a layer of solid rock, the crust, covering the hot stuff in the mantle.The continental crust, then, is mostly coveredwith thick layers of soil.
    • Here we see a slab of Earth taken out ofthe crust with the soil on top showingwhere we live. Soil, from the bedrock to the top, is our subject. Below the top, there are coherent layers of soil—similar in characteristics such as composition, texture, a nd color. Bedrock (crust of the Earth) USDA
    • Thankfully, soil covers most of the Earth’s solidcrust (bedrock); however, in some places it isthin or nonexistent. Why do we care? Plant growth, that allows us to live, occurs on the top layer of soil. layers of Below that layer are soil several other layers, some that provide D minerals and ores for bedrock our use. http://www.nrcs.usda.gov
    • We should understand that soil is a valuable naturalresource that is essential to life on Earth.Since top soil can be nurtured and protected, orsquandered, our well being as a specie may welldepend on which we choose.Listing the good things provided by soil might beuseful for our students: soil is where we grow ourfood; it helps clean our water; it provides thematerial for constructing many of our buildingsand roads; it absorbs rainwater and helps preventfloods.
    • Speaking mainly of top soil and the upper soil horizonsSoil is made of.....? Four things:
    • • weathered pieces of rock made up ofminerals (All rock is a mixture ofminerals.)• organic material (remains of dead plantsand animals)• air (containing oxygen)• water
    • The weathered rock piecesmakes up about half of thetotal mass of most soil. Less than 10% of the mass would be organic matter and living things.The other half is made up of vast numbers ofinterconnecting holes between the rock piecesand organic matter.Depending on location, the time of year orrainfall, these holes, called pores, are filled witheither air or water.
    • SoilTiny pieces of Remains of Air with oxygen Waterweathered dead plants rock and animals In pores, spaces between the pieces of weathered rock air water
    • Those ingredients that make up soil are necessaryfor the plants and micro-organisms that live there.• The weathered pieces of rock and the organicmaterial provide many of the nutrients such asiron, nitrogen, potassium that plants need to growand to carry out their life processes. • Plant roots and soil microorganisms get the oxygen and water they need to live from the spaces (pores) in the soil.• The weathered pieces of rock anchor the plantsroots so a plant doesn’t blow or wash away.
    • Though we usually think of soil as the solidcombination of weathered rock (minerals)and organic material we can hold in ourhands, the liquid and the gas componentsare crucial to the life processes that aresupported by soil. Liquid—the water and the minerals dissolved in the water that plants need. Gas—oxygen is needed for respiration by the roots, the bacteria and other microorganisms.
    • In this presentation we will focus mainly on thesolid parts of soil—weathered rock and thehumus—the organic material (dead plants andanimals). Soil: Weathered rock Humus—organic matter (tiny pieces of rock) (remains of dead plants and animals)
    • First we will explore the weathered rock partof the soil.What is weathered rock?To answer, we will review “weathering”.
    • Weathering can happen as aresult of physical processes
    • Knowing that soil is made up of tiny pieces ofweathered rock, it is obvious that the rocks inthese pictures have a long way to go beforebecoming soil.Weathering will take many years to changethis sized rocks into soil sized pieces.We will find, however, that some rockweathers (breaks down) into soil sizefragments faster than other rock.
    • Some of the reasons for rock to weather atdifferent rates include:Particle size--The smaller the particles makingup the rock, the faster it will weather.Exposed surface area--The more surface areathat is exposed, the more it will weather.Porosity--the more holes in the rock the fasterit weathers.The minerals that make up the rock-- Softerminerals weather faster than harder ones.
    • Knowing that weathering of rock takestime, and that some rocks weather atdifferent rates than others, we will nowconsider what causes weathering.
    • The two major types of weathering arephysical and chemical.1. Physical weathering causes the parentrock to break into smaller fragmentswithout changing the chemical makeup ofthe rock.
    • Physical weathering•Breaking bigger rocks into smallerfragments without changing the chemicalmakeup of the rock can be done bynonliving things and by living things.We will look first at non living things thatbreak up rock.
