2.1a soil physical - food systems organic farming/2.1a_soil... · physical properties such as soil...
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2.1 Soils and Soil Physical PropertiesIntroduction 3
Instructor’s Lecture Outline 5
Detailed Lecture Outline for Students 9
Demonstration 1: Soil Texture Determination 25
Instructor’s Demonstration Outline 25
Demonstration 2: Soil Pit Examination 31
Instructor’s Demonstration Outline 31
Supplemental Demonstrations and Examples 33
Assessment Questions and Key 37
Resources 41
Glossary 43
2 | Unit 2.1Soil Physical Properties
Soil Physical Properties Unit 2.1 | 3
Introduction
Introduction: Soils and Soil Physical Properties
Unit Overview
This unit introduces students to the components of soil and soil physical properties, and how each affect soil use and management. The lecture introduces the components of soil and different concepts of soil and soil physical properties, with special attention to those properties that affect farming and gardening. Through a series of demonstrations and hands-on exercises, students are taught how to determine soil texture by feel and are given the opportunity to examine other soil physical properties such as soil structure, color, depth, and pH. The demonstrations offer an opportunity to discuss how the observed soil properties might affect the use of the soil for farming and gardening.
MOdes Of instrUctiOn
> LEcTURE (1 LEcTURE, 3.0 hOURS)
The lecture outline introduces students to the components of soil, and different concepts of soil and soil physical properties, with special attention to those properties that affect farming and gardening.
> DEmOnSTRATIOn 1: SOIL TExTURE DETERmInATIOn (1.0 hOUR)
Demonstration 1 teaches students how to determine soil texture by feel. Samples of many different soil textures are used to help them practice.
> DEmOnSTRATIOn 2: SOIL PIT ExAmInATIOn (1.0 hOUR)
In Demonstration 2, students examine soil horizons, texture, structure, color, depth, pH, etc. in a large soil pit. Students and the instructor should discuss how the soil properties observed affect the use of the soil for farming, gardening, and other purposes.
> SUPPLEmEnTAL DEmOnSTRATIOnS AnD ExAmPLES (1 hOUR)
These simple demonstrations offer ideas for using objects, samples, or models to illustrate by way of analogy various soil physical properties.
> ASSESSmEnT QUESTIOnS (1.0 hOUR)
Assessment questions reinforce key unit concepts and skills.
Learning Objectives
cOncEPTS
• Componentsofsoil
• Soilphysicalproperties:Whatarethey?
• Howsoilphysicalpropertiesaffectuse
SKILLS
• Howtodeterminesoiltexture
• Howtorecognizedifferenttypesofsoilstructure
Soil Physical Properties4 | Unit 2.1
reqUired readings (see resOUrces sectiOn)
Gershuny, Grace. 1993. Start With the Soil,Chapter1;Chapter2,pp.27–38;Chapter8,pp.187–195;Chapter9,pp.200–205
Brady,NyleC.,andR.R.Weil.1999.The Nature and Properties of Soils.Chapter1,1.1–1.14(pp.1–22)
recOMMended readings
Stell,ElizabethP.1998.Secrets to Great Soil.Chap-ter 1
Introduction
Soil Physical Properties Unit 2.1 | 5
Instructor’s Lecture Outline
Lecture Outline: Soils and Soil Physical Properties
for the instructor
A. Introduction 1.Whatissoil? a) Definitions i.Differentconcepts=Differentdefinitions • Edaphological(inrelationtoplantgrowth) • Engineering • Pedological(seessoilasadistinctentity) b) Functionsofsoil i. Support growth of higher plants ii. Primary factor controlling fate of water in hydrologic system iii. Nature’s recycling system iv. Habitat for organisms v. Engineering medium
B. How Soil Is Made 1. Soil-forming factors a) Time:Howlongthesoilhasbeenforming b) Parentmaterial:E.g.,rock,alluvium c) Bioticfactors:Plants,animals,microorganisms d) Topography:Slopeposition,aspect,shape,andamount e) Climate:Temperature,moisture,seasonaldistribution
2.Soilprofilesandsoildevelopment a) Soilhorizons b) Soilhorizonation
3.Whatisinsoil? a) 40–50%mineral i. Gravel, cobbles, stones, boulders ii.Sand(0.05–2.00mm) iii.Silt(0.002–0.05mm) iv.Clay(<0.002mm) b) 0–10%biological(seetables1and2inDetailedLectureOutline) i.Floraandfauna ii.Liveanddead(organicmatter) iii. Macroscopic and microscopic c) ~50%porespace i. Air ii.Water
4.Soilclassification:12Orders
Soil Physical Properties6 | Unit 2.1
Instructor’s Lecture Outline
C. Soil Properties 1. Texture a) Soilseparates(mineralpartofsoil) i.Sand:Gritty ii.Silt:Flourywhendry,greasywhenwet iii.Clay(seeBaklavademonstrationinSupplementalDemonstrations) b) Texturetriangle i. 12 soil textures
2. Structure a) Whatisit? i. Arrangement of soil particles into aggregates ii.Naturalvs.man-made(pedsvs.clods) iii.Types(shape)(seefigure3,SoilTextureTriangle,inDetailedLectureOutline) iv.Size:veryfine,fine,medium,coarse,verycoarse,thick,thin v. Grade vi.Compoundstructure • Onestructurebesideanother • Onestructurewithinanother(“partsto…”) vii.Persistenceuponwettinganddrying—“Aggregatestability” b) Whatcausesstructure? i. Biological factors/organic matter ii.Clay(typeandamount) iii.Calciumandsodiumeffects iv.Climate(wet/dry,freeze/thaw)
3. Pores a) Whataretheyandwhyaretheyimportant? b) Typesofpores i.Interstitialpores:Spacesbetweenmineralgrainsandpeds ii.Tubularpores:Poresmadebyrootoranimalactivitythatareorwereatonetimecontinuous iii.Vesicularpores:Bubble-shapedpores c) Sizesofpores i.Macropores:Allowfreemovementofairandwater ii.Micropores:Airmovementisgreatlyimpeded,watermovementisrestrictedtocapillaryflow
