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Understanding Soil Moisture
- 2. What is the term for the moisture status of a soil when its pores are 100% full of water? Saturation
- 3. Why do large soil pores (aka macropores) normally drain within a few days? Large soil pores are drained by GRAVITY
- 4. Why doesn’t gravity drain all the water out of soil pores ? Field capacity Wilting point When water is no longer When plants have extracted drained by gravity as much water as they can Capillarity and surface attraction combine to pull more strongly than gravity on: 1) water in “micropores” and 2) water close to the “soil skin”.
- 5. Why doesn’t gravity drain all the water out of soil pores ? Field capacity Wilting point When water is no longer When plants have extracted drained by gravity as much water as they can Some water is held too tightly to be pulled away by roots
- 6. Pull of the soil matrix on H2O surface attraction + capillarity cohesion H + Soil Skin O adhesion H H + pull H2O into small ? O H pores H O H Hydrogen bonding
- 7. Pull of the soil matrix on H2O surface attraction + capillarity cohesion H + Soil Skin O adhesion H H + pull H2O into small ? O Water is pulled into H pores H the micropores and O toward the soil skin H by matric forces Hydrogen bonding
- 8. What do I mean by “soil skin”? Brady and Weil (2002) http://www.ccma.csic.es/dpts/suelos/ clay minerals humus
- 9. Soil circulatory system Unavailable water ~0.2 μm Wilting point available less Plant available water Most available Field Capacity 10-30 μm Gravitational water in drainage pores model soil pore Saturation Adapted from Buol (2000)
- 10. Soil circulatory system Unavailable water ~0.2 μm Wilting point available less Plant available water Most available Field Capacity model soil pore Adapted from Buol (2000)
- 11. Soil circulatory system Unavailable water ~0.2 μm Wilting point model soil pore Adapted from Buol (2000)
- 12. high energy H2O = molecules bouncing around low energy H2O = molecules moving slowly Soil skin Unavailable water Thickness of water film Low energy H2O high energy H2O
- 13. high energy H2O = molecules bouncing around low energy H2O = molecules moving slowly Soil skin Unavailable water Thickness of water film Low energy H2O high energy H2O
- 14. high energy H2O = molecules bouncing around low energy H2O = molecules moving slowly Soil skin Unavailable water Thickness of water film Low energy H2O high energy H2O
- 15. high energy H2O = molecules bouncing around low energy H2O = molecules moving slowly Soil skin There is still some water in air dry soils! Thickness of water film Low energy H2O high energy H2O
- 16. Mars Lander probe finds no water in Martian soils A conductivity probe on the Mars Lander sensed rising and falling humidity levels in the Martian atmosphere, but when stuck into the ground, the probe found “Martian soil” to be completely and perplexingly dry. On Earth, “if you have water vapor in the air, every surface exposed to that air will have water molecules adhering to it that are somewhat mobile, even at temperatures well below freezing," said Aaron Zent , lead scientist for the Lander’s conductivity probe.
- 17. Soil water tension (aka potential) can be visualized as the suction created by a hanging column of water ~1m Field capacity Wilting point Air-dry All of the following are equivalent: 150 m 1 m of H2O There cm of many are water 10,000 m 100 other methods 75 mm of mercury -10 kPa of expressing -0.01 MPa soil water -0.1 bars -0.0987 atmospheres tension -1.45 PSI -1500 kPa -100,000 kPa -15 bars -1000 bars
- 18. Soil water tension (aka potential) can be visualized as the suction created by a hanging column of water ~1m Field capacity Wilting point Air-dry All of the following are equivalent: 150 m 1 m of H2O There cm of many are water 10,000 m 100 other methods 75 mm of mercury -10 kPa You should be of expressing with -0.01 MPa familiar -0.1 bars these units soil water -0.0987 atmospheres tension -1.45 PSI -1500 kPa -100,000 kPa -15 bars -1000 bars
- 19. Saturation Are all of the water molecules in this pore under the same tension ?
- 21. Field Capacity
- 24. Understanding soil water tension Ψtotal = Ψ gravitational + Ψmatric + Ψosmotic Pull of gravity Pull by micropores and soil skin ?
- 25. Understanding osmotic tension Salt added ?
- 26. Understanding osmotic tension Salt added
- 27. What causes fertilizer burn? Osmotic tension
- 28. The same phenomena that causes “dishwashing hands” Osmotic tension
- 29. How do water molecules get from the soil to the top of a plant?
