Soils

The Geography of SoilsThe Geography of SoilsProperties and ProcessesProperties and Processes

OverviewOverviewSoil: Definition, Description, Soil: Definition, Description, CharacteristicsCharacteristicsSoil PropertiesSoil PropertiesSoil ChemistrySoil ChemistrySoil BiologySoil BiologySoil FormationSoil FormationSoil TaxonomySoil TaxonomySoil IssuesSoil Issues

SoilSoil

Soil is a mixture of both mineral and organic matterSoil is a mixture of both mineral and organic matterMaintained by microscopic Maintained by microscopic lifeformslifeforms

Soil ProfilesSoil ProfilesPedonPedon, , PolydedonPolydedonHorizonsHorizons

O: Organic matter: litter and humus: soil moisture retention,O: Organic matter: litter and humus: soil moisture retention,mobilization of nutrients, provision of nutrientsmobilization of nutrients, provision of nutrientsA: Humus and clays (water retention of humus limits A: Humus and clays (water retention of humus limits eluviationeluviation))E: Sand and silt (clays and other oxides E: Sand and silt (clays and other oxides eluviatedeluviated down to B)down to B)B: Clays and oxides B: Clays and oxides illuviatedilluviated from E deposited herefrom E deposited hereC: Weathered bedrock, exclusive of the bedrock, not affected byC: Weathered bedrock, exclusive of the bedrock, not affected bybiotabiotaR: BedrockR: Bedrock

Erosion: Erosion: eluviationeluviationTransport: leachingTransport: leachingDeposition: Deposition: illuviationilluviation

Soil PropertiesSoil PropertiesColorColor

Soils rich in organic matter are dark or blackSoils rich in organic matter are dark or blackIron and Aluminum oxides give soil a reddish colorIron and Aluminum oxides give soil a reddish color

TextureTextureGravelGravelSand Sand SiltSiltClayClay

StructureStructureConsistency (wet, moist and dry)Consistency (wet, moist and dry)Porosity: Porosity:

affects affects inflitrationinflitrationimproved by biotic actionimproved by biotic actionharmed by compaction (increases runoff)harmed by compaction (increases runoff)

Soil MoistureSoil Moisture

2mm2mm50 50 μμmm2 2 μμmm

TextureTexture

StructureStructure

Soil MoistureSoil Moisture

Soil moisture related toSoil moisture related toOrganic matter contentOrganic matter contentTexture (esp. to fine particles)Texture (esp. to fine particles)

Loss of organic matter can lead toLoss of organic matter can lead toLoss of soil moisture retentionLoss of soil moisture retentionLoss of A horizon to E horizonLoss of A horizon to E horizon

EluviationEluviation of fine particles from Eof fine particles from EIlluviationIlluviation of fine particles to Bof fine particles to BFurther loss of soil moisture retentionFurther loss of soil moisture retention

% Soil% SoilMoistureMoisture

% Fine Particles % Organic matter

Soil ChemistrySoil ChemistrySoil Atmosphere (NSoil Atmosphere (N22, CO, CO22))Nutrients take the form of ionsNutrients take the form of ions

CationsCations: Na: Na++, K, K++, Mg, Mg2+2+, Ca, Ca2+2+,, NHNH44++

Anions: NOAnions: NO33--

Colloids acts as nutrient storageColloids acts as nutrient storagesoil particles (especially clay)soil particles (especially clay)organic matterorganic matter

Soil moisture acts as a transport agent by Soil moisture acts as a transport agent by solutionsolutionRoot hairs have a slight negative character Root hairs have a slight negative character and attract positive ionsand attract positive ionsCationCation Exchange Capacity (CEC) is a measure Exchange Capacity (CEC) is a measure of how easily nutrients pass between colloidsof how easily nutrients pass between colloids

Acidity and pHAcidity and pHAcidity refers to the concentration of Acidity refers to the concentration of Hydrogen ions (HHydrogen ions (H++))HH++ ions occupy colloidal spaces, crowding ions occupy colloidal spaces, crowding out other nutrients and lowering fertilityout other nutrients and lowering fertilitypH = pH = --log[Hlog[H++]]

pH < 7, acidicpH < 7, acidicpH = 7, neutral, waterpH = 7, neutral, waterpH > 7, basic (alkaline)pH > 7, basic (alkaline)