4.Bulkdensity a) Whatisit? b) Importance
5.Organicmatter a) Importanceoforganicmatter i. Structure ii.AWC iii.CEC b) Relationshiptoclimate
Soil Physical Properties Unit 2.1 | 7
Instructor’s Lecture Outline
6.Color a) Howitismeasured b) Significanceof/indicatorof: i.Drainageandwetness(redoximorphicfeatures)(showsamples) ii.Organicmatter
7.Soildepth a) Bedrock b) Denselycompactedmaterial(tillagepan) c) Naturalhardpans(soilcementedbyiron,lime,gypsum,silica,etc.)(showexample) d) Stronglycontrastingtextures(poteffect) e) Watertables
8.Soiltemperature a) Factorsinfluencingsoiltemperature i.Localclimate:Soiltemperatureishighlycorrelatedtoairtemperature ii. Slope steepness and aspect iii.Topography:Topographyinfluencesmicroclimates iv.Cover:Plantsshadethesoil,reducingthetemperature v.Soilcolor:Darker-coloredsoilsabsorbheatmorereadilythanlighter-coloredsoils vi.Horticulturalpractices:Influenceofmulches b) Soiltemperatureinfluencesonsoilproperties i. Biological activity ii.Organicmatteraccumulation:Lowertemperature,greaterorganicmatter
accumulation iii.Weatheringofparentmaterials:Fluctuatingtemperatureshelpbreakdown
mineral grains; warmer temperatures increase chemical weathering
9. Drainage a) Excessivelydrained b) Somewhatexcessivelydrained c) Welldrained d) Moderatelywelldrained e) Somewhatpoorlydrained f) Poorlydrained g) Verypoorlydrained
10.Odor a) Indicatorofwetness
11. Permeability a) Rateatwhichwatermovesthroughthesoil b) Measurement(inches/hour) c) Propertiesinfluencingpermeability i.Texture(dopermeabilitydemonstrationinSupplementalDemonstrations) ii. Structure • Salts • Organicmatter • Compactionandpores • Calcium • Soil organisms
Soil Physical Properties8 | Unit 2.1
d) Additionalpropertiesinfluencinginfiltration i. Dryness ii. Surface fragments iii.Fire iii. Slope
12.AvailableWaterCapacity(AWC) a) Fieldcapacity:Theamountofwaterthesoilcanholdagainsttheflowofgravity
(1/3baror33kPa).(AlsoseeUnit1.5,Irrigation:PrinciplesandPractices.) b) Wiltingpoint:Themoisturelevelatwhichthesoilcannolongerprovidemoisture
forplantgrowth.(15baror1500kPa) c) Measurement(inches/inchorinches/foot) d) Propertiesinfluencingwater-holdingcapacity i. Texture ii. Salts iii.Organicmatter iv.Compaction v. Soil depth vi.Coarsefragments vii.EstimatingAWC
Instructor’s Lecture Outline
Soil Physical Properties Unit 2.1 | 9
Students’ Lecture Outline
Detailed Lecture Outline: Soils and Soil Physical Properties
for students
a. introduction 1. What is soil?
a) Definitions
i. Different concepts = Different definitions
• Edaphological(inrelationtoplantgrowth)
Amixtureofmineralandorganicmaterialthatiscapableofsupportingplantlife
• Engineering
Mixtureofmineralmaterial(sands,gravelsandfines)usedasabaseforconstruction
• Pedological(seessoilasadistinctentity)
Theunconsolidatedmineralororganicmaterialonthesurfaceoftheearththathasbeensubjectedtoandshowseffectsofgeneticandenvironmentalfactorsof:climate(includingwaterandtemperatureeffects),andmacro-andmicroorganisms,conditionedbyrelief,actingonparentmaterialoveraperiodoftime.Geosphere-Biosphere-Hydrosphere-Atmosphereinterface.
b) Functionsofsoil
i. Supportgrowthofhigherplants
ii. Primaryfactorcontrollingfateofwaterinhydrologicsystem
iii. Nature’srecyclingsystem
iv. Habitatfororganisms
v. Engineeringmedium
b. How soil is Made 1. Soil-formingfactors
Atonetimeitwasfeltthatsoilswerestatic.Inthelate1800s,Russiansoilscientistsintroducedtheconceptthatsoilsaredynamic—thattheydevelopedtothepointwheretheyarenowandthattheyareevolvingintowhattheywillbe.Theycameupwithfivesoil-formingfactorsthatinfluencehowsoilsturnoutthewaytheydo.Theideaisthatifallfiveofthesoil-formingfactorsarethesame,thenthesoilwillbethesame.Thetechnicaltermusedforsoilformationispedogenesis.
Soil Physical Properties10 | Unit 2.1
Students’ Lecture Outline
Weatheringisthetermusedtorefertothebreakdownofrockintosmallerandsmallerpieces.Twotypesofweatheringarerecognized,chemicalandmechanical(physical).Mechanicalweatheringhastodowiththebreakdownofrockduetophysicalfactorssuchastemperaturefluctuationsandfreeze/thawcyclesofwater.Anexamplewouldbequartzbreakingdowntofinesandsizeparticles.(Sincequartzisresistanttochemicalweathering,itwon’tgetmuchsmallerthanthis.)
Chemicalweatheringreferstothebreakdownofrockduetochemicalreactions.Forexample,limestone(CaCO
3)
andgypsum(CaSO4)dissolveinwaterandbecome
smaller and smaller. micas can lose potassium ions and becomevermiculite.Vermiculite,inturn,canlosemorepotassiumandbecomesmectite.Feldsparslosepotassiumandbecomekaolinite.Inthesecases,rockweatherstoamicroscopicorevenelementalstate.
a) Time:Howlongthesoilhasbeenforming
b) Parentmaterial:E.g.,rock,alluvium
c) Bioticfactors:Plants,animals,microorganisms
d) Topography:Slopeposition,aspect,shapeandamount
e) Climate:Temperature,moisture,seasonaldistribution
2. Soilprofilesandsoildevelopment
figUre 1: ThE 12 mOST cOmmOn ELEmEnTS In ThE EARTh’S cRUST
eLeMent % vOLUMe % weigHt
O2- 90 47
Si4+ 2 27
Al3+ 1 7
Fe2+ 1 4
Mg2+ 1 2
ca2+ 1 3
na+ 1 2
K2+ 1 2
Ti4+ trace 3
h+ trace 1
mn4+ trace 1
P5+ trace 1
a) Soilhorizons
Soilsconsistofoneormoredistinctlayerscalledhorizons.TheselayersarereferredtoasO,A,E,B,CandRdependingontheirpositionandnature
• O:Layersdominatedbyorganicmaterial.Usuallynotpresentunderwarm-dryconditions.