    • Non living thingsthat causeweathering
    • Nonliving things that break rocks into pieces. 1. Water running over the rock 2. Water freezing in cracks in the rocks 3. The temperature of rocks changing from hot to cold 4. The abrasion of rock by the blowing wind carrying sand 5. Salt wedging
    • Running water
    • Rapidly moving water particularly high up in themountains or a canyon can wear away (breakingoff fragments of rock). http://www.ngu.no/en- gb/hm/
    • Seen here, a small,fast runningstream of water isdramaticallycutting throughthis huge granitemonolith.
    • Rocks carried by fast moving water hit other rocksbreaking them into pieces. Moving sand and siltacts like sandpaper on the larger rocks in the riverbed rubbing off pieces of rock. Theses smaller pieces become part of the suspended rock and sediment carried by the fast moving river. http://www.flickr.com kia4067
    • Gradually larger rocks are broken into smallerand smaller rocks eventually becoming sand andsilt. http://www.flickr.com Wildcat Dunny
    • Here you can see fragments of rock createdby water that was moving quickly. http://www.flickr.com Randy OHC
    • Rocks that havebeen tumbled fora long time inrivers and streamsbecome smoothand rounded. http://www.flickr.com Dawn
    • Freezing water
    • 2. Water freezing in cracks in the rocksThis process occurs most often in cold wet climatesusually in high mountain ranges. It does need toperiodically warm up enough for liquid water to runinto the cracks.http://www.flickr.com/reurinkjan
    • Here we can seecracks in largemountain rock. Copyright 2004 by Andrew Alden, geology.about.com, reproduced under educational fair use http://www.ct.gov/
    • Rain fills these cracks with water.When the water freezes, it expands withinthe crack pushing the rock apart. Image courtesy of the geology department umd
    • As the ice melts, the water penetratesmore deeply into the fracture.Then the water freezes again, furthercracking the rock.
    • This repeated cycleof freezing andthawing forces thecrack open moreandmore, eventuallyshattering the largerock into smallerpieces.
    • Changing temperature
    • 3. Changing temperature also causesphysical weathering. As rocks heat up, they expand. As theycool, they contract.This process is repeated over and over againin nature. Eventually this process causes them tobreak apart.
    • The expansion and contraction of rock due tochanges in temperature occurs most often indesert environments where the temperaturefluctuates dramatically from day to night.The sky above a desert is usually clear and dry.As a result, during the day, the ground heats upand at night cools down rapidly.The surrounding air is likewise heated andcooled quickly by the ground. This repeated expansion and contraction of weakens the rock causing it to break.
    • Here we see large rock in a desert environment that has probably been exposed to the freeze- melt cycle.http://www.flickr.comHoggheff aka Hank Ashby aka Mr. Freshtags
    • Abrasion
    • 4. Abrasion by windblown sand alsoweathers rocks particularly in deserts. Itis similar to rubbing sandpaper over apiece of wood or sandblasting concrete. http://www.flickr.com beige alert
    • Frequent sandstorms in the desertweather exposed rocks. http://www.flickr.com nukeit1
    • The scouring of the rock by the windcarrying sand wears off fragments of rock. http://www.flickr.comlumierefl
    • So much for those non living things thatcause physical weathering, now we willconsider the living things that also causephysical weathering.
    • Living thingsthat causeweathering
    • Living things also break rocks intosmaller pieces:1. Plant roots—particularly tree roots2. Lichen growing on rocks3. Burrowing animals
    • Plant roots
    • The roots of plants, particularly treeroots, are amazingly strong. When they startgrowing as tiny root hairs they can fit intothe smallest of cracks.As these tree roots continue to grow indiameter, they put stress on the rock andcause the cracks to expand breaking therock apart.