- 30. H20 H20 H20 H20 Continuous H20 chains of water H20 The chain molecules H20 moves upward move upward if there is a H20 through the negative energy H20 xylem gradient H20 H20 H20 H20 H20 H20 H20 H20 H20 H20
- 31. H20 H20 H20 H20 Continuous H20 chains of water Solar energy H20 molecules drives H20 move upward transpiration H20 through the xylem H20 Plants provide H20 the conduit H20 H20 H20 H20 H20 H20 H20 H20 H20
- 32. Transpiration = air conditioning for plants ~ 4000 gallons H2O per acre on a hot sunny day ~ 30 gallons H2O per corn plant per season
- 33. The tallest living tree is a coast redwood that stands 112 meters (367 feet, 6 in.), or ~ five stories higher than the Statue of Liberty. Why don’t trees grow any taller ? Hydrogen bonding is only strong enough to hold together ~ 400’ of water molecules Cohesion theory
- 34. Soil water is a switch that activates and deactivates soil biology Water is biologically available, when soil organisms are able to win the “tug of war” with the soil
- 35. Up to this point, we have been discussing water tension What is meant by the term water content?
- 36. Determining gravimetric soil moisture content Collect sample. Weigh moist. Weigh after oven drying. g.m.c. = (moist – dry soil mass) / dry soil mass
- 37. Water content can also be expressed volumetrically v.m.c. = volume of water in soil / total soil volume
- 38. Why would you want to do this conversion? Converting from gravimetric to volumetric MC Volumetric Gravimetric moisture moisture Density content content Bulk density of H2O volume mass of H2O mass of dry soil volume of H2O = of H2O mass of dry soil * volume of dry * mass of H2O volume soil of dry soil Gravimetric MC is easier to measure but volumetric MC is more useful inappropriate for expansive soils for managing irrigation
- 39. Translating between water tension (aka potential) and water content using a “characteristic curve” A characteristic curve (aka water release curve) describes the relationship between water tension and water content for a specific soil. 0
- 40. A pressure plate system can be used to bring soil to specific water tensions Why are all those bolts needed? A known positive pressure is applied inside the chamber. Soil water is pushed out through a porous ceramic plate.
- 41. Different soils have different characteristic curves Field capacity Wilting point Brady and Weil, 2002
- 42. Different soils have different characteristic curves Field capacity Wilting point 0.09 – 0.02 = 7% Brady and Weil, 2002
- 43. Different soils have different characteristic curves Field capacity Wilting point 34% - 8% = 26% Brady and Weil, 2002
- 44. Different soils have different characteristic curves Field capacity Wilting point 54% - 24% = 30% Brady and Weil, 2002
- 45. Use the diagram to interpret how much water is held in the clay @ saturation, FC and WP. Calculate how many inches of water are needed to bring a 3’ rooting zone of this soil from 50% of FC to FC. The volumetric water content @ FC = 0.54 0.54 * 36” = 19.4” of water @ FC 50% of 19.4” = 9.7”
- 46. Real soils rarely hold more than 2.5” of plant available water per foot… based on this fact, do you think the characteristic curve for the clay soil is realistic? The volumetric water content @ FC = 0.54 The volumetric water content @ WP = 0.24 PAW = 0.30 0.3* 12” = 3.6” >> 2.5”
- 47. So how does compaction impact soil water relationships ?
- 48. So how does compaction impact soil water relationships ? Loss of drainage pores Gain in small pores
- 49. Which soil texture can hold the most plant available water? Field capacity line ~ 2.5” of plant Plant available water Available (PAW) per foot water Wilting point line Brady and Weil, 2002
- 50. How does SOM affect PAW? Adapted from Brady and Weil
- 51. How does SOM affect PAW? Adapted from Brady and Weil
- 52. Prairie soil Farm field Impressive example of the impact of soil organic matter on water holding capacity
- 53. So when should you irrigate a clay soil? Wimpy crops Tough crops
- 54. So when should you irrigate a clay soil? loam soil? Wimpy crops Tough crops
- 55. Brady and Weil, 2002 So how does one measure soil water tension in the field? A tensiometer is a Tensiometers are water filled tube useful for montioring with a porous tensions between ceramic tip on one 0 and -85 kPa (-0.85 bars) end and a vacuum a range that includes gauge on the other. about half the water in most soils. Water tension in the tube equilibrates When soils are too dry with the water (> -85 KPa), air is drawn tension outside the in through the porous tip porous tip. and the vacuum fails.
- 56. Gypsum block Measuring soil moisture as a function of electrical resistance Brady and Weil, 2002
- 57. Gypsum block Measuring soil moisture as a function of electrical resistance Resistance drops as gypsum starts to Calibration is dissolve critical !! Brady and Weil, 2002
- 58. What is this gizmo?
- 59. What is this gizmo?
- 60. Time Domain Reflectometry The technique involves determination of the propagation velocity of an electromagnetic pulse sent down a fork-like probe installed in the soil. The velocity is determined by measuring the time taken for the pulse to travel down the probe and be reflected back from its end. The propagation velocity depends on the dielectric constant of the material in contact with the probe (i.e. the soil). Water has a much higher dielectric constant than soil.
