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Soil Solution SamplingSoil Solution Sampling

Ralph Oborn

Precisionist

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Precision AgriculturePrecision Agriculture

Grower challengesGrower challenges• New technologies New technologies • Spatial and temporal variabilitiesSpatial and temporal variabilities• Increase labor costsIncrease labor costs• Lower profitsLower profits

• Yield and Quality bonusYield and Quality bonus

• Environmental concernsEnvironmental concerns

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Precision AgriculturePrecision Agriculture

Goal:

Just the right amount

at Just the right place

at Just the right time

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Information Information • As growers have better informationAs growers have better information

• Can make better decisionsCan make better decisions– Agronomically– Economically– Environmentally

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SoilsSoils

Healthy soil is about 50% solids, 25% water, 25% air

Bone DrySoil has no moisture. Pores are empty.

(Only in laboratory at 100° C)

Wilt PointPlant cannot remove any more water. Pores are slightly filled with water film held by surface tension

Field CapacitySoil can hold no more water. Any additional water flows with gravity.

SaturationAll soil pores are totally filled. Water puddles on surface and flows to next lower level

Available waterMost crops do best when soil moisture is between 50% and 100% of available water.

0% 50% 100%

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Soil: Vadose ZoneSoil: Vadose Zone

• Between surface and water tableBetween surface and water table

• Air, Water, Solids, OM etcAir, Water, Solids, OM etc

• Non homogeneous!! Non homogeneous!!

• 3D spatial variability3D spatial variability

• ChemicalsChemicals

• Pores Big and smallPores Big and small

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PoresPores

• MacroporesMacropores• LargerLarger• Freeways for flowFreeways for flow• FastFast• Relatively little Relatively little

interchange with interchange with solidssolids

• PoresPores• SmallerSmaller• City streets for flowCity streets for flow• SlowSlow• TortuosityTortuosity• Large amount of Large amount of

interaction interaction

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Soil SolutionSoil Solution

• QuantityQuantity• Movement (flux)Movement (flux)• ConstituentsConstituents

– Dissolved – Ions– Colloids

• Amount taken up by Amount taken up by roots is very complexroots is very complex

• That which is not That which is not used becomes used becomes problematicproblematic

• Potential to leach into Potential to leach into ground and surface ground and surface waterwater

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Soil SolutionSoil Solution

• Saturated flowSaturated flow– Macro pore– Capillary flow

• Unsaturated flowUnsaturated flow– Capillary flow

• Tightly heldTightly held– Interstitial

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Idealized Soil Water FlowIdealized Soil Water Flow

Life would be easy

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Reality Soil Water FlowReality Soil Water Flow

Variable

Quantity Convergence

Divergence

Variable

Flux RateVariable

Concentration

Obstructions

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Need for MeasurementsNeed for Measurements

• AgronomicAgronomic– Make sure crop is adequately supplied

• EconomicEconomic– Avoid waste

• EnvironmentalEnvironmental– Avoid loose contaminants

If it’s going to be used, it’s a nutrient

If not it's a contaminant

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Areas of ConcernAreas of Concern

• Coarse sandy soilsCoarse sandy soils

• NitratesNitrates

• Available to crops (my interest)Available to crops (my interest)

• Available to leach (environmental concern)Available to leach (environmental concern)

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Needed: Needed: ““Holy Grail of SamplersHoly Grail of Samplers””

• QuantityQuantity

• Flux Flux (movement)(movement)

• ConstituentsConstituents

Star Trek Tricorder

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Needed: Needed: Samplers Samplers

• Integrated areaIntegrated area– Large to be representative– Low cost– Ease of maintenance

• RepeatableRepeatable• NondestructiveNondestructive• ContinuousContinuous• Multiple levelsMultiple levels• AccurateAccurate

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Current ArtCurrent Art

• Moisture quantityMoisture quantity• TraditionTradition• Look and feelLook and feel• GravimetricGravimetric• TensionometerTensionometer• Neutron probeNeutron probe• ET matchET match• TDRTDR• CapacitanceCapacitance

• Solution samplingSolution sampling• Core extractionCore extraction• Pan lysimetersPan lysimeters• Porous cup Porous cup • WickWick

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Moisture QuantityMoisture Quantity• TraditionTradition• Look and feelLook and feel• GravimetricGravimetric• TensionometerTensionometer• Neutron ProbeNeutron Probe• ET MatchET Match• TDRTDR• CapacitanceCapacitance

– Continuous, current, multiple depth, large volume

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Capacitance ProbeCapacitance Probe

Irrigation

Penetration

Free Drainage

Daily Crop Usage

Scheduling

Control LeachingDepth of root zone

Sentek EnviroScan

Calibrate to quantity, Get an idea of flux,

no solution data

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Solution SamplingSolution Sampling

• Core extractionCore extraction• Pan lysimetersPan lysimeters• Porous cup Porous cup • WickWick

• What part of soil What part of soil solution are you solution are you measuring?measuring?

