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143 ch7 06

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143 ch7 06

  1. 1. SOIL WATER CHAPTER 7
  2. 2. SOIL WATER <ul><li>Functions: </li></ul><ul><li>plant cells 50-90% water keeps turgor </li></ul><ul><li>seed germination transpiration </li></ul><ul><li> photosynthesis moves products </li></ul><ul><li> nutrients available lowers soil strength </li></ul><ul><li> chemical reactions microbial activity </li></ul>
  3. 3. Water Stress <ul><li>Initially, decreased photosynthesis . . . </li></ul><ul><li>Continued . . . </li></ul><ul><li>temporary wilting point </li></ul><ul><li>further . . . </li></ul><ul><li>permanent wilting point </li></ul>
  4. 4. Forces on Soil Water <ul><li>Gravitational – pull of gravity downward </li></ul><ul><li>Adhesion – attraction of water to soil </li></ul><ul><li>Cohesion – attraction of water to water </li></ul><ul><li>adhesion and cohesion result from shape of water molecule and sharing of electrons in oxygen-hydrogen covalent bonds </li></ul><ul><li>http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html </li></ul>
  5. 5. Polarity of Water
  6. 6. Polarity of Water <ul><li>Effects of Water Molecule Polarity : </li></ul><ul><li>Hydrogen of one molecule attracted to oxygen of another molecule in a hydrogen bond accounts for cohesion </li></ul><ul><li>Hydrogen bond between hydrogen of water and oxygen of silica (SiO 2 ) accounts for adhesion </li></ul><ul><li>Adhesion water is very tightly held!!! </li></ul><ul><li>Cohesion water can move and is available for use </li></ul>
  7. 7. Capillarity <ul><li>Additive force of adhesion and cohesion </li></ul><ul><li>- can move against force of gravity </li></ul><ul><li>- small pores conduct capillary water </li></ul>
  8. 8. Soil Water Potential <ul><li>Work water can do </li></ul><ul><li>Potential energy </li></ul><ul><li>Tendency of water to flow/move freely in soil </li></ul><ul><li>http://www.fhsu.edu/biology/ranpers/ert/wp_tut.htm </li></ul><ul><li>Water will always try to move from a state of high energy to a low-energy state </li></ul><ul><li>The lower the soil water potential the more tightly water is a d sorbed to soil particles </li></ul>
  9. 9. Water POTENTIAL <ul><li>Refers to the ability of water to move in soil </li></ul><ul><li>More water in soil = More water potential </li></ul><ul><li>At saturation , potential is near 0 (zero) </li></ul><ul><li>As soil dries , values become more negative </li></ul><ul><li>Water is held more tightly by soil!! </li></ul>
  10. 10. WATER FILM – WATER POTENTIAL
  11. 11. Three Forces of Water Potential <ul><li>Gravitational – potential energy due to gravity positive </li></ul><ul><li>Matric – most common force; effect of soil on water </li></ul><ul><li>negative </li></ul><ul><li>Osmotic – special case of salty soils </li></ul><ul><li>negative </li></ul><ul><li>Total water potential is sum of three forces </li></ul>
  12. 12. Units of Potential <ul><li>Official unit is the Pascal (Pa), kilopascal (kPa), or Megapascal (MPa) </li></ul><ul><li>- common usage of older unit bar </li></ul><ul><li>- equivalent to 0.1 MPa or 100 kPa </li></ul><ul><li>Soil water potential is usually negative because of negative matric potential </li></ul>
  13. 13. TYPES OF SOIL WATER <ul><li>Gravitational – at saturation, will drain from larger pores within 24 to 48 hours in well-drained soils </li></ul><ul><li>Available – can be absorbed by plants; held between gravitational water and wilting point </li></ul><ul><li>Cohesion – held between gravitational and adhesion (hygroscopic) water </li></ul><ul><li>Hygroscopic – held tightly by soil particles; air dry </li></ul>
  14. 14. REFRERENCE POINTS RELATED TO SOIL WATER
  15. 15. FOUR CATEGORIES OF SOIL MOISTURE <ul><li>Chemically combined . . . unavailable </li></ul><ul><li>Hygroscopic . . . unavailable </li></ul><ul><li>Gravitational . . . moves downward by gravity </li></ul><ul><li>Capillary . . . taken up by plants </li></ul>
  16. 16. WATER RETENTION <ul><li>Total water-holding capacity and available water-holding capacity are based on soil texture </li></ul>
  17. 17. WATER RETENTION <ul><li>Medium-textured soils have the highest available water-holding capacity e.g. Silt Loam </li></ul><ul><li>Organic matter influences water-holding capacity </li></ul><ul><li>Increases amount of available water </li></ul>
  18. 18. WATER MOVEMENT <ul><li>Gravitational flow – moves by gravity </li></ul><ul><li>occurs only under saturated conditions </li></ul><ul><li>rapid in course soils – large pores </li></ul><ul><li>usually percolation through soil profile </li></ul>
  19. 19. SATURATED SOILS <ul><li>Sandy soil: </li></ul><ul><li>gravitational water moves rapidly downward </li></ul><ul><li>Clay loam: </li></ul><ul><li>gravitational water retained 2-3 days afterward </li></ul>
  20. 20. <ul><li>Once soils lose gravitational water (drain) movement is by . . . </li></ul><ul><li>Capillarity – movement due to attraction between water molecules and soil particles </li></ul><ul><li>Rapid in sandy soils but limited in distance </li></ul><ul><li>Slow in clay soils but may move great distances </li></ul>
