2.1a soil physical - food systems organic farming/2.1a_soil... · physical properties such as soil...

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


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



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



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


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



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.



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%


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


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.)


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)



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


figUre 3. SOIL TExTURE TRIAnGLE



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


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.



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



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.



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.



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


Sandy loam clay loam

distancefromfurrowcenter(inches)

dis

tan

cefr

om

bo

tto

m(i

nch

es)


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



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.




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

2.1a soil physical - food systems organic farming/2.1a_soil... · physical properties such as soil...

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