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Quick Methods for Determining Plant Available Water

Quick Methods for Determining Plant Available Water. Gaylon S. Campbell, Ph. D. Decagon Devices, Inc. Pullman, WA www.decagon.com. Volume Fractions: a “Typical” Soil at Field Capacity. Field capacity or Drained upper limit. Air 24%. Plant Available Water 13%.

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Quick Methods for Determining Plant Available Water

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  1. Quick Methods for Determining Plant Available Water Gaylon S. Campbell, Ph. D. Decagon Devices, Inc. Pullman, WA www.decagon.com

  2. Volume Fractions: a “Typical” Soil at Field Capacity Field capacity or Drained upper limit Air 24% Plant Available Water 13% Unavailable Water 13% Permanent wilt point or lower limit Solid 50%

  3. Our Task Demonstrate a quick method for determining plant available water by • Showing how to find the -1.5 MPa water content of a soil sample • Showing how to find the -33 kPa water content of a soil

  4. WP4 Dew Point Potentiameter Range is 0 to -300 MPa Accuracy is 0.1 MPa Read time is 5 minutes or less Water Potential Measurement in the Dry Range (for PWP)

  5. Chilled Mirror Dew Point • Cool mirror until dew forms • Detect dew optically • Measure mirror temperature • Measure sample temperature with IR thermometer • Water potential is approximately linearly related to Ts - Td Fan Optical Sensor Mirror Infrared Sensor Sample

  6. Equilibrates water under tension with soil water through a porous cup Measures pressure of water Highest accuracy, but limited range (0 to -70 kPa) Water Potential Measurement in the Wet Range (for FC)

  7. Steps for Quick -1.5 MPa Water Content • Air dry, crush and sieve the soil sample • Determine the soil texture • Find the approximate -1.5 MPa and air dry water content for that texture • Calculate the amount of water to add

  8. Steps Continued • Add water, mix and equilibrate • Measure water potential and water content • Compute -1.5 MPa water content • Convert to volumetric water content

  9. How Much Water to Add Silt loam soil: w-1.5 = 0.098 g/g wad= 0.033 g/g 50 g. sample Add 3.15 ml. of water to 50 g of air dry soil

  10. Reading the Water Potential Insert sample Seal chamber Wait 3-5 min. and read the result

  11. Computing the -1.5 MPa Water Content Wm = 0.109 m = -1.01

  12. Convert to Volumetric Water Content • Probes measure volumetric wc • Soil stores in terms of volumetric wc If bulk density were 1.4 g/cm3

  13. Quick Determination of the -33 kPa Water Content • Do it in the field or take field samples after heavy rain or irrigation • Measure water potential with a tensiometer • Measure water content with a probe (volumetric) • Calculate -33 kPa water content

  14. Measuring Water Potential Remove cap from tensiometer Insert the probe and read

  15. Water Content with EC-5 and Echo Check

  16. Extrapolating to get the -33 kPa water content

  17. b values for Soils

  18. Calculating the -33 kPa water content Assume • Sandy loam • Water potential reading is -18 kPa • Water content reading is 0.25 m3 m-3

  19. Conclusions • Upper and lower limits for plant available water are soil specific • They are approximated by the -33 kPa and -1.5 MPa water contents • Quick methods for determining these values are available

  20. Conclusions • The -1.5 MPa water content is found by measuring water content and potential on a sample near -1.5 MPa and extrapolating • The -33 kPa water content is found by measuring water content and potential on a sample near -33 kPa and extrapolating

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