Soil Solution Sampling Ralph Oborn Precisionist 1 Precision Agriculture Grower challenges New technologies Spatial and temporal variabilities Increase labor costs Lower profits
Yield and Quality bonus Environmental concerns 2 Precision Agriculture Goal: Just the right amount at Just the right place at Just the right time 3 Information As growers have better information Can make better decisions Agronomically Economically
Environmentally 4 Soils Healthy soil is about 50% solids, 25% water, 25% air 0% Bone Dry Wilt Point Plant cannot remove any more water. Pores are slightly filled with water film
(Only in laboratory at held by surface tension 100 C) Soil has no moisture. Pores are empty. 50% 100% Available water Most crops do best when soil
moisture is between 50% and 100% of available water. Field Capacity Saturation All soil pores Soil can hold no are totally filled. Water more water. puddles on Any additional water flows with surface and
flows to next gravity. lower level 5 Soil: Vadose Zone Between surface and water table Air, Water, Solids, OM etc
Non homogeneous!! 3D spatial variability Chemicals Pores Big and small 6 Pores Macropores Larger Freeways for flow Fast
Relatively little interchange with solids Pores Smaller City streets for flow Slow Tortuosity Large amount of
interaction 7 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 8 Soil Solution Saturated flow Macro pore Capillary flow Unsaturated flow Capillary flow Tightly held Interstitial 9
Idealized Soil Water Flow Life would be easy 10 Reality Soil Water Flow Obstructions Variable Flux Rate Convergence Divergence Variable Concentration
Variable Quantity 11 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
12 Areas of Concern Coarse sandy soils Nitrates Available to crops (my interest) Available to leach (environmental concern) 13 Needed: Holy Grail of Samplers
Quantity Flux (movement) Constituents Star Trek Tricorder 14 Needed: Samplers Integrated area Large to be representative Low cost Ease of maintenance
Repeatable Nondestructive Continuous Multiple levels Accurate 15 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 16 Moisture Quantity
Tradition Look and feel Gravimetric Tensionometer Neutron Probe ET Match TDR Capacitance Continuous, current, multiple depth, large volume 17 Capacitance Probe
Free Drainage Irrigation Scheduling Daily Crop Usage Depth of root zone Calibrate to quantity, Get an idea of flux, no solution data Penetration
Control Leaching Sentek EnviroScan 18 Solution Sampling Core extraction Pan lysimeters Porous cup Wick
Placement of all samplers is extremely critical What part of soil solution are you measuring? Free water Large pore Small pore Interstitial in clay 19 Reality Soil Water Flow 20
Destructive Non repeatable Difficult to extract What portion are you extracting? Quantity - maybe Flux no Solution - maybe 21 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 22 Porous Cup Ceramic interface Similar to soil Hydraulically Vacuum applied to extract solution Continuous
Intermittent Saturated and unsaturated flow 1904 Artificial Root Gradient of suction 23 Porous Cup Diversions Can divert streamflows Uncertain sampling volume Ineffective for clay
24 Porous Cup Diversions Intermittent sampling may not match intermittent flow May miss flux front May miss solution front 25 Porous Cup Diversions Too much suction removes nearby,
tightly held, high concentration water Wilt point 26 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 27 Porous Cup Conclusions
Quantity - no Flux no Solution - maybe 28 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 29 Wick 30 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 31 Wick 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 32 Wick Research
Constitutes _ Yes Becoming just a tool 34 Conclusion A large cross section together with a low extraction rate can yield a sample large enough for chemical analysis 35 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 America Journal 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. 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. 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. Journal. 58:721-729. 58:721-729. 36
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