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Quick Methods for Determining Plant Available Water

This guide presents efficient techniques for determining plant available water (PAW) in soil. It details methods for measuring water content at -1.5 MPa and -33 kPa. Utilizing devices like the WP4 Dew Point Potentiometer, the procedure includes measuring water potential and content in both dry and wet ranges. Key steps involve sample preparation, equilibrium mixing, and calculating volumetric water content. Results help define upper and lower limits for PAW specific to soil types, facilitating effective irrigation management.

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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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