Knowledge brief detailing the advantages and function of pumice as a soil amendment and conditioner. SoilRox is a brand name for select pumice grades for use in preparing potting soils, garden soil, and even large-scale landscaping and reclamation efforts when poor native soil needs improvement—especially structurally (as in the case with heavy clay soils)—to support vibrant plant life.

1. Conditioning Soil
withPumice
K N O W L E D G E B R I E F
close-up of a 1-inch pumice stone
crushed pumice (G2) at 100x magnification
(208)766-4777x111
email:rd@hesspumice.com
www.hesspumice.com
Pumice, by virtue of its natural structure (best described as a
foamed stone), makes an ideal soil conditioner for those looking to
permanently improve root zone structure, mitigate compaction,
and retain moisture and nutrients.
FORASLONGasmanhasbeencultivating
andplanting,hehasunderstoodthe
importanceofconditioningthesoil—
eitherbyaddingneedednutrients,byimprovingpoornativestructure,orboth.Organiccomposts
workwonderfully,butforpoorsoilsthatneedtobestructurallyamended,organicsarenota
long-termsolution.Instead,aclassofinorganicamendmentscanbeusedtoengineerpermanent
performancestructureintosoils.Pumice,byvirtueofitsnaturally-occurring,foamed-stonestructure,
makesanideal,long-livedsoilamendment.Evencrushedtosoil-blendingparticlesize,pumiceretains
itslight,frothy,functionalcharacter,contributingtosoilperformanceinthefollowingways:
IMPROVES SOIL STRUCTURE. Afunctionalrootzoneiscriticaltosupportvibrant,deep-rooted
plants.Forexample,heavyclaysoils,consistingofatightmatrixofplate-likeparticles,offerapoor
growingmediumifnotamended.Thefertilitymaybethere,butthestructureiswrong.Addingpumice
physicallydisruptsclay’stendencyforcompaction,forcingapartthedense,stickyparticlesand
improvingthesoil’sabilitytobreath,absorbanddrainwater,filterrunoff,andsupportlushvegetation.
Astudy1 publishedintheJournalofCentralEuropeanAgriculture(2005)measuredstrawberry
seedlinggrowthinpumice-amendedsoil(at15,30,and45%amendmentlevels)anddetermined
“importantlevel”increaseingrowth,attributedtothestudiedenhancedmoistureretentionandpore
sizedistributionfactorscontributedbythepumiceamendment,especiallyat45%amendmentratio.
Anotherstudy2 concluded“thatpumicemaybeeffectivelyusedinspecificamountsforimproving
aerationandbulkdensityconditionsofpoorlystructuredsoils.”
IMPROVE SOIL SUBSTRUCTURE. Creatingorrepairingcompactedsubsoilstructureisas
essentialtocropproductivityasisamendingtheuppersoilarea.Openingupsubsoilporespacemeans
vegetativevitalityisnotrestrictedbypooraerationanddrainage.Workingpumiceintothesubsoil
lightenssoildensity,allowingdeeperaerationandmicrobialaction,whichinturnbringsgreaterroot
massanddepth.
KEYCHARACTERISTICSof
HESSPUMICE:
■ Specificgravityweightof2.35
■ Lowunitweight:40-50lbscubic
foot(dependingongrade)
■ MOHS-scalehardness:6
■ pHneutral:7.2
■ Duetoitsamorphouscharacteristics,
pumiceisnotconsideredahealthriskto
theworkerswhohandleit.
■ Chemically,pumiceisprimarilySilicon
Dioxide(AmorphousAluminumSilicate),
someAluminumOxide,andtraceamounts
ofotheroxides.
[1] Effect Of Pumice Amendment On Physical Soil Properties and Strawberry Plant Growth.
Ustun SAHIN, Selda ORS, Sezai ERCISLI, Omer ANAPALI, Ahmet ESITKEN; Atatürk University, Department of Agricultural Structures and Irrigation, Erzurum-Turkey
[2] Addition of Pumice Affects Physical Properties of Soil Used for Container Grown Plants.
Ustun SAHIN, Omer ANAPALI; AgriculturaeConspectusScientificus (ACS) 01/2006; 71(2)

2. C O N D I T I O N I N G S O I L W I T H P U M I C E | 2
MITIGATES COMPACTION.
