Soil Solution Sampling

1. Soil Solution Sampling Ralph Oborn Precisionist

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

3. Precision Agriculture Goal: Just the right amount at Just the right place at Just the right time

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

5. Soils 0% 50% 100% Saturation All soil pores are totally filled. Water puddles on surface and flows to next lower level Field Capacity Soil can hold no more water. Any additional water flows with gravity. 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.

6. Soil: Vadose Zone • Between surface and water table • Air, Water, Solids, OM etc • Non homogeneous!! • 3D spatial variability • Chemicals • Pores Big and small

7. Pores Macropores Larger Freeways for flow Fast Relatively little interchange with solids Pores Smaller City streets for flow Slow Tortuosity Large amount of interaction

8. Soil Solution Quantity Movement (flux) Constituents Dissolved Ions Colloids Amount taken up by roots is very complex That which is not used becomes problematic Potential to leach into ground and surface water

9. Soil Solution • Saturated flow • Macro pore • Capillary flow • Unsaturated flow • Capillary flow • Tightly held • Interstitial

10. Idealized Soil Water Flow Life would be easy

11. Reality Soil Water Flow Obstructions Variable Flux Rate Variable Concentration Variable Quantity Convergence Divergence

12. Need for Measurements • Agronomic • Make sure crop is adequately supplied • Economic • Avoid waste • Environmental • Avoid loose contaminants If it’s going to be used, it’s a nutrient If not it's a contaminant

13. Areas of Concern • Coarse sandy soils • Nitrates • Available to crops (my interest) • Available to leach (environmental concern)

14. Needed: “Holy Grail of Samplers” Quantity Flux (movement) Constituents Star Trek Tricorder

15. Needed: Samplers Integrated area Large to be representative Low cost Ease of maintenance Repeatable Nondestructive Continuous Multiple levels Accurate

16. 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

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

18. Capacitance Probe Free Drainage Irrigation Scheduling Daily Crop Usage Penetration Depth of root zone Control Leaching Calibrate to quantity, Get an idea of flux, no solution data Sentek EnviroScan

19. 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

20. Reality Soil Water Flow

21. 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

22. 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

23. 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”

24. Porous Cup Diversions • Can divert streamflows • Uncertain sampling volume • Ineffective for clay

25. Porous Cup Diversions • Intermittent sampling may not match intermittent flow • May miss flux front • May miss solution front

26. Porous Cup Diversions • Too much suction removes nearby, tightly held, high concentration water • Wilt point

27. 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

28. Porous Cup Conclusions • Quantity - no • Flux – no • Solution - maybe

29. 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

30. Wick

31. Wicks • Wicks must be prepared • Heat to 400°C • Splay and secure on collector plate • Must be held tightly to soil • Measure collected volume • Capture solution for analysis • Doesn’t sorb or slow down collection • Large integrated sampling area

32. Wick Sizing Ksatsoil x Plate Area Number of Wicks = Ksatwick x Wick Area K Sat Soil ~ 2.54 cm/hr KsatWick ~ 36 cm/hr Wick area ~ 1.2 cm2

33. Wick Research • Flux rates • Sorption properties • Installation methods • Sampling methods

34. Wick Conclusion • Quantity – Yes • Flux – Yes • Constitutes _ Yes • Becoming “just” a tool

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

36. 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 66:37-43. • 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 59:88-92. • 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 AmericaJournal 64:1330-1336. • 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. 32:1173-1182. • Barbee, G. C., and K. W. Brown. 1986. Comparison Between Suction and Free Drainage Soil Solution Samplers. Soil Science. 141:149-154. • 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 56:701-707. • 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. 58:721-729.