• A:ThemineralsoilhorizonthatisusuallyatthesurfaceorbelowanOhorizon.Itusuallyhasmoreorganiccarbonthanunderlyinglayers.Sometimesthislayerismissingortruncatedduetoerosionorremoval.Also,allsurfacesresultingfromplowing,pasturing,orsimilardisturbancesarereferredtoasAhorizons.
• E:Horizoncharacterizedbyeluviation(removalofmaterialssuchassilicateclay,iron,aluminum,ororganicmatter),ifdistinctfromtheAhorizon.Frequentlynotpresent.UsuallymorepalecoloredthantheAhorizon.
• B:Ahorizon,formedbelowanA,E,orOhorizon,whichisdominatedbyobliterationofallormuchoftheoriginalrockstructureandwhichshowsevidenceofsoilformationsuchasilluvial(moveddownfromanabovehorizon)concentrationofsilicateclay,iron,aluminum,humus,carbonates,gypsum,orsilica;developmentofsoilcolororstructure;orbrittleness,etc.
• C:Horizonsorlayers,excludinghardbedrock,thatarelittleaffectedbypedogenic(soilforming)processesandthatlackpropertiesofO,A,EorBhorizons.
• R:Hardbedrock
b) Soilhorizonation
(Talkthroughapossiblescenario,useblackboard)
3. Whatisinsoil?(Seefigure2)
figUre 2. SOIL cOmPOSITIOn: An IDEALIzED SOIL
ORGAnIc mATTER 5%
PORE SPAcE 50%
mInERAL 45%
Soil Physical Properties Unit 2.1 | 11
a) 40–50% mineral
i. Gravel,cobbles,stones,boulders
ii. Sand (0.05–2.00 mm)
iii. Silt (0.002–0.05 mm)
iv. Clay(<0.002mm)
b) 0–10%biological(Seetables1and2)
i. Floraandfauna
ii. Liveanddead(organicmatter)
iii. macroscopic and microscopic
c) ~50% pore space
Porespaceconsistsofthe“empty”spacesinthesoil.Whilethismightseemtomaketheporespaceunimportant,inrealityitisaveryimportantpartofthesoil.Porespacemightbefilledwithoneoftwothings:
i. Air
ii. Water
tabLe 1.SOILFAUNAANdTHEIREATINGHABITS
MicrOpHytic feeders carnivOres carnivOres secOndary cOnsUMers tertiary cOnsUMers
OrganisM MicrOfLOra predatOr prey predatOr prey cOnsUMed
Springtails Algae* Mites Springtails* Ants Spiders
Bacteria* Nematodes* Centipedes
Fungi* Enchytraeids Mites*
Mites Fungi Centipedes Springtails Scorpions
Algae Nematodes Centipedes Spiders
Lichens Snails* Mites
Protozoa Bacteriaand Slugs* Centipedes
othermicroflora Aphids* Beetles Spiders
Flies Mites
Nematodes Bacteria Moles Earthworms* Beetles*
Fungi Insects
Termites Fungi
tabLe 2.COMMONPOPULATIONSOFSOMESOILMICROORGANISMS
Students’ Lecture Outline
OrganisM nUMber per graM Of sOiL
Bacteria 108 –109
Actinomycetes 107 –108
Fungi 105 –106
Algae 104 –105
Protozoa 104 –105
nematoda 10 –102
*feedonliveplants/plantresidues,and/orsoilorganicmatter
12 | Unit 2.1Soil Physical Properties Students’ Lecture Outline
4. Soil classification: 12 Orders
Soilscientistshavecomeupwithsystemsforclassifyingsoils,muchlikeplantsandanimalsareclassified.Therearecurrently4mainclassificationschemes:Russian,FAO,Canadian,andSoilTaxonomy(Euro-Americaninorigin,butusedworldwide).SoilTaxonomyissimilartoplantandanimalclassificationinthatthisclassificationisbasedongenesis—howitisthoughtthesoildeveloped(plantsandanimalsarealsoclassifiedbyhowitisthoughttheyoriginated—genetics).Also,likeplantandanimalclassificationsystems,SoilTaxonomyisnotstatic.Asmoreislearned,thesystemchangessomewhat.
ThehighestcategoryofthissystemiscalledOrders.Currentlythereare12soilorders(seeTable3).
tabLe 3. 12 ORDERS In SOIL TAxOnOmy
Alfisols highbasesaturation—areaswithlowrainfall,butwetterthandeserts
Andisols volcanicashaffected
Aridisols deserts
Entisols “young”soils(floodplains,mountains,deserts,etc.)
Gelisols permafrost-affectedsoils
Histosols organicsoils,commoninwetandcoldareas(marshes,muskeg,etc.)
Inceptisols fairly“young”soils—soildevelopmentmoreadvancedthanEntisols
Mollisols thick,darksurfaces—humidandsub-humidgrasslands(cornbelt)
Oxisols verylowfertility,very“old”soils—humidtropics
Spodosols humidtemperatewoodlands,acidic
Ultisols lowbasesaturation—humidwarm-temperate,sub-tropicsandtropics;lowfertility,acidic
Vertisols highshrink-swell
Theothercategoriesoftheclassificationsystemaresuborder,greatgroup,subgroup,family,andseries.Theseriescorrespondstospeciesinbiologicalclassificationsystems.Seriesnamesareusuallytakenfromlocalgeographicfeaturesorplacenames.Thereareover20,000recognizedsoilseriesintheU.S.Thisisanindicatorofthetremendousamountofvariabilitythereisinsoils.
c. soil properties 1. Texture
Non-technicaldefinition:Howthesoilfeels
Technicaldefinition:Anexpressionthatcharacterizestherelativeamountsofsand,siltandclayinthesoil.
a) Soil separates (mineral part of soil)
i. Sand:gritty
ii. Silt:flourywhendry,greasywhenwet
iii. clay
• Morphology
Mostclaymineralsconsistofmicroscopiclayers(seeBaklavademonstrationinSupplemental Demonstrations and Examples). These are called phyllosilicate minerals.(Phyllo-isfromGreekforleaf,asinphyllodoughusedtomakebaklava.)