    • Here theroots of thetree aregrowing inthe cracks inthe rocksmaking thefractureslarger. http://www.flickr.com Chazz Layne
    • http://www.flickr.comiandavid
    • http://media.photobucket.com/ city bumpkins
    • Lichen growth
    • 2. Lichen growing on rocks.Lichens are unusual creatures, theyare not single organisms the waymost other living things are.Rather, lichens are a combination of twoorganisms—a fungus and an algae-- whichlive together in a symbiotic relationship.A little memory prodder for this, “Freddyfungus met Alice algae and took a lichen toher.”
    • Lichens appear in the form of small patchy crusty outgrowths in colors of green, brown, orange and other colors. They often grow on rocks.http://www.flickr.combrian http://www.flickr.comSeaDavid
    • Lichens expand and contract as they aremoistened and then dry out. In thisprocess they loosen microscopic rockfragments creating tiny cracks in the rock.Their rock-breaking power is greatlyenhanced when seeds fall into cracksand then germinate. This is particularlytrue in the case of woody shrubs andconifers with powerful expanding rootsystems.
    • Burrowing animals
    • 3. Burrowing animalsWhen animals burrow in rocks or betweenthe rocks, they introduce seeds whichgerminate, or water that freezes, both ofwhich encourage rock breakup.
    • So far, we have looked at the agents ofphysical weathering—the process ofbreaking up parent rock into smallerpieces without changing the chemicalmakeup of the rock. Now we will investigate chemical weathering.
    • Chemical weathering
    • Chemical weathering creates holes orsoft spots in rock, so the rock breaksapart more easily.Causes of chemical weathering: 1. Acidic water2. Oxidation (rusting of iron in rock) 3. Hydrolysis
    • 1. Acidic water:Carbonic acid is very common in nature. It isproduced when carbon dioxide is releasedfrom decaying material or when carbondioxide in the air combines with water.When this weak carbonic acid tricklesinto cracks in limestone, it can dissolvelarge amounts of calcium carbonate(calcite) in the form of calciumbicarbonate.
    • The mildly acidic rainwater seeps intocracks in the ground.The carbonic acid inthe water dissolvesthe calciumcarbonate inlimestone rockleaving behind holesin the rock. http://www.esi.utexas.edu
    • Carbonic acid also works on limestone foundon the surface. We don’t see the classicholes here, but there is evidence that softerparts of the rock have been dissolved awayallowing the rock to break apart more easily. http://www.geolab.unc.edu/
    • 2. Oxidation, another type of chemicalweathering, is the process in which oxygenchemically combines with anothersubstance.Many rocks contain iron. When water andair are present, this iron often oxides(rusts), similar to an iron bar rusting.The rust formed by oxidation makesrocks softer. As a result, the rockdeteriorates.
    • When you see redrocks you knowoxidation (rusting)is happening andthat those rocksare weathering.This process iscommon is aridregions. pics4learning
    • 3. Hydrolysis occurs when minerals in rocksreact with water to form other weakercompounds.These weaker compounds are then moreeasily worn away by physical weathering. Feldspar changes to clay.
    • To review, both physical and chemical weatheringare important in creating soil out of rock.Physical Weathering Chemical Weathering Wind with sandFreezing andthawing of Running waterwater Acids Heat and cold sand Water Ice silt clay soil Weathering breaks the rock intoparent rock smaller and smaller pieces
    • In the study of soil, the rock that is weathered toproduce soil is called the “parent rock”. The parentrock may be igneous, sedimentary or metamorphic. Soil: Weathered rock Humus (remains of dead plants & animals)igneous metamorphic sedimentary
    • Igneous rock:is rock that is formed fromsolidified magma.It may be cooled above thesurface, usually on theocean floor (as in basalt). http://rst.gsfc.nasa.gov/Or the magma may beextruded undergroundwhere it gradually cooledand solidified (as in granite) http://rst.gsfc.nasa.gov/
    • Sedimentary rock:is rock that is formedfrom sediment depositedby water or wind orminerals deposited whenwater evaporated.Seventy-five to eightypercent of the rockexposed on the Earth’ssurface is sedimentary rock; therefore most of theparent rock is sedimentary.
    • Metamorphic rock: changed chemically byis rock that has beenheat and pressure deep under the surface ofthe Earth. GniessIgneous and metamorphic rock tend to beburied underground and therefore, generally notsubject to weathering.