Becausethefoamy,pore-richnatureof
pumiceisinherentlystructural,muchofthe
compactionresistantbenefitof
pumice-blendedsoilscomesfromthe
matrixoftinypumicestonesthemselves.
Thatmeansdesirablefactorslikerootzone
airexchangecapacityandmoisture
retentionarenotwhollyreliantonthe
spacesbetweensoilparticles.Andthe
grippynatureofthepumicegranules
stabilizessurroundingsoilparticles,greatly
increasingresistancetothecompressive
forcesofcultivation,watering,harvesting
and,inthecaseofturf, footandcarttraffic.
RETAINSWATER AND NUTRIENTS. Frothypumicegranules
grabandholdwaterandnutrientsandthengivethembacktothesoil
(androots)overtime.
TheAustralianJournalofCropSciencepublishedastudy3 thatlooked
ataddingpumicetoachievegainsinsoilmoistureabsorptionand
retentionformaize(corn)crops.Theresultsshowedthat“pumice
significantly(p>0.05)increasedtheamountofsoilmoistureretention
comparedtocontrol.Growthcharacteristicsofmaize(vegetativegrowth
andyield)weresignificantlyimprovedwithincreasingamountof
pumiceconcentration”withthemaxresultsobtainedwiththe30%
pumiceapplication.
CREATES ELEMENTALTILTH. Mixingpumiceintopoornative
soilstransformsthesoilstructureonanelementallevel,restructuring
thesoilmatrixanddiversifyingtheporesizestocreatemorefavorable
conditionsforseedgerminationandrootproliferation.Thistypeoftilth
iselementalandenduring,unlikethetemporary,unstabletilthcreated
solelybytillagepractices.
ENGINEERED SOILS
Engineeredlandscapes—reclamationprojects,runoff-filteringecology
embankments,drainageandstormwatercontrollingcontours,
constructedwetlands,high-trafficparksandsportsfields—demand
fine-tunedsoilsdesignedtooptimizeperformance.Theseprojectsare,
ofnecessity,fix-and-forgetprojects,andifthesoilprofileisnot
improvedinthelong-term,4 mostfailin
sustainingtheplantlifethatisseededin
them.Engineeringproperlong-termsoil
structureandwater-holdingand/or
drainagecapacityincreasestheoddsof
successsignificantly.Pumiceistheideal
inorganicfoundationamendmentupon
whichtobuildthesetypesoflarge-scale
constructs,asthesoil-enhancingbenefits
ofpumicecanberealizedwithalittleasa
10%pumice-to-soilratio.
A LIGHTWEIGHT SOIL
AMENDMENT. Thefrothy-stone
characterofpumicemakesitanideal
lightweightamendmentforroof-gardensandotherweight-sensitive
grow-bedapplicationslikelargepotsandraisedplanterbeds.In
greenhouseculture,alowerbulkdensityisdesirableduetoeasier
handlingandlessrootlossduringpickupandtransportation.
PUMICE-BLENDED COMPOST
Thebeautyofpumice-blendedcompostisthatlongaftertheorganic
matterisconsumed, thepumiceremains,afunctional,physical
presenceimprovingthesoilprofile.
Inlarge-scalefix-and-forgetrevegetationprojectslikethose
mentionedabove,repeatcompostapplicationsarenotpossibleor
feasible. Thatbecomesespeciallyproblematicwhenthecompostwas
usedasarestructuringagentinthesoilandnotsolelytoprovidea
kick-startingnutrientandmicrobialcharge.
Pumice-enhancedcompostprovidesrevegetationcontractorswith
aviable,cost-effectivepre-seedingsolutiontobothenrichthesoiland
amendstructurallypoorordamagedsoilstoachievelong-term,
self-sustainingvegetativecover.
Pumice-blendedcompostisavalue-addedproductforregionswith
poororproblematicnativesoils—asthepumice-enhancedcompostis
applied,aresidual,cumulativeimprovementistakingplaceindefinite-
ly—animprovementthatcontinuestobuildcycleaftercycle.Whenthe
physicalsoilstructureisfinallydeemedideal,thepumice-enhanced
formulationcanbediscontinued.
[3] SoilWater Retention and Maize (Zea mays L.) Growth as Effected by Different Amounts of Pumice.