Soil Physical Properties Unit 2.1 | 13
differenttypesofclayhavedifferentkindsoflayersanddifferentproperties
Someclaymineralsareamorphous—withoutshape.Commoninhumidtemperatewoodlands(Spodosols)andvolcanicsoils(Andisols)
• Propertiesofclays
Sticky(adhesion—stickstootherthings)(targetdemonstration)
Plastic(cohesion—stickstoitself )(ribbondemonstration)
Shrink-swell(slinkydemonstration)
Largesurfacearea,duetolayersandtosize(blockdemonstration)
CationExchangeCapacity(CEC)
Clayhasnetnegativecharge,attractscations(positiveions;ionsarebrokenmolecules.Certainionsserveasplantnutrients)
• Characteristicsofsomeclayminerals
Kaolinite 1:1,noshrink-swell,lowCEC(3-15meq/100g),lowsurfacearea(5-20m2/g);prevalentinwarm,humidareassuchasthesoutheastUS
illite (hydrousmica),2:1,moderateshrink-swell,mediumCEC(15-40meq/100g);mediumsurfacearea(100-120m2/g)
smectite (montmorillonite)2:1highshrink-swell,highCEC(80-110meq/100g),highsurfacearea(700-800m2/g).Foundinyounger(lessweathered)soils.CommoninCalifornia.
iron and aluminum Oxides:somewhatcrystalline,verylowCEC(pHdependent),prevalentintropicsandsemi-tropics,fixesphosphorus
allophane and imogolite:amorphous,moderateCEC(pHdependent,10-30meq/100g),prevalentinvolcanicashderivedsoils,fixesphosphorus
b) TextureTriangle(seeFigure3,nextpage)
i. 12soiltextures(seeTable4)
tabLe 4.12SOILTExTURESNAMESANdTHEIRABBREVIATIONS
clay c sandy loam SL
sandy clay Sc loam L
silty clay SIc silt loam SIL
clay loam cL loamy sand LS
sandy clay loam ScL sand S
silty clay loam SIcL silt Sl
2. Structure
a) What is it?
i. Arrangementofsoilparticlesintoaggregates
ii. Naturalvs.man-made(pedsvs.clods)
iii. Types(shape)(SeeFigure4,p.15)
• Granular
• Blocky(angularandsub-angular)
• Platy
• Columnarandprismatic
• Singlegrain(non-structure)
• Massive(non-structure)
Students’ Lecture Outline
Soil Physical Properties14 | Unit 2.1
iv. Size:veryfine,fine,medium,coarse,verycoarse,thick,thin
v. Grade
Weak:Pedsbarelyobservableinplace,difficulttodistinguishfrommassiveorsinglegrain.
Moderate:Pedswellformedandevidentinundisturbedsoil
Strong:Pedsdistinctinundisturbedsoil.Pedshavedistinctivesurfacefeatures.
vi. Compoundstructure
• Onestructurebesideanother
• Onestructurewithinanother(“partsto…”)
vii. Persistenceuponwettinganddrying—“Aggregatestability”
tabLe 5.SIzECLASSESOFSOILSTRUCTURALUNITS.THINANdTHICk,RATHERTHANFINEANdCOARSE,AREUSEdFORPLATySTRUcTURES.
size cLass pLaty cOLUMnar/ bLOcKy granULar prisMatic
veryfine(thin) <1mm <10mm <5mm <1mm
fine (thin) 1 – 2 mm 10 – 20 mm 5 –10 mm 1 – 2 mm
medium 2 – 5 mm 20 – 50 mm 10 – 20 mm 2 – 5 mm
coarse(thick) 5–10mm 50–100mm 20–50mm 5–10mm
verycoarse(thick) >10mm >100mm >50mm >10mm
Students’ Lecture Outline
figUre 3. SOIL TExTURE TRIAnGLE
Soil Physical Properties Unit 2.1 | 15
Students’ Lecture Outline
b) Whatcausesstructure?
i. Biologicalfactors/organicmatter
• Bacterialexudates
• Rootactivityandexudates
• Macrofaunaactivityandwaste
ii. clay (type and amount)
iii. calcium and sodium effects
iv. Climate(wet/dry,freeze/thaw)
figUre 4. SOILSTRUCTUREANdITSEFFECTSONPERMEABILITy
3. Pores
a) Whataretheyandwhyaretheyimportant?
Poresarethe“holes”orvoidsinthesoils.Theyareimportantbecauseairandwatermovethroughandarestoredinpores.Withoutair,rootscannotlivenorcanmostmicrobesthatareessentialtotheproperfunctioningofahealthysoil.
b) Typesofpores:Threetypesofporesaregenerallyrecognized
i. Interstitialpores:Spacesbetweenmineralgrainsandpeds
ii. Tubularpores:Poresmadebyrootoranimalactivitythatareorwereatonetimecontinuous
iii. Vesicularpores:Bubble-shapedpores
Soil Physical Properties16 | Unit 2.1
c) Sizesofpores—twobasicsizeclassesofporesarerecognized,thoughthereisnotaparticularsizelimitbetweenthem
i. Macropores:allowfreemovementofairandwater
ii. Micropores:airmovementisgreatlyimpeded,watermovementisrestrictedtocapillaryflow.
4. Bulkdensity
a) What is it?
