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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

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
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%

our task
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
water potential measurement in the dry range for pwp
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)
slide5

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

water potential measurement in the wet range for fc
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)
steps for quick 1 5 mpa water content
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
steps continued
Steps Continued
  • Add water, mix and equilibrate
  • Measure water potential and water content
  • Compute -1.5 MPa water content
  • Convert to volumetric water content
how much water to add
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

reading the water potential
Reading the Water Potential

Insert sample

Seal chamber

Wait 3-5 min. and

read the result

convert to volumetric water content
Convert to Volumetric Water Content
  • Probes measure volumetric wc
  • Soil stores in terms of volumetric wc

If bulk density were 1.4 g/cm3

quick determination of the 33 kpa water content
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
measuring water potential
Measuring Water Potential

Remove cap from tensiometer

Insert the probe and read

calculating the 33 kpa water content
Calculating the -33 kPa water content

Assume

  • Sandy loam
  • Water potential reading is -18 kPa
  • Water content reading is 0.25 m3 m-3
conclusions
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
conclusions1
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