    • In science, it is often found that things occur incycles. The rock cycle is one of the moreinteresting ones, but it takes a very long timeto complete. This picture of the rock cycle shows that each type of rock can be weathered and changed into sediments including soil (small pieces of weathered rock).
    • Looking again at the solid part of soil we will consider the three types of soil: Soil: Weathered rock Humus (remains of dead plants & animals) Sand Silt Claylargest smallest particles
    • These three types of soil, sand, silt and clayare identified largely based of the size oftheir weathered rock pieces. 1. Sand—largest particles (created by physical weathering.2. Silt—medium sized particles (created byphysical weathering.3. Clay—smallest particles (created bychemical weathering.
    • The weathered rock in soil probablystarted out as a huge boulder.In the process of being broken down, thesize of the particles of rock becomesmaller and smaller—boulders to largerocks, to smaller pieces of rock topebbles to sand, silt and clay.The following particle size guidelines areused to differentiate the various rocks aswell as the three types of soil.
    • Size of Particles of Rock (Diameter)•2 m boulders• coarse fragments such as pebbles > 2 mm• sand < 2 mm to 0.05 mm• silt< 0.05 mm to 0.002 mm• clay < 0.002 m
    • Wikipedia Commons
    • Still looking at the solid part of soil, we now examine the humus: Soil: Weathered rock Humus (remains of dead plants & animals) Sand Silt Claylargest smallest particles
    • What is humus? The organic part of soil.How is it formed? Humus is formed when dead plants and animals decay.
    • What causes these dead things to changeinto soil?Special organisms in the soil, calleddecomposers, cause dead plants and animalsto decay or rot changing their bodies into thehumus part of soil.When plants and animals die, they become foodfor these decomposers--bacteria, fungi,arthropods, nematodes and earthworms.Decomposers recycle dead plants and animalsinto chemical nutrients like carbon and nitrogenthat are released back into the soil, air and water.
    • Bacteria are the smallest living organisms,and the most numerous of thedecomposers; they make up 90% of thebillions of microorganisms typically foundin a gram of soil.A teaspoon of fertile soil generallycontains between 100 million and 1 billionbacteria.They carry out the majority of decomposingthat occurs in the soil.
    • Bacteria are important in the carbon cycle.During respiration, bacteria release carbondioxide into the atmosphere. USDA
    • Bacteria are an essential parts of the nitrogen cycle. Wikipedia commons
    • http://soils.usda.govMagnified bacteria found in the soil.
    • http://soils.usda.govBacteria dot the surface of strands of fungal hyphae.
    • http://soils.usda.gov/Actinomycetes are a large group of bacteria that growas hyphae like fungi. They are especially important indegrading hard-to-decompose compounds, such aschitin and cellulose.
    • Fungi is the name for simple organismsincluding mushrooms, molds and yeasts.Next to bacteria, fungi are the most efficientdecomposers.Fungi are not plants; they cant make their own food.They absorb their nutrients from the organismsthey are decomposing. In the process they releaseenzymes that decompose dead plants and animalsMany fungi can break down tough organic materialssuch as the cellulose in leaves and the cellulose andlignin in wood.Many plants depend on fungi to help extractnutrients from the soil.
    • http://www.flickr.comBenimoto http://www.flickr.comFuturilla Mushrooms growing on logs http://www.flickr.comscoobygirl
    • Mushrooms growing on a forest floorhttp://www.flickr.com mill56 Mushrooms growing in dead grasshttp://www.flickr.com photogirl7
    • http://soils.usda.gov/Fungus beginning to decompose leaf veinsin grass clippings.
    • Other important decomposers found inthe soil are numerous invertebrates—animals without backbones.Thus, the saying “F B I ungi ” as the acteria nvertebratesthree main decomposers.Included under the invertebrategrouping are worms callednematodes, mites, pillbugs andmillipedes.