Ashraf Malekian, EinollahValizadeh, Mona Dastoori, Sohaila Samadi,Vahid Bayat; Department of Agriculture, Payame Noor University, PO Box 19395-3697,Tehran, Iran
[4] Shortcomingsof Native Seeding Project Implementation;Western Chapter of IECA Newsletter, Spring 2006

3. C O N D I T I O N I N G S O I L W I T H P U M I C E | 3
OTHER CONSIDERATIONS
BLENDING. Theuniformityofthepumicesoil-amendmentgrades
makesqualitycontroleasyduringtheblendingoperation,aspumiceis
dryandflowable.
TOP DRESSING FORTURF. Asatopdressingforexistingturf
lawnsandplayingsurfaces,pumiceout-performssandforenhancingair
andwatermovementintherootzone,holdingontomoistureand
nutrients,resistingcompactionforces,andsupportingbeneficialsoil
microorganismsthatbreakdownthatchandenrichthesoil.Pumicealso
readilymixeswithcompost,peatmoss,sand,andsoilsinblended
topdressingapplications.
PUMICEVS. MANUFAC-
TURED SOIL AMENDMENTS.
Thepracticeofblendingbitsoflight,
frothyrockwithgrowingsoilsis
alreadyawell-establishedpracticein
themarketplace.Thereareseveral
productsonthemarketdesignedto
mimicthestructuralcharacterand
in-soilperformanceofpumice—
materialssuchasexpandedperlite,
expandedvermiculite,expandedclays
(likeHydrockTM)andexpandedshale
(suchasPerma-TillTM
andUteliteTM).
Notetheuseoftheword“expanded”
witheach.Theoresfortheseproductshavetoundergoanenergy-inten-
sivesuperheatingprocessthatexpandsorpuffstheoretoitsuseful,
pumice-likestate.Pumice,ontheotherhand,wassuperheatedina
volcanoandfoamedwhenejectedintotheatomsphere.
Pumice,initsnaturalminedstate,improvessoilstructurewithout
theenvironmentalimpact/fuelburnandprocessingcostsofthe
expandedproducts.Thisfactnotonlymakespumicethegreenchoice
butalsomakespumicetheeconomicalchoice—bringingitinata
quarter-to-halfthepriceofexpandedperlite,forexample.Thatyields
significantvalue,especiallyinlarge-scaleapplications.
PUMICE AND PERLITE: PHYSIOCHEMICALTWINS.
Perliteiswidelyusedinhorticulture,especiallyinpottingandgarden
soils.Itisvaluedforitscontributiontodesirablesoilstructureandwater
andnutrientretention.Buttheprocessofflash-heatingperliteoreto
expanditaddstothecostandthecarbonfootprintofperlite.
AUniversityofIllinoisstudy5 evaluatedpumiceas
aperlitesubstituteforamendingsoil.Specifically,chemicalproperties
andsurfacecharacteristicswerecomparedandprovedanalogous,with
pumiceexhibitingagreaterporesizespan.Fromthereportsummary:
“Pumiceandperlitewereshowntohavesimilarphysicochemical
propertieswhichsubsequentlytranslatedintosimilarbehaviorin
blendedsoilmixtures.Itproved
equally,ifnotevenmoreeffective
insomeways,thanperlite.
Asubsequentcompanionplant
growthstudy(notreportedherein)
furtherconfirmedthesuitabilityof
pumiceasasoilamendment.Plants
grewequallywellinpumiceand
perlitemedia.”
HESS PUMICE
PRODUCTS
Pumiceisfoundallovertheworld,but
thequality,physicalcharacteristics
andchemicalcompositionvaryfrom
deposittodeposit.TheHesspumicedepositinsoutheastIdahois
recognizedasthepurestcommercialdepositofpumiceontheplanet,
andHessrefinesandshipsthatpumiceworld-wideforuseinvarious
industrialandagriculturalprocesses.HessPumiceProductshasbeen
miningandrefiningthischoicepumicedepositsince1958andenjoysa
well-earnedreputationforgettingitwhereitneedstobeon-timeand
on-spec.
SoilRoxHorticulturalPumiceisavailablein2.5lb.bags,20lb.boxes,
supersacksorshippedinbulkviatruckortraincars.
[5] Evaluation of Pumice as a Perlite Substitute for Container Soil Physical Amendment.
Dianne A. Noland , L. Art Spomer & David J.Williams; Department of Horticulture, University of Illinois, 1201 South Dorner Drive, Urbana, 61801–4720
www.soilrox.com