• Free waterFree water• Large poreLarge pore• Small poreSmall pore• Interstitial in clayInterstitial in clay

Placement of all samplers is

extremely critical

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Reality Soil Water FlowReality Soil Water Flow

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Soil Core – Solution ExtractionSoil Core – Solution Extraction

• Remove Soil CoreRemove Soil Core• Extract soil solutionExtract soil solution• AnalyzeAnalyze

• Fixed volume (good)Fixed volume (good)• DestructiveDestructive• Non repeatableNon repeatable• Difficult to extractDifficult to extract• What portion are What portion are

you extracting?you extracting?• Quantity - maybeQuantity - maybe• Flux – noFlux – no• Solution - maybeSolution - maybe

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Pan LysimetersPan Lysimeters

• Needs good soil Needs good soil contactcontact

• Drips - only gets Drips - only gets saturated flow (macro saturated flow (macro pore)pore)

• Divergence of Divergence of unsaturated flow unsaturated flow around sampleraround sampler

• Saturated flow can be Saturated flow can be more dilutemore dilute

• Create capillary fringeCreate capillary fringe• Unsure of sampling Unsure of sampling

volumevolume

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Porous CupPorous Cup

• Ceramic interfaceCeramic interface• Similar to soilSimilar to soil

– Hydraulically

• Vacuum applied to Vacuum applied to extract solutionextract solution– Continuous– Intermittent

• Saturated and Saturated and unsaturated flowunsaturated flow

• Gradient of suctionGradient of suction1904 “Artificial Root”

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Porous Cup DiversionsPorous Cup Diversions

• Can divert Can divert streamflows streamflows

• Uncertain sampling Uncertain sampling volumevolume

• Ineffective for clayIneffective for clay

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Porous Cup DiversionsPorous Cup Diversions

• Intermittent Intermittent sampling may not sampling may not match intermittent match intermittent flowflow

• May miss flux frontMay miss flux front

• May miss solution May miss solution frontfront

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Porous Cup DiversionsPorous Cup Diversions

• Too much Too much suction suction removes removes nearby, nearby, tightly held, tightly held, high high concentration concentration waterwater

• Wilt pointWilt point

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In a NutshellIn a Nutshell

“One cannot be sure from what macroscopic volume

of soil the sample was extracted nor from which

pores it was drained”

England

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Porous Cup ConclusionsPorous Cup Conclusions

• Quantity - noQuantity - no

• Flux – noFlux – no

• Solution - maybeSolution - maybe

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Wick SamplerWick Sampler

• Hanging water columnHanging water column• Wick designed to match Wick designed to match

soil suctionsoil suction• Continuous samplingContinuous sampling• No distortion of No distortion of

streamlinesstreamlines• Only samples available Only samples available

waterwater• Relatively easy to install, Relatively easy to install,

maintain, use, samplemaintain, use, sample• No continuous powerNo continuous power

Brown 1986

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WickWick

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WicksWicks

• Wicks must be preparedWicks must be prepared

• Heat to 400°CHeat to 400°C

• Splay and secure on collector plateSplay and secure on collector plate

• Must be held tightly to soilMust be held tightly to soil

• Measure collected volumeMeasure collected volume

• Capture solution for analysisCapture solution for analysis

• Doesn’t sorb or slow down collectionDoesn’t sorb or slow down collection

• Large integrated sampling areaLarge integrated sampling area

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Wick SizingWick Sizing

Number of Wicks

=Ksat soil x Plate Area

Ksat wick x Wick Area

K Sat Soil ~ 2.54 cm/hr

Ksat Wick ~ 36 cm/hr

Wick area ~ 1.2 cm2

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Wick ResearchWick Research

• Flux ratesFlux rates

• Sorption propertiesSorption properties

• Installation methodsInstallation methods

• Sampling methodsSampling methods

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Wick ConclusionWick Conclusion

• Quantity – YesQuantity – Yes

• Flux – YesFlux – Yes

• Constitutes _ YesConstitutes _ Yes

• Becoming “just” a toolBecoming “just” a tool

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ConclusionConclusion

““A large cross section together with a A large cross section together with a low extraction rate … can yield a low extraction rate … can yield a sample large enough for chemical sample large enough for chemical analysis”analysis”

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For More InformationFor More Information• Knutson, J. H. and J. S. Selker. 1996. Fiberglass wick sampler effects on measurements of solute Knutson, J. H. and J. S. Selker. 1996. Fiberglass wick sampler effects on measurements of solute

transport in the vadose zone. transport in the vadose zone. Soil Science Society of America JournalSoil Science Society of America Journal 60: 420-424.  60: 420-424. • Zhu, Y, R. H. Fox, and J. D. Toth. 2002. Leachate Collection Efficiency of Zero-tension Pan and Passive Zhu, Y, R. H. Fox, and J. D. Toth. 2002. Leachate Collection Efficiency of Zero-tension Pan and Passive