    • Nematodes, a group of invertebrate decomposersliving in the soil are tiny non-segmented wormstypically 1/500 of an inch in diameter and 1/20 ofan inch in length.One square yard of woodland or agricultural soilcan contain up to several million nematodes. Nematodes are important in releasing nutrients into the soil in forms plants can use. Nematodes are play an crucial role in the return of nitrogen to the soil (nitrogen cycle).
    • http://soils.usda.gov/Nematodes magnified in soil.
    • Other importantinvertebratedecomposersPill bug http://www.flickr.comzimpdenfis
    • Decomposing mitesMillipede
    • Organisms such pill bugs, millipedes and mitesare important to the soil because they stir upand churn the soil, mixing in air which isneeded by other organisms in the soil habitat.They shred organic matter into smallpieces, assisting other soil organisms in thedecomposition process.Finally, along with other decomposers, theychange nutrients into forms plants can use.The lowly earthworm is also an importantdecomposer.
    • Earthworms eat dead plants andanimals, thereby, absorbing thenutrients that they need tosurvive.Earthworms excrete wastes inthe form of casts whichare rich in nutrients such asnitrogen, phosphorous andpotash that plants need.In addition to breaking down organic materials andadding nutrients to the soil, earthworms also helploosen the soil, thereby, creating space for the oxygenthat plant roots and microorganisms need to live.
    • Decomposer food web
    • All these decomposers along with thedead plants and animals form a soil foodweb.And how are soil food webs different thanfood webs on the surface of the Earth?
    • In a food web above ground, thedecomposers are at the end of the energytransfer in the food web.In a food web below ground the decomposersare at the beginning of the web.While living plants are the largest source ofenergy above ground, the most abundantenergy resource in the soil is dead plant andanimal matter.
    • http://www.blm.gov/
    • The primary consumers in soil are oftenmicrobes such as bacteria, fungi andnematodes (tiny microscopic worms).They consume the remains of dead plantand animal.In turn, these primary consumers are thefood for many other secondary consumersthat live in the soils such as mites, protozoaand other nematodes.
    • http://soils.usda.gov/
    • The natural cycle ofplants absorbingminerals from the soiland and these mineralsbeing returned to thesoil throughdecomposition isrepeated over andover in nature. http://www.flickr.com/ angus clyne
    • Decomposition creates fertile soil containingthe nutrients plants need in a form that theycan use to carry out their life processes.There are 13 main mineral nutrients that whendissolved in water and absorbed through a plantsroots provide what is needed by plants to growhealthy forests, prairies, or even front lawns.These are: nitrogen, phosphorous, potassium,calcium, magnesium, sulfur, boron, copper, iron,copper, manganese, molybdenum and zinc.
    • USDA
    • Where there is lots of vegetation to decay andenrich the soil, such as in deciduous forests andgrasslands, the soil is rich in humus and veryfertile. Wikipedia Commons
    • Wikipedia Commons http://www.flickr.com/ Cory Leopold Desert in Saudi Arabia The Chihuahuan Desert along the Rio GrandeWhere there is little or no vegetation toprovide the organic debris, such as at theseashore or in the desert, the soil has little orno humus and is not very fertile.
    • Now that we have explored howthe two solid parts of soil areformed (weathered rock andhumus), let’s move on to anothertopic.What are the physical propertiesof these kinds of soil, sand, silt,clay and humus?
    • Remember physical properties are thethings we can observe about a substanceusing our five senses.For soil, the two main physical propertiesare: •color •texture
    • Soil ColorThe most obvious property when lookingat soil is its color.Geologist officially recognize over 170different soil colors.The most common color of soils are shadesof black, brown, red, gray, and white.
    • Generally speaking, the darker a soilis, the more nutrients it contains.The darker color often indicates an increase indecomposed organic matter known as humus.Gray soils often indicate poor drainage, whilered soils may indicate a lack of nutrients.
