Soil Solution Sampling

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Soil Solution Sampling
Ralph Oborn Precisionist
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Precision Agriculture

• Grower challenges
• New technologies
• Spatial and temporal variabilities
• Increase labor costs
• Lower profits
• Yield and Quality bonus
• Environmental concerns

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Precision Agriculture
Goal Just the right amount at Just the right
place at Just the right time
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Information

• As growers have better information
• Can make better decisions
• Agronomically
• Economically
• Environmentally

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Soils
Healthy soil is about 50 solids, 25 water, 25
air
Wilt Point Plant cannot remove any more water.
Pores are slightly filled with water film held by
surface tension
Bone Dry Soil has no moisture. Pores are empty.
(Only in laboratory at 100 C)
Available water Most crops do best when soil
moisture is between 50 and 100 of available
water.
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Soil Vadose Zone

• Between surface and water table
• Air, Water, Solids, OM etc
• Non homogeneous!!
• 3D spatial variability
• Chemicals
• Pores Big and small

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Pores

• Macropores
• Larger
• Freeways for flow
• Fast
• Relatively little interchange with solids

• Pores
• Smaller
• City streets for flow
• Slow
• Tortuosity
• Large amount of interaction

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Soil Solution

• Amount taken up by roots is very complex
• That which is not used becomes problematic
• Potential to leach into ground and surface water

• Quantity
• Movement (flux)
• Constituents
• Dissolved
• Ions
• Colloids

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Soil Solution

• Saturated flow
• Macro pore
• Capillary flow
• Unsaturated flow
• Capillary flow
• Tightly held
• Interstitial

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Idealized Soil Water Flow
Life would be easy
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Reality Soil Water Flow
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Need for Measurements

• Agronomic
• Make sure crop is adequately supplied
• Economic
• Avoid waste
• Environmental
• Avoid loose contaminants

If its going to be used, its a nutrient If not
it's a contaminant
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Areas of Concern

• Coarse sandy soils
• Nitrates
• Available to crops (my interest)
• Available to leach (environmental concern)

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Needed Holy Grail of Samplers

• Quantity
• Flux (movement)
• Constituents

Star Trek Tricorder
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Needed Samplers

• Integrated area
• Large to be representative
• Low cost
• Ease of maintenance
• Repeatable
• Nondestructive
• Continuous
• Multiple levels
• Accurate

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Current Art

• Moisture quantity
• Tradition
• Look and feel
• Gravimetric
• Tensionometer
• Neutron probe
• ET match
• TDR
• Capacitance

• Solution sampling
• Core extraction
• Pan lysimeters
• Porous cup
• Wick

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Moisture Quantity

• Tradition
• Look and feel
• Gravimetric
• Tensionometer
• Neutron Probe
• ET Match
• TDR
• Capacitance
• Continuous, current, multiple depth, large volume

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Capacitance Probe
Calibrate to quantity, Get an idea of flux, no
solution data
Sentek EnviroScan
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Solution Sampling

• Core extraction
• Pan lysimeters
• Porous cup
• Wick

• What part of soil solution are you measuring?
• Free water
• Large pore
• Small pore
• Interstitial in clay

Placement of all samplers is extremely critical
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Reality Soil Water Flow
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Soil Core Solution Extraction

• Remove Soil Core
• Extract soil solution
• Analyze

• Fixed volume (good)
• Destructive
• Non repeatable
• Difficult to extract
• What portion are you extracting?
• Quantity - maybe
• Flux no
• Solution - maybe

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Pan Lysimeters

• Needs good soil contact
• Drips - only gets saturated flow (macro pore)
• Divergence of unsaturated flow around sampler
• Saturated flow can be more dilute
• Create capillary fringe
• Unsure of sampling volume

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Porous Cup

• Ceramic interface
• Similar to soil
• Hydraulically
• Vacuum applied to extract solution
• Continuous
• Intermittent
• Saturated and unsaturated flow
• Gradient of suction