Thebulkdensityofthesoil(orofanythingelse)isthe(ovendry)weightofagivenvolumeofsoildividedbythevolume.Itisexpressedingramspercubiccentimeter.Theformulaisusuallywrittenlikethis:
db=Ms/Vt
Wheredb=bulkdensity
ms = mass of solids
Vt=totalvolume
Soilbulkdensityvaluesrangefrom0.5to3.0butmostvaluesarebetween0.8and1.8.Anythingdenserthanabout1.8isrootlimiting.
Bulkdensityisusuallydeterminedbycoatingamassofsoilwithathinlayerofplastic;weighingthesoil,correctingformoisturecontent;thendeterminingthevolumeofthatsoilbywaterdisplacement.
Thebulkdensityofthesoilisareflectionoftheamountofporespaceinthesoil.Otherfactorsaffectingthebulkdensityaretheytypesofmineralspresent(someareheavierthanothers),thetexture(claysarelighterthansiltsandsands)andtheamountoforganicmatter(organicmatterhasareallylowbulkdensitycomparedtomineralgrains).
b) Importance
Compactedsoilshavehigherbulkdensitiesthannon-compactedsoils
5. Organicmatter
Organicmatterconsistsofdeadplantpartsandanimalandmicrobialwasteproductsinvariousstagesofdecomposition.Eventually,thesethingsbreakdownintohumus,whichisrelativelystableinthesoil.
a) Importanceoforganicmatter:Althoughorganicmattermakesupaminorpartofthesoil,ithasaverystrongimpactonit
i. Structure
Organicmatteractslikegluethathelpsholdsoilaggregatestogether.Thiswillevenholduponwetting.
ii. AvailableWaterCapacity(AWC)
Organicmatterhelpsbindwatertothesoiltokeepitfrombeinglostthroughpercolation. This is especially important in sandy soils.
iii. CationExchangeCapacity(CEC)
WhilethehighestCECyouwillfindinaclayis160meq/100g(cmol/kg),humushasaCECof100to300meq/100g(cmol/kg)ormore
b) Relationshiptoclimate
youcannotaddlargeamountsoforganicmattertothesoilandexpectittopersist.Thereisamaximumequilibriumamountanygivensoilcanhold.Thisamountisinverselyproportionaltosoiltemperatureandmoisture.Thatistosay,wetterandcoldersoilscanmaintainhigherequilibriumamountsoforganicmatter.Anythingaddedbeyondthatamountwillbreakdowntocarbondioxideandwater.Theequilibriumamountcanberaisedtosomedegreebyadditionsoforganicmatter,suchasinorganicgardeningsituations,buteventhenitwillonlygosohigh.Tropicalsoils,forexample,tendtobenutrient-andorganic-matterpoor;thenutrientpooltendstobestoredintheabove-groundbiomass(leavesandbranches).ThemuskegareasofnorthernCanada,however,containlargeamountsoforganicmatter.
Students’ Lecture Outline
Soil Physical Properties Unit 2.1 | 17
Anewpracticerelatedtothisisbeingemployedtoreducegreenhousegasemissions.Carbonsequestrationisthepracticeofburyingorganicmatterdeepinthesoiltomaximizetheamountoforganiccarbonitcontains.Totheextentthatthesoilcanholdtheorganiccarboninequilibrium,itwillreduceemissionsofCO
2 from the soil.
6. color
a) Howitismeasured
MunsellColorNotation(showcolorbook)
b) Significanceof/indicatorof:
i. drainageandwetness(Redoximorphicfeatures)(showsamples)
Greenish,bluish,andgraycolorsinthesoilindicatewetness.Thesecolorsmayoccurasthedominantcolor(matrix)orinpatches(mottles).Thecolorsarecausedbythereductionofironbybacteriainanaerobicconditions.(Thebacteriagettheelectronstheyneedforenergyfromironratherthanfromoxygen.)Thesecolorswillpersisteveniftheareaisdrained,sotheyserveasindicators,notproof.
Brightcolors(redsandyellows),ontheotherhand,indicatewell-drainedsoils.youdon’twanttohavefreewater(waterinexcessoftheavailablewatercapacity)withintherootingdepthofyourplantsduringthegrowingseason.Itispossibleforasoilwithbrightcolorstostillhavewetnessproblemsifthegroundwaterismovingfastenoughandifithassufficientoxygenorifitistoocoldforbiologicalactivity.
ii. Organicmatter
darkcolorsinthesoilusuallyindicateorganicmatter.Theymayalsoindicatewetness(remember,wettersoilscanaccumulatemoreorganicmatter).Sometimes,thecolormaybederivedfromtheparentmaterial.Thisisoftenthecaseinsoilsderivedfrombasic(dark-colored)igneousrock.
7. Soil depth
Itisimportanttoknowthedepthofthesoil.Thedepthdetermineshowfartherootscangrowandhowmuchwaterthesoilcanhold.depthismeasuredtotheshallowestroot-limitinglayer.Somethingsthatmaylimitdepthare:
a) Bedrock
b) denselycompactedmaterial(tillagepan)
Tillagepansareformedwhenfarmimplementsrepeatedlypassthroughthesoilatthesamedepth.Thisactioncausessoilparticlestobepressedclosertogether,reducingtheamountofporespaceandthesizeofthepores.Consequently,thesepanshavepermeabilityrateslowerthanthatofthesoilaboveandbelowit.
Asoilmaybeplowedorrippedtotearupnaturalortillagepansandtoincreasetheporespaceinthesoil.Also,deep-rootedcovercropsmightbeused(seeUnit1.6,SelectingandUsingCoverCrops).Inagardeningcontext,doublediggingmightbeused(seeUnit1.2,GardenandFieldTillageandCultivation).