    • black fertile soil infertile red soil Photos courtesy of USDA
    • • Soil texture is the identifying characteristicof sand, silt and clay, the three types of soilformed from weathered rock. Texture isbased on the size of the particles making upthe soil and is the single most importantphysical property of the soil. The size of theparticles making up a soil, therefore, greatlyaffects: • 1) how much water will flow through it • 2) water holding capacity • 3) ability to sustain plant growth
    • Sand• particles are visible without microscope• < 2 mm to > 0.05 mm• rounded or angular in shape• if the sand grains look white it is usually weathered quartz; if they look brown it contains many minerals Images from Wikipedia Commons Particles under a microscope
    • Sand• feels gritty• does not stick together in a mass unless it is very wet.• pores between sand particles promote free drainage and entry of air• fewer nutrients for plants than silt or clay• holds little water and prone to drought
    • Sandy soilSandy soil has large particles with largeair spaces between them.Therefore, it drains very quickly anddries out faster than the other soils.Also, water-soluble nutrients leach outrapidly before the plants can use them.Humus added to sandy soil acts like asponge, absorbing and holding moisture andany nutrients dissolved in it.
    • Silt• particles not visible without a microscope• particles < 0.05 mm to > 0.002 mm• erosion by glaciers often responsible for formation of silt
    • • does not feel gritty.• floury powdery feel – smooth like silly putty when wet.• wet silt does not stick together and cannot mold it into different shapes.• bigger particles than sand -- retains more water for plants and have slower drainage than sand.• easily washed away by flowing water – highly erosive.• has more plant nutrients than sand.
    • Clay• particles are flat plates or tiny flakes• < 0.002 mm• when stirred in water small particles of clay donot settle but remain suspended. Clay particles under a microscope http://photojournal.jpl.nasa.gov
    • • wet clay is very sticky and slippery and can be molded readily into any shape like a rod or a long ribbon.• pores (spaces between particles), like the particles are very small somovement of water and air through clay is slow giving clay a tremendous ability to hold water.• clay swells when water is added and shrinks when the water evaporates. This results in bothersome shrinking and swelling that can adversely affect buildings, roads and walls.
    • Clay soilThe tiny particles that make up clay tend to sticktogether causing water to fill up the air spaces.Since moisture does not drain from this soilwell, if clay is too wet plant roots cannotrespire so they rot.Adding humus to clay soils discourages thesmall particles from sticking so tightlytogether, resulting in larger spaces that drainwater more easily and hold more air.
    • Determining Soil Texture - Feel Method• Wet a sample of the soil and place in your hand• Try to roll it into a ribbon.• If it makes a ribbon, it contains mostly clay.• The longer the the more clay in the soil.• If it does not roll into a ribbon, it is sand or silt• If it feels grainy or gritty it is mostly sand• If it feels smooth, silky or flowery, it is mostly silt
    • Properties of humusHumus is the soil formedwhen dead plants andanimals decay. http://www.edupic.net/sci_gr.htm Free clip are for educational useIt is a dark brown or black (color).It feels crumbly and loose when dry andspongy when wet (texture).
    • When dead plants andanimals decay leaving behindthe humus, it accumulatesjust under the top layer ofsoil (shown as Horizon “A”)Humus inherits the minerals that werecontained in the decayed plants and animals.Humus therefore contains the nutrients(minerals such as, nitrates, phosphates,potassium, copper, zinc dissolved in water)that plant roots can absorb.
    • Comparing soil... ability to hold waterClay soils hold more water than sand, silt orhumus. For growing plants, clay holds too much water for most kinds to survive.Humus holds more water than sand or silt. But water does drain through humus quickly.Sand and silt hold the least water, often toolittle for many kinds of plants to survive.
    • Comparing soil...nutrients (minerals) provided Humus soils have an abundance of nutrients for plants. Adding humus to infertile soil increases its fertility. Clay has more nutrients (minerals) than sandy soils mainly because it hangs on to its mineral abundant water for longer. Since clay soils hold a lot of water, fewer nutrients are leached out of the soil. However, as clay soil ages and plants remove its nutrients, it becomes infertile.