1904 Artificial Root
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Porous Cup Diversions

• Can divert streamflows
• Uncertain sampling volume
• Ineffective for clay

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Porous Cup Diversions

• Intermittent sampling may not match intermittent
  flow
• May miss flux front
• May miss solution front

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Porous Cup Diversions

• Too much suction removes nearby, tightly held,
  high concentration water
• Wilt point

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In 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 Conclusions

• Quantity - no
• Flux no
• Solution - maybe

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Wick Sampler

• Hanging water column
• Wick designed to match soil suction
• Continuous sampling
• No distortion of streamlines
• Only samples available water
• Relatively easy to install, maintain, use, sample
• No continuous power

Brown 1986
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Wick
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Wicks

• Wicks must be prepared
• Heat to 400C
• Splay and secure on collector plate
• Must be held tightly to soil
• Measure collected volume
• Capture solution for analysis
• Doesnt sorb or slow down collection
• Large integrated sampling area

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Wick Sizing
K Sat Soil 2.54 cm/hr Ksat Wick 36
cm/hr Wick area 1.2 cm2
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Wick Research

• Flux rates
• Sorption properties
• Installation methods
• Sampling methods

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Wick Conclusion

• Quantity Yes
• Flux Yes
• Constitutes _ Yes
• Becoming just a tool

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Conclusion

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

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For More Information

• Knutson, J. H. and J. S. Selker. 1996.
  Fiberglass wick sampler effects on measurements
  of solute transport in the vadose zone. Soil
  Science Society of America Journal 60 420-424. 
• Zhu, Y, R. H. Fox, and J. D. Toth. 2002. Leachate
  Collection Efficiency of Zero-tension Pan and
  Passive Capillary Fiberglass Wick Lysimeters.
  Soil Science Society of America Journal 6637-43.
• Rimmer, Alon, Tammo S. Steenhuis, and John S.
  Selker . 1995. One Dimensional Model to Evaluate
  the Performance of Wick Samplers in Soils. Soil
  Science Society of America Journal 5988-92.
• Goyne, Keith W., Rick L. Day, and Jon Chorover.
  2000. Artifacts caused by collection of soil
  Solution with Passive Capillary Samplers. Soil
  Science Society of America Journal
  641330-1336.
• Brandi-Dohrn, Florian, Richard P. Dick, Mario
  Hess, John S. Selker. 1996. Suction Cup Sampler
  Bias in Leaching Characterization of and
  Undisturbed Field Soil. Water Resources Research.
  321173-1182.
• Barbee, G. C., and K. W. Brown. 1986. Comparison
  Between Suction and Free Drainage Soil Solution
  Samplers. Soil Science. 141149-154.
• Wood, Warren W. 1973. A Technique Using Porous
  Cups for Water Sampling at Any Depth in the
  Unsaturated Zone. Water Resources Research.
  9(2)486-488.
• England, C. B., Comments on A Technique Using
  Porous Cups for Water Sampling at Any Depth in
  the Unsaturated Zone by Warren Wood. 1974. Water
  Resources Research. 10(5)1049.
• Boll, J., J. S. Selker, B. M. Nijssen, T. S.
  Steenhuis, J. Van Winkle. and E. Jolles. Water
  Quality Sampling Under Preferential Flow
  Conditions. In p290-298. R. G. Allen et al. (ed.)
  Lysimeters for Evapotranspiration and
  Environmental Measurements. Procedings ASCE
  International Symposium. Lysimetry, Honolulu,
  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 fiberglass wick soil solution samplers.
  Soil Science Society of America Journal 56
  1751-1753.
• Boll, J., T. S. Steenhuis, and J. S. Selker,
  1992. Fiberglass Wicks for Sampling of Water and
  Solutes in the Vadose Zone. Soil Science Society
  of America Journal 56701-707.
• Knutson, John H., and John S. Selker. 1994.
  Unsaturated Hydraulic Conductivities of
  Fiberglass Wicks and Designing Capillary Wick
  Pore-Water Samplers. 1994. Soil Science Society
  of America Journal. 58721-729.