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Soil Science CurriculumContent and lab derived from the USDA-NRCS Guides for Educators. Go to https://www.sdsoilhealthcoalition.org/soil-health-buckets/ for the Guides and additional pictures and diagrams.
South DakotaNatural Resources Conservation Service South Dakota FFA
July 2019
Approximately 60 minutes
ObjectivesBy the end of the lesson, students will know or be able to:
•Definesoilaggregate,mycorrhizalfungi,hyphae,plantrootexudates,glycoprotein•Explainwhyasoilaggregateisimportanttosoilhealth•Demonstrateand/ormeasuresoilaggregatestability•Describemanagementpracticesthatsustainorimproveaggregatestability
Materials•3-inchfoamballs•1-inchorsmallerfoamballs•Post-itnotesor3x5indexcards•Slenderwoodendowels,toothpicksand/orcoffeestirrers•Stringlicorice(differentcolors)•Sillystring•Rollofmonofilamentstring(fishingline)•Smallplasticanimals(farmanimalset&jungleanimalset-optional)•Babyorfootpowder•Sinkstrainers(atleast2)•Disposablecerealbowls•Water(aboutaquart)•Flatplasticlidorcuttingboard
Preparatory Work•Printallnecessarycopies•Securepermissiontocollectsoilfromthelandowner(s)•Collectsurfacesoilfromawell-managedcroplandfieldthatisnottilled,andfromonethatisconventionallytilled,about2cupsofeach•Setupatablewithadisposabletableclothorpaper•Providesheetwithdefinitions
Soil Aggregates: Role in soil health and measuring stability
Soil Aggregates: Role in soil health and measuring stability
2
Enroll the Participants (Approximately 30 minutes)Handoutabout10-15,3-4-inch-longdowels/toothpicks/stirrers,onesetperstudent;aboutfourtofive3-inchfoamballs,aboutseventononeofthesmallerfoamballs,andtwototheeindexcards/Post-itnotesroughlytornintofourpieces.Haveastudentwithadowelinsertitintothefoamballfromanotherstudent,andplaceonthetablecloth.Haveastudentwithanindexcardpieceplaceitonthedowel(youmayhavetopiercethepaperwithasmallholetogetitstartedonthedowel).Addanotherfoamballto
the other end of the dowel and insert another dowel to one of thefoamballsthatisalreadyattachedtotheotherdowel.Itmayhelptohaveglueonhandandapplytotheendsofthesticksasyoubuildtheaggregate.Theobjectistobuildasoilaggregate“lattice”,somethinglikewhatisshowninFigure1.
Asyouarebuildingthesoilaggregate,explainthatthelargefoamballsrepresentsandparticles,thesmallerfoamballsrepresentsiltparticles,andthePost-itnote/indexcardpiecesrepresentclayparticles.Thedowelsrepresentweakbondsbetweenthesoilparticlesthatwouldnormallybreakveryeasilywhenthesoilgetswet.
Figure 1
Next,handoutafewoftwocolorsofthestringlicorice,andhavethestudentslaythemon/inthelattice.Explainthatonecolorrepresentsmycorrhizalfungi,andtheothercolorrepresentsplantroots.
Thengiveacanofthesillystringtoastudent(ortwocans,onetoeachoftwostudents),andhavethemspraysomesillystringonthesoilaggregate.sillystringrepresentsglycoproteinsthathelpgluethesoilaggregatetogether.
(Optional)Takesomeofthefarmanimalsandjungleanimals,andhavethestudentsplacethemon/inthesoilaggregate.Youcanaskanadditionalquestionaboutwhattheanimalsrepresent.Theyrepresentsoilmicrobesthatmaketheaggregatetheirhome(habitat).Somesoilmicrobesarepredators,likethelionortiger,andsomeareprey,likethesheep.Youcanusepapercut-outanimalsifyoudon’thavetheplasticsonesavailable.
Takeoneendofthemonofilamentstringandtieitofftoa“plantroot”orjustattachittothesoilaggregate,thenrollthestringoutacrossthefloor.Thisrepresentsmycorrhizalfungihypae,whicharebasicallyliketherootsofthefungi.Theyconnecttotheplantrootsandgreatlyincreasetheareaofthesoilfromwhichtheplantrootscangetwaterandnutrients.YoucaninitiallywrapthestringaroundyourarmandexplainthatthehyphaearelikeanIVthatyougetwhenyougointosurgery,andactsthesameway,supplyingfluidsandnutrients/medicinesbetweentheplantsandfungi.
Youcanalsoattachadollarbilltooneofthefoamballs,andwhenyoudestroytheaggregate(below),youcanseethemoney(fertilizer,organicmatter,etc.)erodingaway.
Soil Aggregates: Role in soil health and measuring stability
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Label the Information (Approximately 3 minutes)Reviewandidentifyeachofthesuppliesthatwillbeusedduringthelabactivity.
• Sink strainers• Disposablecerealbowls• Water• Twosoilsamples,onefromno-tillandonefromconventionaltill
Provide the Experience (Approximately 3 minutes)
Reviewthelaboratoryscenariowithstudents.