    • Comparing soil...nutrients (minerals) providedSandy soils have fewer mineralsbecausewhen water drains through sandysoils, it often dissolves the minerals in therock and carries them along with it. This conditionis called leaching. When nutrients leach out of the soil, they are not available for plants to use.
    • Comparing soil...nutrients (minerals) providedLoam is a mixture of clay, silt, sand and humusand is the best soil for growing plants.Because loam is a mixture of four kinds ofsoil, it holds the proper amount of water andprovides all the nutrients plants need. Loam is formed in nature when the dead plants and animals are left to rot and mix in clay, sand or silt.
    • We previously mentioned that humus isfound in the “A” soil horizon.What do we mean by soil horizons?
    • A soil profile is a verticalcross-section of soil.It shows the variouslayers or horizons,beginning at the layers ofsurface with the “O” soilhorizon andcontinuing downwardthrough the “A”, “B”, bedrock R“C”, and “R” horizons http://www.nrcs.usda.govto the parent material. Soil horizons
    • The layers in across-section of soilare easy to spothere but they areusually muchharder to find. Photo coutesy of USDA
    • The horizons are named O, A, B, C and R.“O” is the tophorizon. Itsabout an inchthick and ismade up ofdead plants,animals andsome humus.
    • The A horizon called topsoil is made up ofhumus (decomposed organic matter) mixedwith mineral materials.The A horizon is alive with plant rootsand tiny microorganism like bacteria,fungi, and other decomposers such asnematodes, mites, pill bugs and worms.It is generally dark-colored because itcontains large amount of humus.
    • E Horizon -- the Eluviaton LayerThis layer, between the A and B Horizons islight in color and made up mostly of sand andsilt.Due to a process known as leaching inwhich water with dissolved minerals dripsthrough the soil above, Horizon E losesmost of its minerals and clay.
    • Horizon BThe B Horizon is commonly called “subsoil”.It contains clay and mineral (such asiron, aluminum oxides, and calciumcarbonate).Horizon B is formed when clay and mineralsmigrate down through Horizon A and E sosub-soils generally have more clay than top-soils.
    • Horizon CThat layer beneath the B Horizon andabove the D Horizon consists of broken-up bedrock that is only slightly.weathered .Plant roots do not grow down into thislayer and organic material is prettymuch nonexistent.
    • Layer RLayer R is theunweathered rock(bedrock) layer thatis beneath all theother layers.Layer R is the crust,the outermost layer ofthe Earth, that liesbelow the soil. NASA image
    • In discussing soil texture, one of the maincharacteristics we discussed was how eachwas able to hold or pass through water.Which brings us to the water table, aphenomenon dependant upon watersupply but also affected by soil texture.Surface runoff water seeps through emptyspaces in the soil, sand, or rocks until itreaches an impermeable layer of rock.
    • The water then fills the pores, emptyspaces and cracks above that rock layer. Water tableThe upper “surface” of the water fillingthose spaces in the soil, sand, or rocks iscalled the water table.
    • Again, the upper “surface” of theground water is called thewater table. usgs
    • It is in this unsaturatedzone above the watertable that plants absorbthe moisture throughtheir roots and return itto the atmospherethrough transpiration.The dirt and rocks in thiszone contain air andusually some water. Indry spells, this layersometimes dries out.
    • A large amount ofsurface water canresult in waterseeping throughthe upper soillayers andaccumulatingunderground asground water.
    • Ground water of recharged from rain water A large body is ground water is called anand snowmelt or from water that leaks from aquifer.lakes and rivers. usgsGround water is the water trapped in theempty spaces under ground.
    • Thinking long term aboutsoil• Over long periods (thousands of years) the soil horizon textures change.• As soils get older, sand weathers to silt and silt weathers to clay. Consequently, old soils have more clay than younger soils.•Soil, along with oxygen, water, plants and animals, is considered a renewable natural resource. This is because soil can be replaced in a relatively short period of time.•”Relatively” short because it may take a 1,000 years toform one inch of soil as compared to the 350 millionyears for fossil fuels and around 100 million years formost rocks on the surface of the Earth.
    • So there it is, the dirt or soil.