Soils

SoilsAg I Objectives*Outline the process involved in soil formationDescribe a mature soil profileDiscuss the 8 land capability classesSoilThe top layer of the Earths surface which is suitable for the growth of plant life

Soil FormationSlowForms from parent materials- mineral & rocks, glacial deposits, loess deposits, alluvial and marine deposits, organic deposits

Parent MaterialMaterials underlying the soil from which it was formed5 categoriesMineral and rocksGlacial depositsLoess depositsAlluvial and marine depositsOrganic deposits

Minerals & RocksMineralsSolidInorganicchemically uniformFeldspars, micas, silica, iron oxides, calcium carbonatesRocksAggregates of minerals3 groups-Igneous, Sedimentary, Metamorphic

Glacial DepositsDeposits of rock, parent material and already formed soils that were left behind as the glaciers moved across the northern hemisphereSoils found in much of the Midwestern US were formed from glacial deposits

Loess DepositsWind-blown siltMake up much of the soil in the eastern Mississippi Valley

Alluvial & Marine DepositsAlluvial DepositsLeft by moving fresh waterMarine DepositsFormed on ancient ocean floorsBoth are found in the forms of flood plains and deltas

Organic DepositsForm as vegetation dies and falls into the water where it decays slowlyMuck Soils- plants are decayed to the point they are no longer recognizablePeat Soils- plant materials are still recognizable

Weathering

Major ForcesTemperature changeWater actionPlant rootsIce expansionMechanical grindingProcessContinuousEven as erosion is occurring, parent material is breaking down forming new soilBiggest problem is balanceactions of erosion happen much more quicklySoil Organic MatterMake-UpDecaying plants and animals2 formsOriginal Tissue- organic matter that is still recognizableEx- Leaves and twigs on a forest floorHumus-organic matter that has decomposed beyond recognition. Gives topsoil its brown color

FunctionsAffects soil structure by acting as a cementing agentReturns nutrients to the soil- most notably phosphorous, sulfur and nitrogenHelps store soil moistureMakes soil more tillableProvides food for soil microorganisms which make the soil capable of plant productionPhysical, Chemical and Biological CharacteristicsSoil Profile4 or more horizons OABC

C HorizonParent materialStores and releases water to the upper layersDoesnt contribute much to plant nutritionContains larger soil particles Substantial amounts of gravel or rockLayer below is bedrock

B HorizonSubsoilMineral content is similar to the A Horizon but particle sizes and properties differLess organic matter than in the O and A horizonsLighter in color than A horizonA HorizonTopsoilNear the surface Contains desirable proportions of organic matter, and sand, silt and clayGenerally supports plant growthO HorizonSurfaceComposed of organic matter and a small amount of mineral matterLand Capability ClassesCapability ClassesDeveloped by the USDA and NRCS to classify lands based on their limitations. 8 classes

Class IVery good, productive landCan be safely cultivated with minimal erosion control methodsNearly levelWell drainedDeepMedium texturedNot subject to erosion or floodingEasily cultivatedVery little land falls into Class I even in the most productive farming areas.

Class IIGood for all types of farmingHas some limitationsGentle slopeSuffers from past effects of erosionMay have less than ideal soil depth and drainage problems that can be corrected by installation of drainage systemsLand requires careful soil adjustment to prevent erosion damageClass IIIModerately good landMay be cultivated and farmed regularly as long as limitations are followedMay be highly susceptible to erosion or have problems with drainageNeeds to be farmed with great care to control erosionClass IVSevere limitations but can be cultivated with careful soil managementMay have a strong slope and be subject to severe erosionMay suffer from past erosionMay be very shallow, very dry or very wetIf wet, even extensive drainage systems may still allow occasional water loggingClass VNearly level but has some properties that make it unsuitable for cultivationMay be in a frequent floodplainMay be very wet, dry or rockySwampy areas frequently are Class 5Best suited for pasture, wildlife habitat, or forest production

Class VISerious limitationsMay have very rocky or shallow usable surface soilTree production, permanent pasture and wildlife habitat are the best uses

Class VIISeverely limiting propertiesMay be very steep or severely eroded with large gulliesMay have very course soil combined with strong slopesUsing native grasses for controlled grazing is the best useCan also be used for forest production, wildlife and recreation

Class VIIIOne or more extreme limitationsRock outcroppings, areas of almost solid surface rockBest examples are river washes, streambeds, lake bottoms, sand dunes and other nearly barren areasLittle agricultural valueBest preserved in its natural state for recreation and wildlifeSummary*The Earths naturally occurring rocks and minerals are broken down into parent material by weathering forces.Parent materials combine with organic matter through further weathering into soil.Soils may be classified based on the lands capability to produce plants while resisting erosion.There are 8 capability classes- class I is the best and class VIII is the worst from the standpoint of plant productionClasses I-IV are tillable, classes V-VIII are generally not considered tillable.