Thebenefitsofusingsomekindoftillagetobreakupsoilcompactiondon’tlastforever.Andwhileitismoredifficulttobreakupcompactioninafinertexturedsoil,thebenefitswilllastlongerthantheywillinacoarsetexturedsoil.Inacoarsetexturedsoil,suchasasandyloam,mostoftheporespaceaddedbyplowingorrippingwillbelostbytheendofonecroppingseason.
c) Naturalhardpans(soilcementedbyiron,lime,gypsum,silica,etc.)(Showexample)
d) Stronglycontrastingtextures(poteffect;usuallyfoundinfloodplainsoils)
e) Watertables
Students’ Lecture Outline
Soil Physical Properties18 | Unit 2.1
8. Soiltemperature
Soiltemperatureisimportanttogardeners,especiallywhenitcomestospringplanting.Manyseedsneedacertainminimumtemperaturebeforetheywillgerminate.
a) Factorsinfluencingsoiltemperature
i. Localclimate:Soiltemperatureishighlycorrelatedtoairtemperature
ii. Slopesteepnessandaspect:IntheNorthernHemisphere,north-facingaspectstendtobecoolerthansouth-facingaspects.Theeffectismorepronouncedwithsteeperslopesandlowerrelativehumidity.
iii. Topography:Topographyinfluencesmicroclimates.Forexample,coolairflowsdownfrommountaintopsalongdrainagesandsettlesinlowpartsofvalleys.Soilandairtemperatureinthesedrainagesandlowareasmaybelowerthantheelevatedareasadjacenttothem.Thisisreadilyapparentinthe“citrusbelt”intheSanJoaquinValley.
iv. Cover:Plantsshadethesoil,reducingthetemperature.Inaddition,growingplantscoolthetemperaturethroughtranspiration.
v. Soilcolor:darker-coloredsoilsabsorbheatmorereadilythanlightedcoloredsoils
vi. Horticulturalpractices:Mulchingreducesheatbyreducinginsolation—the absorbtionofheatwhenit’ssunnyandcanalsoactasaninsulator —holdinginheatinextremelycoldweather
b) Soiltemperatureinfluencesonsoilproperties
i. Biologicalactivity:Lowertemperature=lowerbiologicalactivity.Belowabout40°Fthereislittlebiologicalactivity.
ii. Organicmatteraccumulation:Lowertemperature=higherorganicmatteraccumulation.
iii. Weatheringofparentmaterials:Fluctuatingtemperatureshelpbreakdownmineralgrains.Warmertemperaturesincreasechemicalweathering.
iv. Nutrientavailability:Manynutrientsareunavailableorpoorlyavailableatlowtemperatures,especiallyphosphorus.(Thisisprimarilyrelatedtobiologicalactivity.)
9. drainage
Soildrainageordrainageclassesisawayofexpressingthefrequencyanddurationofperiodsinwhichthesoilissaturated(hasfreewaterorwaterinexcessoffieldcapacity).Excessfreewaterintherootzonecankillplantsorkeepthemfrombecomingestablished.TheU.S.departmentofAgriculturerecognizessevennaturaldrainageclasses(fromtheSoilSurveyManual):
a) Excessivelydrained
Waterisremovedveryrapidly.Theoccurrenceofinternalfreewatercommonlyisveryrareorverydeep.Thesoilsarecommonlycoarse-texturedandhaveveryhighhydraulicconductivityorareveryshallow.Thesesoilstendtobedroughty.
b) Somewhatexcessivelydrained
Waterisremovedfromthesoilrapidly.Internalfreewateroccurrencecommonlyisveryrareorverydeep.Thesoilsarecommonlycoarse-texturedandhavehighsaturatedhydraulicconductivityorareveryshallow.
c) Well drained
Waterisremovedfromthesoilreadilybutnotrapidly.Internalfreewateroccurrencecommonlyisdeeporverydeep;annualdurationisnotspecified.Waterisavailabletoplantsthroughoutmostofthegrowingseasoninhumidregions.Wetnessdoesnotinhibitgrowthofrootsforsignificantperiodsduringmostgrowingseasons.Thesoilsaremainlyfreeofthedeeptoredoximorphicfeaturesthatarerelatedtowetness.
Students’ Lecture Outline
Soil Physical Properties Unit 2.1 | 19
d) Moderatelywelldrained
Waterisremovedfromthesoilsomewhatslowlyduringsomeperiodsoftheyear.Internalfreewateroccurrencecommonlyismoderatelydeepandtransitorythroughpermanent.Thesoilsarewetforonlyashorttimewithintherootingdepthduringthegrowingseason,butlongenoughthatmostmesophyticcropsareaffected.Theycommonlyhaveamoderatelyloworlowersaturatedhydraulicconductivityinalayerwithintheupper1m,periodicallyreceivehighrainfall,orboth.
e) Somewhatpoorlydrained
Waterisremovedslowlysothatthesoiliswetatashallowdepthforsignificantperiodsduringthegrowingseason.Theoccurrenceofinternalfreewatercommonlyisshallowtomoderatelydeepandtransitorytopermanent.Wetnessmarkedlyrestrictsthegrowthofmesophyticcrops,unlessartificialdrainageisprovided.Thesoilscommonlyhaveoneormoreofthefollowingcharacteristics:loworverylowsaturatedhydraulicconductivity,ahighwatertable,additionalwaterfromseepage,ornearlycontinuousrainfall.
f ) Poorly drained
Waterisremovedsoslowlythatthesoiliswetatshallowdepthsperiodicallyduringthegrowingseasonorremainswetforlongperiods.Theoccurrenceofinternalfreewaterisshalloworveryshallowandcommonorpersistent.Freewateriscommonlyatornearthesurfacelongenoughduringthegrowingseasonsothatmostmesophyticcropscannotbegrown,unlessthesoilisartificiallydrained.Thesoil,however,isnotcontinuouslywetdirectlybelowplow-depth.Freewateratshallowdepthisusuallypresent.Thiswatertableiscommonlytheresultofloworverylowsaturatedhydraulicconductivityornearlycontinuousrainfall,orofacombinationofthese.
g) Verypoorlydrained
Waterisremovedfromthesoilsoslowlythatfreewaterremainsatorverynearthegroundsurfaceduringmuchofthegrowingseason.Theoccurrenceofinternalfreewaterisveryshallowand persistent or permanent. Unless the soil is artificially drained, most mesophytic crops cannot begrown.Thesoilsarecommonlylevelordepressedandfrequentlyponded.Ifrainfallishighornearlycontinuous,slopegradientsmaybegreater.