Soil BiologySoil BiologyCritical for cycling nutrientsCritical for cycling nutrients

Performed largely by bacteriaPerformed largely by bacteriaFungaeFungae, nematodes, springtails, earthworms, etc. (form , nematodes, springtails, earthworms, etc. (form food chains)food chains)

Nitrogen cycle, Carbon cycleNitrogen cycle, Carbon cycleSources of NitrogenSources of Nitrogen

Atmosphere (NAtmosphere (N22))Litter, Litter,

Through decomposition, also biological, provides NHThrough decomposition, also biological, provides NH44++ from organic from organic

compoundscompounds

Inorganic fertilizer (NPK)Inorganic fertilizer (NPK)

UtilizabilityUtilizability of Nitrogenof NitrogenNitrogen gas, inert, unusable by plantsNitrogen gas, inert, unusable by plantsNitrite, toxic to plantsNitrite, toxic to plantsNitrate and Ammonium utilizable by plantsNitrate and Ammonium utilizable by plants

Nitrogen CycleNitrogen CyclePlants must rely on soil Plants must rely on soil microbiotamicrobiota to process to process nitrogen into a form it can usenitrogen into a form it can use

Nitrogen fixation: bacteria change atmospheric N Nitrogen fixation: bacteria change atmospheric N into various nitrogen compoundsinto various nitrogen compounds

legumeslegumes

Nitrification: Ammonium changed into nitrite by Nitrification: Ammonium changed into nitrite by ““Type IType I”” organisms (organisms (nitrosomonanitrosomona); nitrite turned ); nitrite turned into nitrate by into nitrate by ““Type IIType II”” organisms (organisms (nitrobacternitrobacter))DenitrificationDenitrification: nitrogen compounds turned back : nitrogen compounds turned back into nitrogen gas by bacteria (20 into nitrogen gas by bacteria (20 –– 40% of 40% of fertilizer is wasted this way)fertilizer is wasted this way)

Plants return nitrogen to the cycle through Plants return nitrogen to the cycle through leaf litter and decompositionleaf litter and decompositionA diverse soil biota is necessary for proper A diverse soil biota is necessary for proper functioningfunctioning

Carbon CycleCarbon Cycle

Plants take in carbon dioxide, add it to Plants take in carbon dioxide, add it to their tissuestheir tissues

Other nutrients, including N, are taken inOther nutrients, including N, are taken in

Tissues becomes deposited in soil Tissues becomes deposited in soil through leaf litterthrough leaf litter

Builds up soil organic matterBuilds up soil organic matterActs as a form of carbon storageActs as a form of carbon storage

Contributes N to the soilContributes N to the soil

Soil FormationSoil Formation

Dynamic factorsDynamic factorsClimatic Climatic –– weathering of parent material; texture; weathering of parent material; texture; pedogenicpedogenic regimesregimesBiologic Biologic –– organic matter additionsorganic matter additions

Passive factorsPassive factorsparent material: affects nutrient availabilityparent material: affects nutrient availabilitytopography and relief: accumulation and depthtopography and relief: accumulation and depthtimetime

Lots of it (hundreds to thousands of years), under stable Lots of it (hundreds to thousands of years), under stable conditionsconditionsPaleosolsPaleosols

Human factorHuman factorRapid loss of soilRapid loss of soil

Classification of Soil TypesClassification of Soil Types

PedogenicPedogenic regimesregimesAn earlier system of classification based on An earlier system of classification based on climatic factorsclimatic factorsReplaced earlier texture classification system with Replaced earlier texture classification system with one that was more focused on developmentone that was more focused on development

USDA Soil TaxonomyUSDA Soil TaxonomyReplaced Replaced pedogenicpedogenic regimes as a system, but regimes as a system, but includes them as a diagnostic featureincludes them as a diagnostic featureUses diagnostic horizons to classify soilsUses diagnostic horizons to classify soils

epipedonepipedon: Upper most surface layers, exclusive of C and : Upper most surface layers, exclusive of C and RRdiagnostic subsurface horizons: A and B horizonsdiagnostic subsurface horizons: A and B horizons