Alocalfarmerhasbeenfarmingwitharotationofcorn,soybeans,wheat,andsunflowers,andhasbeenpracticingno-tillfor14years.Asoftenaspossible,covercropsareincorporatedintotherotation.Thefarmerrecentlydecidedtorentsomeadjacentlandbutfoundthatthefieldcannotbeenteredintoasearlyasthelandcurrentlybeingfarmed.Waterisoftenpondingintherentedfieldafterarain,anderosionishappening,evenformingsmallgulliesinsomeplaces.
Usethefollowingquestionstofacilitateaconversationwiththestudents:
Whatdoesthesillystringrepresent?Answer:Plantrootsgiveoffexudates,whicharebasicallysugars–sugarissticky,andtheseactlikegluesthathelpholdtheaggregatetogether.Mycorrhizalfungigiveoffglomalin,whichisanother“glue”thathelpsholdtheaggregatetogether.Earthwormsareslimy,andthisslimeisanotherglue–allthesegluesmakethesoilaggregateverystrongandstable.
Whatdoyouthinkwouldhappentotheaggregateifthesoilisdisturbed(tilled)?Answer:Afteryougetsomeanswersfromthestudents,youcandemonstratewhatwillhappenbysmashingthesoilaggregatewithyourhand,andsaythisisadiscorplow.
Whatwillhappentotheindividualsoilparticleswithdisturbance/tillage?Answer:Erosion–youcandemonstratebytakingoneofthefoamballsandrollingitacrossthefloor,andsay“thisislikewindorwatererosion”.Also,thefirstthingtoleaveduringanerosioneventislightorganicmatterparticlesintheairorwater.Youcandemonstratethiswiththebaby/footpowder,bysqueezingthebottleupintotheair–itislikethecarbondioxideleavingwithdisturbancebutyoucan’tseecarbondioxide.
Whathappenstothemycorrhizalfungiandtheirhyphaewithdisturbance/tillage?Answer:Youcandemonstratetheanswerbyshowingthatthehyphaearedisconnectednowfromtheplantroot,andyoucanpulloutalicoricestringandbreakitintopieces.
Thereareotheranalogiesyoucandrawfromthisdemonstration,useyourimagination.
Soil Aggregates: Role in soil health and measuring stability
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Demonstrate the Relevance (Approximately 25 minutes)Steps:1.Fillonesinkstrainerleveltothetopwithsoilfromtheno-tillfield.Asyouarefillingthesinkstrainer,gently
breakapartthesoilintothenaturallyoccurringaggregates.Donotoverlyworkthesoilsampleintofineparticlesorcrushtheaggregates.
2.Repeatstep1withtheconventionaltilledsoilintotheothersinkstrainer.3.Fillthedisposablecerealbowlswithwatertothebrim.4.Placeeachsoilsampleinthesinkstrainersgentlyintothebowlswiththewater.Ensurethattheentire
soilsampleisinfullcontactwiththewater.Letthesamplessoakforapproximately30-60seconds.5.Removeeachsoilsamplefromthebowlsandturnthesampleoverontotheflatplasticlidorplasticcuttingboardsothattheentiresampleremainsonthelid/board.Placethesamplesseveralinchesapart.
6.Raiseonesideoftheboard/lidslightlysothewaterstartstomoveawayfromthesamplesandobservethedifferences.
Thisisonemethodtotestforsoilaggregatestabilityandisalsoknownastheslumptest.Thesamplewithgoodaggregatestabilitywill“standup”higherandtheindividualsoilaggregateswillstillbevisible.Thesamplewithpooraggregatestabilitywilloozedowntheboard/lidandwillbe“soupy”.Oneanalogythatworksisthegoodaggregatestabilitysamplewilllooklikeachocolatebrownie,andthepooraggregatestabilitysamplewilllooklikechocolatepudding.Oftentimesthegoodsamplewillbedarkerthanthepoorersample,andtheywilloftensmelldifferent(thegoodsamplewillhaveanearthysmell,orthesmellafterarain,whilethesmellofthepoorersamplewillbevariable).Figure2showsanexampleoftheresultsofthistest.
Glycoprotein:astickysubstanceproducedabundantlybyhyphaeandsporesofmycorrhizalfungiinsoils–itisanexudateofthefungi;actsasaglueholdingsoilparticlestogetherintoaggregates.
Hyphae:long,branchingfilamentousstructuresofthemycorrhizalfungithattransportwaterandnutrientsfromthesoiltothemainfruitingbodyofthefungiandtotheplantrootswithwhichtheyareassociated–similartotherootsofaplant.
Mycorrhizal fungi:thesearefungithatformasymbioticrelationshipwithplants,andattachtoorintotheplantrootandprovidewaterandnutrientstotheplant–theplantinturnsuppliesfoodtothefungi;theseassociationsgreatlyincreasetheextensionoftherootsintothesoilprofileareainthesoilwhichisavailabletotheplant.
Plant root exudates:thesearepolysaccharidesorbasicallysugarsproducedbyplantrootsthatareusedbymanysoilorganismsforfood,andhelpbuildsoilaggregates.
Soil aggregate:soilparticlesheldtogetherbyorganicmatterandrelatedsubstances.Wellaggregatedsoilshavehigherinfiltrationratesandarelesspronetoerosion.Aggregatesalsoprovidehabitatforsoilmicroorganisms.
Definition
Figure 2