10. Odor
a) Indicatorofwetness
Whensoilsarewaterlogged,bacteriawillgettheiroxygenfromsulfur.Thiswillreleasehydrogensulfidegas.Thisaccountsforthesulfursmellprevalentaroundsaltmarshes.
11. Permeability
a) Rateatwhichwatermovesthroughthesoil
Permeabilityistherateatwhichwatermovesdownthroughthesoil.Itisusuallymeasuredininchesperhour.Infiltrationistherateatwhichwaterentersthesoil.Itissimilartopermeability,exceptthatitalsotakesintoaccountsurfaceconditionssuchassoilcrusting.Permeabilityandinfiltrationratesaffecttherateatwhichyoucansafelyapplywatertothefield.Applyingwaterfasterthanthepermeabilityandinfiltrationratescanleadtosealingofthesoilsurface,whichfurtherdecreasesinfiltrationrates;itcanalsocauseponding,whichincreasesthepossibilityofdiseases;anditcanleadtorunoff,whichcauseserosionandpossiblefertilizerloss.
Thepermeabilityofasoilcanbenofasterthanthepermeabilityoftheslowestlayer.Forexample,sandyloamhasapermeabilityof2.0to6.0inchesperhour.Sandyclayloamhasapermeabilityof0.2to0.6inchesperhour.Asoilthathasasandyloamsurfaceoverasandyclayloamsubsoilwillhaveapermeabilityof0.2to0.6inchesperhour.
b) Measurement(inches/hour)
Permeabilityisnormallymeasuredininchesperhour.AnewerexpressionyouwillseeisSaturatedHydraulicConductivity(ksat).Itismeasuredinµm/secorcm/hr.
Students’ Lecture Outline
Soil Physical Properties20 | Unit 2.1
c) Propertiesinfluencingpermeability
i. Texture
Soiltextureisusuallythedominantsoilpropertyaffectinginfiltration.Soilsthatarehighinclaycontenttendtohaveaslowerpermeability.Soilsthatarehighinsandcontenttendtohaveafasterpermeability(seeTable6).
tabLe 6.SOILPERMEABILITyCHART THESEARENORMALVALUESFORNON-COMPACTEdSOILS,SUCHASINGRASSLANdSITUATIONS
textUre cLass textUre perMeabiLity rate perMeabiLity cLass
Coarse gravel,coarsesand >20inches/hour veryrapid
sand,loamysand 6–20inches/hour rapid
ModeratelyCoarse coarsesandyloam 2–6inches/hour moderatelyrapid sandy loam fine sandy loam
Medium veryfinesandyloam 0.60–2inches/hour moderate loam silt loam silt
Moderatelyfine clayloam 0.20–0.60inches/hour moderatelyslow sandy clay loam silty clay loam
Fine sandyclay 0.06–0.20inches/hour slow silty clay clay(<60%)
Veryfine clay(>60%) <0.06inches/hour veryslow clay pan
Soiltexturenotonlyaffectshowfastwatermovesthroughthesoil,italsoaffectsthepatternbywhichwatermovesthroughthesoil.Waterwillmovealmoststraightdownthroughasandysoil,whereasitwillhavemorelateralmovementinaheaviersoil(onewithmoreclay).(Seefigure5)
figUre 5. MOVEMENTOFWATERTHROUGHSANdyANdCLAySOILS
Students’ Lecture Outline
Sandy loam clay loam
distancefromfurrowcenter(inches)
dis
tan
cefr
om
bo
tto
m(i
nch
es)
Soil Physical Properties Unit 2.1 | 21
ii. Structure
Soilstructurehasperhapsthegreatesteffectonpermeability.Thecracksandporesbetweenaggregatesallowforthemovementofairandwaterthroughthesoil.Anythingthatimprovesstructureimprovespermeabilityandviceversa.Tillageandirrigationaffectsoilstructure.Forexample,heavyoverheadirrigationorfloodirrigationbreaksdownsoilstructure,whichcanleadtoasealingofthesoilsurface.Thisinturnmakesitmoredifficultforanyfurtherwatertoenterthesoil.Tillagecanhelpbreakupasoilthathasbecomesealed,providingitisnotdonewhilethesoilistoowetortoodry.
Allofthefollowingpropertiesrelatetosoilstructure:
• Salts:Sodiumsaltscausesoilparticlestodisperseandclogpores,whichhasanegativeeffectonsoilstructure.Suchsoilstendtosealwhenwet,whichslowsinfiltrationandpermeabilityratesdrastically.
• Organicmatter:Organicmatter(decayedplantmaterial)isdesirableinthesoil,notonlybecauseitimprovessoilfertility,butitalsoimprovessoilstructure,whichhasbeneficialeffectsonpermeabilityandinfiltration.
• Compactionandpores:Allsoilscontainpores.Theporespacesareoccupiedbyeitherairorwater(plantrootsneedbothairandwater).Fine-texturedsoils(soilswithhighclaycontent)containmoretotalporespacethancoarse-texturedsoils(suchassandyloamandsand),howevertheporespacesaresmaller.Becauseofthis,watermovesmoreslowlythroughafine-texturedsoil.
• Calcium:Calciumimprovessoilstructurebyencouragingaggregationandincreasingporesize.Asaresultitimprovespermeabilityandinfiltration.