- 63. Why do the wetting fronts have different shapes? Capillarity pulls the water farther in finer textured soils http://www.ext.colostate.edu/mg/gardennotes/images/213-7.jpg
- 64. Capillary rise in a sandy soil http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf
- 65. Capillary rise in a silt loam http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf
- 66. What happens when capillary rise lifts water to the soil surface? http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf
- 68. How fast does water move through soil ? Hydraulic conductivity Darcy’s Law Flow rate = Area*Ksat *pressure/length Brady and Weil, 2002
- 70. Hydraulic conductivity = permeability Flow rate ~ pore radius4
- 72. How does the presence of a coarse textured layer under a fine textured layer affect percolation ? Fine textured layer Coarse textured layer
- 73. Water will not enter the coarse textured layer until the upper layer is near Coarse textured layer saturation After water enters the coarse textured layer, it will percolate more quickly. http://www.personal.psu.edu/asm4/water/drain.html
- 74. Does a layer of sandy soil improve Layer with sandy texture drainage ? NO ! Layer with sandy texture
- 75. Soil suitability for septic drainfields
- 76. What happens if a septic drainfield does not drain adequately? Can a drainfield drain too well? http://organicearthsolutions.wordpress.com/2012/02/16/
- 77. Impact of topography on drainage Poorly drained Interstream divide Somewhat Moderately poorly LANDSCAPE well drained drained POSITIONS Well drained Poorly drained Shoulder Common in IL Valley floor SOIL Backslope DRAINAGE CLASSES N.C. Agric. Res. Bull. 467
- 78. Illinois’ natural drainage classes http://www.il.nrcs.usda.gov/technical/soils/Suite_Maps.html
- 79. What is a hydric soil? http://www.il.nrcs.usda.gov/technical/soils/Suite_Maps.html A hydric soil is a soil that formed under conditions of saturation, flooding or ponding long enough during the warm season to develop anaerobic conditions in the upper horizons. Soils in which the hydrology has been artificially modified are still considered hydric if the soil, in an unaltered state, was hydric.
- 81. Hydric soils are dominated by low chroma colors
- 82. http://www.wtamu.edu/~crobinson/soils/clayskn05s.jpg Mottles are indicative of a fluctuating water table.
- 83. Some hydric soils in McDonough Cty
- 84. Artificial drainage in the United States % of land drained http://www.ars.usda.gov/SP2UserFiles/Place/36251500/TheExtentofFarmDrainageintheUnitedStates.pdf
- 85. IL has experienced some very wet springs in recent years ?
- 86. Yield maps have made drainage problems more obvious
- 87. Could this story be about your farm? Increasing yield by installing drainage By Mindy Ward, Missouri Farmer Today BOONVILLE --- For more than 100 years, the Hoff family has fought to farm wet areas of their fields. For Eddie Hoff, the fourth generation to farm the creek bottom ground in Cooper County, the loss of yield and added expense of working the ground was ultimately affecting his bottom line. “We were losing 60 to 70 bushels per acre in some spots,” he says. We were working the ground over and over. I just wanted to no-till and save some cost.” So, he decided to drain the soils with pattern tile.
- 88. Pattern Tiling in Ontario http://www.omafra.gov.on.ca/english/engineer/facts/10-091.htm
- 91. Installing corrugated plastic tile with a tile plow http://www.fastline.com/flimages/internet/032/169/3959312_4.jpg
- 92. Why do crops on tiled-drained land tend to be more drought resistant ? Ontario Ministry of Ag and Food
- 93. The current guide reflects recent developments in drainage science and technology. Most of these are related to new equipment and materials, widespread use of computers, and ? water quality considerations. It includes information not in the previous edition on pipeline crossings, water and sediment control basins, drain fields for septic systems, design of drainage water management systems, and design charts for smooth-walled pipes.
- 94. Conservation Drainage Maximum conveyance Crop productivity Environmental quality Controlled drainage system Bioreactor filled with woodchips http://wrc.umn.edu/prod/groups/cfans/@pub/@cfans/@wrc/documents/asset/cfans_asset_212844.jpg
- 95. Artificial drainage has greatly increased the number of days when soils in the Upper Midwest are suitable for field operations and deep root growth but has also contributed Pollution of to some water resources environmental Loss of SOM problems
- 96. Which is worse?? Compaction Saturated soil is probably extends less compressible several feet deep than wet soil
- 97. What is the optimum soil moisture for compacting soil? Soils are most compactible near field capacity because the particles are well lubricated and the large pores are empty and most collapsible
- 98. Soil resistance to penetration is very related to soil moisture content. Healthy crops tend to use more water which can result in higher penetrometer readings.
- 99. Understanding Heat Capacity A heat capacity of water = 1 calorie / gram / degree C B How much will the temperature of the water increase in cup A if 300 calories of thermal energy are added? How about cup B?
- 100. Why does soil heat up faster than water ? The heat capacity of water is ~ 5 times higher than the heat capacity of dry soil. As a result, moist soils heat up and cool down more slowly than dry soils.
- 101. Water has a high thermal conductivity Air has a low thermal conductivity What can be done to maximize geothermal heat transfer ?