  21. 21. WATER MOVEMENT <ul><li>Unsaturated flow – lateral movement; capillary flow </li></ul><ul><li>depends on unbroken films of water spreading through connected capillary pores </li></ul><ul><li>moves from moist to dry soil </li></ul><ul><li>can move in any direction </li></ul>
  22. 22. WETTING FRONT <ul><li>A distinct “line” where water is moving in soil – </li></ul><ul><li>Wet behind, Dry ahead </li></ul><ul><li>Soils must be nearly saturated in order for the front to advance; Why? </li></ul><ul><li>Dry soil cannot “pull” the water deeper </li></ul><ul><li>All the soil must be wet in order for the front to advance </li></ul>
  23. 23. CAPILLARY RISE <ul><li>Upward movement of water from higher to lower potentials </li></ul><ul><li>Explains evaporation of water from soil to atmosphere </li></ul><ul><li>Continuation of capillary rise when entire soil column dries </li></ul><ul><li>Boundary in soil serves to protect from further losses </li></ul><ul><li>Unsaturated flow only moves over short distances </li></ul><ul><li>Saturated soil near the surface encourages capillary rise </li></ul><ul><li>Responsible for accumulation of salts at surface of soils in dry climates and in potted plants </li></ul>
  24. 24. <ul><li>Effect of Soil Horizons </li></ul><ul><li>water flows differently in different textures . . . </li></ul><ul><li>stratified layers will slow percolation </li></ul>
  25. 25. <ul><li>Vapor Flow </li></ul><ul><li>occurs when water vapor moves from moist to drier soil . . . </li></ul><ul><li>- condenses on cooler soil particles </li></ul><ul><li>- very slow </li></ul><ul><li>- minimal water moved </li></ul>
  26. 26. <ul><li>Preferential Flow </li></ul><ul><li>Saturated soil conditions . . . </li></ul><ul><li>water enters biopores or other soil channels </li></ul><ul><li>Increases infiltration and percolation </li></ul><ul><li>May also move pollutants!!! </li></ul>
  27. 27. How Roots Gather Water <ul><li>Governed by Soil Water Potential </li></ul><ul><li>Root hairs draw from higher potential regions </li></ul><ul><li>Capillary flow moves water </li></ul>
  28. 28. <ul><li>Soil – Plant – Atmosphere continuum </li></ul><ul><li>Plants create “unbroken” column of water </li></ul><ul><li>Driven by plant transpiration </li></ul>
  29. 29. Patterns of Water Removal <ul><li>Plants will use water near the surface first </li></ul><ul><li>Oxygen is highest . . . Respiration drives uptake </li></ul><ul><li>As surface dries, plant roots grow deeper . . . </li></ul><ul><li>absorption shifts downward </li></ul><ul><li>If surface is rewetted, absorption shifts upward </li></ul>
  30. 30. Measuring Soil Water <ul><li>Four methods: </li></ul><ul><li>- gravimetric measurements </li></ul><ul><li>- potentiometers </li></ul><ul><li>- resistance blocks </li></ul><ul><li>- neutron probes (mainly research) </li></ul>
  31. 31. Gravimetric <ul><li>measures soil water content by weight </li></ul><ul><li>water content = moist wt – dry wt dry wt </li></ul><ul><li>Example: soil sample at field capacity 162 grams dry sample 135 grams </li></ul><ul><li>water content = 162g – 135g = .20 </li></ul><ul><li> 135g </li></ul>
  32. 32. Volume Basis <ul><li>More useful – utilizes gravimetric water content </li></ul><ul><li>volumetric water content = </li></ul><ul><li>gravimetric water content x soil bulk density </li></ul><ul><li> water density </li></ul><ul><li>From previous gravimetric example . . . </li></ul>
  33. 33. <ul><li>If bulk density of soil is 1.4 grams per cubic cm, and we know density of water is 1.0 g/cc </li></ul><ul><li>Volumetric water content = </li></ul><ul><li>.20 x 1.4g/cc = .28 </li></ul><ul><li> 1.0 g/cc </li></ul>
  34. 34. Soil Depth Basis <ul><li>Measures “inches of water” per foot of soil </li></ul><ul><li>- Uses volumetric water content </li></ul><ul><li>- Simple calculation . . . </li></ul><ul><li>Inches water per foot = </li></ul><ul><li>12 inches x volumetric water content </li></ul><ul><li>Continue from previous example . . . </li></ul>
  35. 35. <ul><li>Inches water per foot soil = </li></ul><ul><li>12 inches x .28 = 3.36 </li></ul><ul><li>Or simply stated . . . Each foot of soil depth contains 3.36 inches of water assuming constant soil conditions </li></ul>
  36. 36. Practical Measuring Devices <ul><li>Gravimetric method not very practical management </li></ul><ul><li>More useful and practical are . . . </li></ul><ul><li>Potentiometers (tensiometers) </li></ul><ul><li>Resistance Blocks (gypsum blocks) </li></ul>
  37. 37. Potentiometers <ul><li>Measure soil moisture potential at given levels </li></ul><ul><li>Water exiting tube creates vacuum </li></ul><ul><li>Measured by gauge/instrument </li></ul><ul><li>Function best at higher potentials </li></ul>
  38. 38. Resistance Blocks <ul><li>Measure resistance of electrical flow between two electrodes embedded in block buried in soil </li></ul><ul><li>- moist soil with ions of salts in solution carry electrical flow </li></ul><ul><li>- resistance blocks designed to buffer salt effects (gypsum accomplishes this) </li></ul><ul><li>- works well between field capacity and WP </li></ul>

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