Twelve orders and multiple subordersTwelve orders and multiple suborders

PedogenicPedogenic RegimesRegimesRelates soil Relates soil formation to formation to climateclimateSoil types are Soil types are closely related closely related to these to these regimesregimes

climate is not climate is not the sole factor the sole factor in determining in determining soil type, soil type, howeverhowever

Soil Taxonomy: OrdersSoil Taxonomy: OrdersOxisolsOxisolsAridisolsAridisolsMollisolsMollisolsAlfisolsAlfisolsUltisolsUltisolsSpodosolsSpodosolsEntisolsEntisolsInceptisolsInceptisolsVertisolsVertisolsHistosolsHistosolsAndisolsAndisolsGelisolsGelisols

OxisolsOxisolsTropical soilsTropical soilsLaterizationLaterization

Iron and aluminum oxides (oxidation; reddish color)Iron and aluminum oxides (oxidation; reddish color)Heavily leached A horizon (leaching occurs throughout the Heavily leached A horizon (leaching occurs throughout the year)year)Loss of nutrients and colloidal materialLoss of nutrients and colloidal materialpoor fertility and low CECpoor fertility and low CEC

Exposure to air and water creates hardpan (Exposure to air and water creates hardpan (plinthiteplinthite, , lateritelaterite))Rainforest soilRainforest soilCan be farmed sustainablyCan be farmed sustainably

slash and burn, with long fallow (low demand)slash and burn, with long fallow (low demand)

Vulnerable to erosion if activities not done right Vulnerable to erosion if activities not done right (1000s of tons per sq. km per year)(1000s of tons per sq. km per year)

AridisolsAridisolsDesert soils (arid climates)Desert soils (arid climates)

shallow horizonsshallow horizonspoor colloidal structure (fewer fine poor colloidal structure (fewer fine particles)particles)lacking in organic matterlacking in organic matterpoor water retentionpoor water retention

contributes to flash floodingcontributes to flash floodingleaching highly seasonalleaching highly seasonal

High High evapotranspirationevapotranspirationSalinizationSalinization

Evaporation deposits salts near the soil Evaporation deposits salts near the soil surfacesurface

MollisolsMollisolsGrassland soilsGrassland soils

worldworld’’s richest soil types richest soil typedeep and rich organic layer (humus) due to deep and rich organic layer (humus) due to deep grass rootsdeep grass roots

CalcificationCalcificationMore COMore CO22 in the soil atmosphere causes in the soil atmosphere causes leaching of calcium and magnesium leaching of calcium and magnesium carbonates carbonates illuviationilluviation into B and C horizonsinto B and C horizons

calichecaliche, , kunkurkunkur

AlfisolsAlfisolsName taken from Aluminum and Iron Name taken from Aluminum and Iron content (Al, Fe)content (Al, Fe)Moderately weathered forest soilModerately weathered forest soilConsidered to be a moist version of Considered to be a moist version of mollisolsmollisolsNote similarities to Note similarities to OxisolsOxisols

UltisolsUltisols

Highly weathered forest soilsHighly weathered forest soilsHeavily leached A horizon with residual Fe Heavily leached A horizon with residual Fe and Al oxidesand Al oxidesIntermediate between Intermediate between OxisolsOxisols and and AlfisolsAlfisols

SpodosolsSpodosolsNorthern coniferous forest soilsNorthern coniferous forest soilsHumid continental mild summerHumid continental mild summer

cool, moist climatescool, moist climates

Humus and fine particles Humus and fine particles eluviatedeluviated from A from A horizonhorizonUppermost horizon lacks clay and humus, Uppermost horizon lacks clay and humus, tends to have a sandy texture and has a light tends to have a sandy texture and has a light colorcolorPodzolizationPodzolization

Low pH (acidic)Low pH (acidic)leaching of fine particles and nutrientsleaching of fine particles and nutrients