Capillary Fiberglass Wick Lysimeters. Capillary Fiberglass Wick Lysimeters. Soil Science Society of America JournalSoil Science Society of America Journal 66:37-43. 66:37-43.• Rimmer, Alon, Tammo S. Steenhuis, and John S. Selker . 1995. One Dimensional Model to Evaluate the Rimmer, Alon, Tammo S. Steenhuis, and John S. Selker . 1995. One Dimensional Model to Evaluate the

Performance of Wick Samplers in Soils. Performance of Wick Samplers in Soils. Soil Science Society of America JournalSoil Science Society of America Journal 59:88-92. 59:88-92.• Goyne, Keith W., Rick L. Day, and Jon Chorover. 2000. Artifacts caused by collection of soil Solution with Goyne, Keith W., Rick L. Day, and Jon Chorover. 2000. Artifacts caused by collection of soil Solution with

Passive Capillary Samplers.Passive Capillary Samplers. Soil Science Society of America Soil Science Society of America Journal Journal 64:1330-1336. 64:1330-1336.• Brandi-Dohrn, Florian, Richard P. Dick, Mario Hess, John S. Selker. 1996. Suction Cup Sampler Bias in Brandi-Dohrn, Florian, Richard P. Dick, Mario Hess, John S. Selker. 1996. Suction Cup Sampler Bias in

Leaching Characterization of and Undisturbed Field Soil. Leaching Characterization of and Undisturbed Field Soil. Water Resources ResearchWater Resources Research. 32:1173-1182.. 32:1173-1182.• Barbee, G. C., and K. W. Brown. 1986. Comparison Between Suction and Free Drainage Soil Solution Barbee, G. C., and K. W. Brown. 1986. Comparison Between Suction and Free Drainage Soil Solution

Samplers. Samplers. Soil Science.Soil Science. 141:149-154. 141:149-154.• Wood, Warren W. 1973. A Technique Using Porous Cups for Water Sampling at Any Depth in the Wood, Warren W. 1973. A Technique Using Porous Cups for Water Sampling at Any Depth in the

Unsaturated Zone. Unsaturated Zone. Water Resources ResearchWater Resources Research. 9(2):486-488. . 9(2):486-488. • England, C. B., Comments on ‘A Technique Using Porous Cups for Water Sampling at Any Depth in the England, C. B., Comments on ‘A Technique Using Porous Cups for Water Sampling at Any Depth in the

Unsaturated Zone’ by Warren Wood. 1974. Unsaturated Zone’ by Warren Wood. 1974. Water Resources Research. Water Resources Research. 10(5):1049.10(5):1049.• Boll, J., J. S. Selker, B. M. Nijssen, T. S. Steenhuis, J. Van Winkle. and E. Jolles. Water Quality Sampling Boll, J., J. S. Selker, B. M. Nijssen, T. S. Steenhuis, J. Van Winkle. and E. Jolles. Water Quality Sampling

Under Preferential Flow Conditions. Under Preferential Flow Conditions. InIn p290-298. R. G. Allen et al. (ed.) Lysimeters for Evapotranspiration p290-298. R. G. Allen et al. (ed.) Lysimeters for Evapotranspiration and Environmental Measurements. Procedings ASCE International Symposium. Lysimetry, Honolulu, and Environmental Measurements. Procedings ASCE International Symposium. Lysimetry, Honolulu, Hawaii. 23-25 July 1991. ASCE, New York.Hawaii. 23-25 July 1991. ASCE, New York.

• Poletika, N. N., Roth, K., and W. A. Jury. 1992. Interpretation of solute transport data obtained with Poletika, N. N., Roth, K., and W. A. Jury. 1992. Interpretation of solute transport data obtained with fiberglass wick soil solution samplers. fiberglass wick soil solution samplers. Soil Science Society of America JournalSoil Science Society of America Journal 56: 1751-1753. 56: 1751-1753.

• Boll, J., T. S. Steenhuis, and J. S. Selker, 1992. Fiberglass Wicks for Sampling of Water and Solutes in the Boll, J., T. S. Steenhuis, and J. S. Selker, 1992. Fiberglass Wicks for Sampling of Water and Solutes in the Vadose Zone. Vadose Zone. Soil Science Society of America JournalSoil Science Society of America Journal 56:701-707. 56:701-707.

• Knutson, John H., and John S. Selker. 1994. Unsaturated Hydraulic Conductivities of Fiberglass Wicks and Knutson, John H., and John S. Selker. 1994. Unsaturated Hydraulic Conductivities of Fiberglass Wicks and Designing Capillary Wick Pore-Water Samplers. 1994. Designing Capillary Wick Pore-Water Samplers. 1994. Soil Science Society of America JournalSoil Science Society of America Journal . 58:721-. 58:721-729.729.