• Soilorganisms:Microorganisms(e.g.,bacteriaandfungi)andmacroorganisms(e.g.,insectsandearthworms)inthesoilcontributetoimprovedpermeabilityandinfiltration.Theyhaveabeneficialeffectonsoilstructurebecausetheyencouragetheformationofsoilaggregatesandtheymakeporesinthesoil.
d) Additionalpropertiesinfluencinginfiltration
i. dryness:Frequently,drysoilswillrepelwateruntiltheybecomemoistenedtosomedegree.Thisisespeciallytrueofsoilsthathavehighamountsoforganicmatter.(Peatmossdemonstration)
ii. Surfacefragments:Aheavycoverofgravelandstoneswillpreventwaterfromenteringthesoilandincreaserunoff.However,thesetypesofsoilarenotusuallycultivated.
iii. Fire:Ahotfirecanproduceresinsandwaxymaterialsthatrepelwater
iv. Slope:Slopemaycausewatertorunoffratherthanenterthesoil
12. AvailableWaterCapacity(AWC)
definition:Amountofwaterthatthesoilcanholdthatisavailableforplantgrowth
AWCisthedifferencebetweentheamountofwaterinthesoilatfieldcapacityandtheamountofwaterinthesoilatwiltingpoint.(Spongedemonstration)
a) Fieldcapacity:Theamountofwaterthesoilcanholdagainsttheflowofgravity.(1/3baror33kPa)
b) Wiltingpoint:Themoisturelevelatwhichthesoilcannolongerprovidemoistureforgrowthofmostagronomicplants.(15baror1500kPa)
c) Measurement(inches/inchorinches/foot)
AWCisusuallymeasuredininchesperfootorinchesperinch.Ifittakestheadditionoftwoinchesofwatertowetadrysoil(atPWP)toadepthof1foot,thentheAWCis2inchesperfoot(0.16inchesperinch).Theavailablewatercapacityisthenexpandedtothenumberofinchesofwaterthesoilcanholdwithintherootingdepthofthecrop—usuallyrangingfrom4–60inchesoraroot-restrictinglayer,whicheverisshallower.
d) Propertiesinfluencingwaterholdingcapacity
Students’ Lecture Outline
Soil Physical Properties22 | Unit 2.1
i. Texture
Soilsthatarehighinsandcontenttendtohavealoweravailablewatercapacity.Soilsthatarehighinclaycontenttendtohaveahigheravailablewatercapacity.However,iftheclaycontentistoohighortheclayparticlesaretoofine,thentheAWCmightbereducedbecausethetinyporesmayholdontothewatersotightlythattheplantscan’tgetit(seeexamplesinTable7).
tabLe 7.TyPICALAVAILABLEWATERCAPACITy(AWC)FORVARIOUSSOILTExTURESFORSOILSHIGHIN2:1MINERALS (SOILSHIGHINkAOLINITEORGIBBSITEAREABOUT20%LOWER)
sOiL textUre avaiLabLe MOistUre range average inches/foot inches/foot
VeryCoarsetoCoarseTextured (sands and loamy sands) 0.50 – 1.25 0.90
moderately coarse Textured (coarse sandy loam, sandy loam and fine sandy loam) 1.25 – 1.75 1.50
medium Textured (veryfinesandyloam,silt,siltloam,loam,sandyclayloam, clay loam and silty clay loam) 1.50 – 2.30 1.90
FineandVeryFineTextured (silty clay, sandy clay and clay) 1.60 – 2.50 2.10
OrganicSoils (peatsandmucks) 2.00–3.00 2.50
ii. Salts
SaltsreducetheAvailableWaterCapacityofthesoil.Asoilthatissaltycanbewetandyetnothaveanywateravailableforplantgrowth.Thisisbecausethesaltshavesuchastrongattractionforthewaterthattherootscannotovercomeit(seeTable8).
tabLe 8.REdUCTIONINAWCFORSALTS
ECofsoil 4 6 12 16 18 20 22 25 30
%ReductionAWC 10 20 30 40 50 60 70 80 90
iii. Organicmatter:Organicmatterisdesirableinthesoil,notonlybecauseitimprovessoilfertility,butbecauseitcanalsoimprovetheAvailableWaterCapacity
iv. Compaction:Whenasoiliscompacted,thesoilparticlesarepressedtogether,whichreducestheporespace.Thismeansthereislessspaceforthewatertooccupy.
v. Soildepth:Rootingdepthisthedepthtorockorotherlayerinthesoilthatrestrictsrootdepth.Naturalhardpansaswellasman-madepansmayrestrictrootgrowth.Thepresenceofaroot-restrictinglayerreducestheavailablewatercapacityofthesoil,sinceitreducestheamountofsoilthatisavailableforplantroots.
Onealsoneedstoconsiderthenaturalrootingdepthoftheplants.Forexample,iftheplantrootswillonlygotoadepthoftwofeetinasoilwithnorootrestrictions,thensoilbelowtwofeetshouldnotbeconsideredwhencalculatingavailablewatercapacityforthatcrop.
Students’ Lecture Outline
Soil Physical Properties Unit 2.1 | 23
Students’ Lecture Outline
vi. Coarsefragments:“Coarsefragments”referstogravel,cobbles,stonesandbouldersinthesoil—anythinglargerthan2mm.Sincecoarsefragmentsdonotholdwater,theirpresenceinthesoilreducesitswaterholdingcapacity.(SeeTable9)
tabLe 9.REdUCTIONINAWCFORCOARSEFRAGMENTS
textUre MOdifier % cOarse fragMents % awc redUctiOn
Nomodifier 0-15% 0-15%
Gravelly,cobbly,stony,bouldery 15-35% 15-35%
Very(gravelly,cobbly,stony,bouldery) 35-60% 35-60%
Extremely(gravelly,cobbly,stony,bouldery) 60-90% 60-90%
vii. EstimatingAWC:SeeExample1
exaMpLe 1.CALCULATIONOFTOTALAVAILABLEWATERCAPACITyINTHEROOTzONE
ESTIMATINGAVAILABLEWATERCAPACITy
Determine AWc for each layer soil texture.
ReduceAWCforeachlayerforgravel.
Reduce AWc for each layer for salts.
calculate AWc for entire soil.
(Inthisexampleweassumenosaltsorcoarsefragments)
deptH textUre Layer tHicKness awc per fOOt avaiLabLe MOistUre (fOOt) (incHes/fOOt) (incHes)
0to8inches sandyloam 8/12 x 1.5 = 1.0
8to20inches sandyclayloam 12/12 x 1.9 = 1.9
20to48inches loamysand 28/12 x 0.9 = 2.1
48inches rock(rootingdepth)
TOTALAVAILABLEMOISTURE 5.0InchES
Ifyouwantedtoirrigateat50%depletion,whichisoftenthecase,theninthiscaseyouwouldirrigatewith2.5inchesofwaterwhentheavailablewaterreached2.5inches(50%of5inches).
24 | Unit 2.1Soil Physical Properties