Why doesn’t gravity pull all the water out of soil ?

2. Why doesn’t gravity pull all the water out of soil ? Field capacity Wilting point When plants have extracted as much water as they can When water is no longer drained by gravity Capillarity and surface interactions combine to pull more strongly than gravity on the water in micropores and the water close to the surfaces of soil particles.

3. Matric forces surface interactions + capillarity cohesion + adhesion pull H2O into small pores = H2O H O + H Soil Skin Water is pulled into the micropores and toward the soil skin by matric forces Hydrogen bonding

4. Soil circulatory system Field Capacity Saturation Drainage pores Most available 10-30 μm Wilting point Plant available water less available ~0.2 μm Unavailable water Adapted from Buol (2000)

5. Soil skin Unavailable water Distance from soil skin Low energy H2O high energy H2O

6. Relationship between water film thickness and moisture tension Adhesive water Air dry Soil feels wet Plant available water Gravitational water 0 0 Low energy H2O high energy H2O http://piru.alexandria.ucsb.edu/~geog3/concept_illus/279_sc.jpg

7. Soil water tension (aka potential) can be visualized as the suction from a hanging column of water 1-3.3 m 150 m 10,000 m Field capacity Wilting point Air-dry All of the following are equivalent: -1 m of H2O -100 cm of water -75 mm of mercury -10 kPa -0.01 MPa -0.1 bars -0.0987 atmospheres -1.45 PSI There are many other methods of expressing soil water potential You should be familiar with these units -10 to -33 kPa -0.1 to -0.33 bars -1500 kPa -15 bars

8. Field Capacity Saturation Do all of the water molecules in this pore have the same energy status ? -20 KPa -10 KPa

9. -1500 kPa Wilting point

10. -100,000 kPa Air-dry

11. Understanding soil water potential Ψtotal = Ψgravitational + Ψmatric + Ψosmotic

12. Osmotic potential Salt added

13. H20 A continuous chain of water molecules is pulled up through the plant Solar energy drives the process Plants provide the conduit H20 H20 H20

14. Soil water is a switch that activates or deactivates soil biology Water is considered biologically available, when soil organisms are able to win the “tug of war” with the soil

15. Tortuous, loosely connected and highly constricted porosity Structural rigidity The soil matrix presents its inhabitants with many challenges Low quality nutritional resources Moisture fluctuations

16. Translating between water potential (aka tension) and water content using a “characteristic curve” A characteristic curve describes the relationship between water tension and water content for a specific soil. 0

17. A pressure plate system can be used to bring soil to specific water tensions After intact cores of soil are brought to specific tensions, the moisture content at those tensions can be determined. Why are all those bolts needed? A known positive pressure is applied inside the chamber. Soil water is pushed out through a porous plate.

18. Different soils have different characteristic curves Field capacity Wilting point 54% - 24% = 30% 34% - 8% = 26% 0.09 – 0.02 = 0.07 Brady and Weil, 2002

19. So how does compaction impact soil water relationships ? Loss of drainage pores Gain in small pores

20. Impact of texture on soil water 35 - 14 Available water 21% 21% of 12” ~ 2.5” Brady and Weil, 2002

21. SOM increases plant available H20 Adapted from Brady and Weil (2002)

22. Determining gravimetric soil moisture content Collect sample. Weigh moist. Weigh after oven drying. g.m.c. = (moist – dry soil mass) / dry soil mass

23. Converting from gravimetric to volumetric volume of H2O volume of dry soil volume of H2O mass of H2O mass of dry soil volume of dry soil mass of H2O mass of dry soil = * * inappropriate for expansive soils

24. So when should you irrigate ? Wimpy crops Tough crops

25. Brady and Weil, 2002 Tensiometer

26. Gypsum block Measuring soil moisture as a function of electrical resistance Calibration is critical !! Brady and Weil, 2002

27. “An affordable and practical substitute for tensiometric measurement in most agricultural and landscape irrigation environments.”

28. 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.

29. Neutron probe A neutron probe contains a source of fast neutrons and slow neutron detector.  Neutrons are emitted from a radioactive source (e.g. Radium or Americium-beryllium) at a very high speed.  When these fast neutrons collide with a small atom such as the hydrogen contained in soil water, their direction of movement is changed and they lose part of their energy.  These “slowed” neutrons are measured by a detector tube and are calibrated to indicate the amount of water present in the soil.

30. Measuring infiltration rate

31. Visualizing water in a 1 foot layer of soil Macropores 50% porosity saturation 6” Plant available H2O 2.5” Total water at field capacity 3.5” 50% plant available H2O 1.25” 12” 50% solids 6”

32. What will happen if more than 1.25” of water infiltrates into this soil ? How much water is need to bring the soil to field capacity ? 1.25” 50% plant available H2O Water will percolate deeper than 1’

33. How fast does water move through soil ? Darcy’s Law Hydraulic conductivity Flow rate = Area*Ksat *pressure head/length Brady and Weil, 2002

34. Permeability = Hydraulic conductivity Flow rate ~ pore radius4

35. How does the presence of a coarse textured layer under a fine textured layer affect percolation ? Fine textured layer Coarse textured layer

36. Water will not enter the coarse textured layer until the upper layer is near saturation Coarse textured layer After water enters the coarse textured layer, it will percolate more quickly. http://www.personal.psu.edu/asm4/water/drain.html

37. Does a thin layer of coarse material improve drainage ? Thin layer with coarser texture NO !

38. Systems for rapidly draining surface water should be open to the surface Soil capped slit Slit filled with coarse material

39. Slit trenching equipment Outlets are needed !!

40. 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.

41. In Illinois soil drainage groups are assigned a number (1 to 4) and a capital letter (A or B). The number indicates the degree of soil permeability. The letter indicates the natural drainage. IL Permeability classes

43. Fields at the Allison Farm

44. Open field ditch

45. Bioreactor vs. standard tile outlet

49. One calorie is the amount of thermal energy required to raise the temperature of one gram of water by one Celsius degree. 3000 calories of thermalenergy enters each cup.The temperature of thewater on the left rises by30 Celsius degrees.By how much does thetemperature of thewater in the cup on theright rise ??

50. Why does soil heat up faster than water ? The heat capacity of water is ~ 5 times higher than the heat capacity dry soil. As a result, moist soils heat up and cool down more slowly than dry soils.