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SOIL WATER

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  1. SOIL WATER CHAPTER 7

  2. SOIL WATER Functions: plant cells 50-90% water keeps turgor seed germination transpiration photosynthesis moves products nutrients available lowers soil strength chemical reactions microbial activity

  3. Water Stress Initially, decreased photosynthesis . . . Continued . . . temporary wilting point further . . . permanent wilting point

  4. Forces on Soil Water Gravitational – pull of gravity downward Adhesion – attraction of water to soil Cohesion – attraction of water to water adhesion and cohesion result from shape of water molecule and sharing of electrons in oxygen-hydrogen covalent bonds http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html

  5. Polarity of Water

  6. Polarity of Water Effects of Water Molecule Polarity: Hydrogen of one molecule attracted to oxygen of another molecule in a hydrogen bond accounts for cohesion Hydrogen bond between hydrogen of water and oxygen of silica (SiO2) accounts for adhesion Adhesion water is very tightly held!!! Cohesion water can move and is available for use

  7. Capillarity Additive force of adhesion and cohesion - can move against force of gravity - small pores conduct capillary water

  8. Soil Water Potential Work water can do Potential energy Tendency of water to flow/move freely in soil http://www.fhsu.edu/biology/ranpers/ert/wp_tut.htm Water will always try to move from a state of high energy to a low-energy state The lower the soil water potential the more tightly water is adsorbed to soil particles

  9. WaterPOTENTIAL Refers to the ability of water to move in soil More water in soil = More water potential At saturation, potential is near 0 (zero) As soil dries, values become more negative Water is held more tightly by soil!!

  10. WATER FILM – WATER POTENTIAL

  11. Three Forces of Water Potential Gravitational – potential energy due to gravity positive Matric – most common force; effect of soil on water negative Osmotic – special case of salty soils negative Total water potential is sum of three forces

  12. Units of Potential Official unit is the Pascal (Pa), kilopascal (kPa), or Megapascal (MPa) - common usage of older unit bar - equivalent to 0.1 MPa or 100 kPa Soil water potential is usually negative because of negative matric potential

  13. TYPES OF SOIL WATER Gravitational – at saturation, will drain from larger pores within 24 to 48 hours in well-drained soils Available – can be absorbed by plants; held between gravitational water and wilting point Cohesion – held between gravitational and adhesion (hygroscopic) water Hygroscopic – held tightly by soil particles; air dry

  14. REFRERENCE POINTS RELATED TO SOIL WATER

  15. FOUR CATEGORIES OF SOIL MOISTURE Chemically combined . . . unavailable Hygroscopic . . . unavailable Gravitational . . . moves downward by gravity Capillary . . . taken up by plants

  16. WATER RETENTION Total water-holding capacity and available water-holding capacity are based on soil texture

  17. WATER RETENTION Medium-textured soils have the highestavailablewater-holding capacity e.g. Silt Loam Organic matter influences water-holding capacity Increases amount of available water

  18. WATER MOVEMENT Gravitational flow – moves by gravity • occurs only under saturated conditions • rapid in course soils – large pores • usually percolation through soil profile

  19. SATURATED SOILS Sandy soil: gravitational water moves rapidly downward Clay loam: gravitational water retained 2-3 days afterward

  20. Once soils lose gravitational water (drain) movement is by . . . Capillarity – movement due to attraction between water molecules and soil particles Rapid in sandy soils but limited in distance Slow in clay soils but may move great distances

  21. WATER MOVEMENT Unsaturated flow – lateral movement; capillary flow • depends on unbroken films of water spreading through connected capillary pores • moves from moist to dry soil • can move in any direction

  22. WETTING FRONT A distinct “line” where water is moving in soil – Wet behind, Dry ahead • Soils must be nearly saturated in order for the front to advance; Why? • Dry soil cannot “pull” the water deeper • All the soil must be wet in order for the front to advance

  23. CAPILLARY RISE Upward movement of water from higher to lowerpotentials • Explains evaporation of water from soil to atmosphere • Continuation of capillary rise when entire soil column dries • Boundary in soil serves to protect from further losses • Unsaturated flow only moves over short distances • Saturated soil near the surface encourages capillary rise Responsible for accumulation of salts at surface of soils in dry climates and in potted plants

  24. Effect of Soil Horizons water flows differently in differenttextures. . . stratified layers will slow percolation

  25. Vapor Flow occurs when water vapor moves from moist to drier soil . . . - condenses on cooler soil particles - very slow - minimal water moved

  26. Preferential Flow Saturated soil conditions . . . water enters biopores or other soil channels Increases infiltration and percolation May also move pollutants!!!

  27. How Roots Gather Water Governed by Soil Water Potential Root hairs draw from higher potential regions Capillary flow moves water

  28. Soil – Plant – Atmosphere continuum Plants create “unbroken” column of water Driven by plant transpiration

  29. Patterns of Water Removal Plants will use water near the surface first Oxygen is highest . . . Respiration drives uptake As surface dries, plant roots grow deeper . . . absorption shifts downward If surface is rewetted, absorption shifts upward

  30. Measuring Soil Water Four methods: - gravimetric measurements - potentiometers - resistance blocks - neutron probes (mainly research)

  31. Gravimetric • measures soil water content by weight water content = moist wt – dry wt dry wt Example: soil sample at field capacity 162 grams dry sample 135 grams water content = 162g – 135g = .20 135g

  32. Volume Basis More useful – utilizes gravimetric water content volumetric water content= gravimetric water content x soil bulk density water density From previous gravimetric example . . .

  33. If bulk density of soil is 1.4 grams per cubic cm, and we know density of water is 1.0 g/cc Volumetric water content = .20 x 1.4g/cc = .28 1.0 g/cc

  34. Soil Depth Basis Measures “inches of water” per foot of soil - Uses volumetric water content - Simple calculation . . . Inches water per foot = 12 inches x volumetric water content Continue from previous example . . .

  35. Inches water per foot soil = 12 inches x .28 = 3.36 Or simply stated . . . Each foot of soil depth contains 3.36 inches of water assuming constant soil conditions

  36. Practical Measuring Devices Gravimetric method not very practical management More useful and practical are . . . Potentiometers (tensiometers) Resistance Blocks (gypsum blocks)

  37. Potentiometers • Measure soil moisture potential at given levels • Water exiting tube creates vacuum • Measured by gauge/instrument • Function best at higher potentials

  38. Resistance Blocks • Measure resistance of electrical flow between two electrodes embedded in block buried in soil - moist soil with ions of salts in solution carry electrical flow - resistance blocks designed to buffer salt effects (gypsum accomplishes this) - works well between field capacity and WP