EntisolsEntisols

recENTrecENT soilssoilsNot climate dependentNot climate dependentInsufficient time passage to develop Insufficient time passage to develop horizonshorizons

Insufficient weathering of parent materialInsufficient weathering of parent materiallacking fine particles and nutrientslacking fine particles and nutrients

OccurenceOccurence: areas of recent : areas of recent depositiondepositionslopes, floodplains, tidal mudflats, dune slopes, floodplains, tidal mudflats, dune and desert sandsand desert sands

InceptisolsInceptisols

Soils in earliest stages of developmentSoils in earliest stages of developmentEluviationEluviation and and illuviationilluviation are evident, but are evident, but distinct horizons have not yet formeddistinct horizons have not yet formedLow fertilityLow fertility

GelisolsGelisols

Cold and frozen soilsCold and frozen soilsthe presence of permafrost is a defining characteristicthe presence of permafrost is a defining characteristic

Form very slowly due to low temperatures Form very slowly due to low temperatures slowing chemical and physical weatheringslowing chemical and physical weathering

disturbance to these soils has long lasting effectsdisturbance to these soils has long lasting effects

cryoturbationcryoturbationfreezefreeze--thaw cycle of frost tends to churn the soilthaw cycle of frost tends to churn the soil

AndisolsAndisols

Named after the AndesNamed after the AndesVolcanic parent materialVolcanic parent material

pyroclasticpyroclastic materialmaterial

Rapidly weather into colloidal materialRapidly weather into colloidal materialhigh fertility, high CEChigh fertility, high CEC

Distributed around Distributed around subductionsubduction zoneszonesPacific Ring of Fire especiallyPacific Ring of Fire especially

VertisolsVertisols

Expandable clay soilsExpandable clay soilsAt least 30% expandable clay (shrink and swell)At least 30% expandable clay (shrink and swell)

Climatic variablesClimatic variablesSubhumidSubhumid to semito semi--arid moisture, moderate to high arid moisture, moderate to high temperaturetemperatureRegions with highly variable seasonal moisture Regions with highly variable seasonal moisture balancesbalancesDistinct dry season following wet seasonDistinct dry season following wet season

Form under tropical and subtropical Form under tropical and subtropical grasslands and savannagrasslands and savannaDisastrous for construction (donDisastrous for construction (don’’t buy a house t buy a house built on built on vertisolsvertisols))

HistosolsHistosols

Organic soilsOrganic soilsFormed from the accumulation of Formed from the accumulation of organic matterorganic matter

lakebeds > bogs, marsh and peat > lakebeds > bogs, marsh and peat > histosolshistosolsPoor drainagePoor drainage

HumanHuman--Environment Environment Interaction Issues with SoilsInteraction Issues with Soils

Irrigation and Irrigation and SalinizationSalinizationMore common in arid environmentsMore common in arid environmentsThe accumulation and evaporation of irrigated The accumulation and evaporation of irrigated water causes salts to build up in the soils, and water causes salts to build up in the soils, and ultimately ruins its fertilityultimately ruins its fertilityDrainage tilesDrainage tilesGroundwater issues, Colorado River Basin Groundwater issues, Colorado River Basin issuesissues

Soil degradationSoil degradationresults from various practices that destroy agriculturally results from various practices that destroy agriculturally viable landviable land

agricultural practices that leave soil vulnerable to erosionagricultural practices that leave soil vulnerable to erosionovergrazing, vegetation removal, over exploitationovergrazing, vegetation removal, over exploitation20% drop in yield over next 20 years at moderate erosion rates20% drop in yield over next 20 years at moderate erosion rates

UrbanizationUrbanization

3 3 –– 5 million acres of US farmland are lost each year5 million acres of US farmland are lost each year38% decrease in world38% decrease in world’’s farmable land since 1950s farmable land since 1950

1380 million acres1380 million acres12 12 –– 15 million acres per year15 million acres per year

Agriculturally viable soil takes hundreds to thousands of Agriculturally viable soil takes hundreds to thousands of years to formyears to form

Soil as a nonSoil as a non--renewable resource?renewable resource?Agriculture as soil mining?Agriculture as soil mining?