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

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

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. 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)

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. Texture
w-1.5
(g/g)
wad
Sand
0.008
0.003
loamy sand
0.015
0.005
sandy loam
0.045
sandy clay loam
0.143
0.048
Loam
0.106
0.035
sandy clay
0.204
0.068
silt loam
0.098
0.033
Silt
0.075
0.025
clay loam
0.174
0.058
silty clay loam
0.166
0.055
silty clay
Clay
0.234
0.078

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

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

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

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

14. 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

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

16. Water Content with EC-5 and Echo Check

17. Extrapolating to get the -33 kPa water content

18. b values for Soils Texture b Sand 1.6 loamy sand 2.7 sandy loam 3.7
sandy clay loam
7.7
Loam
5.5
sandy clay 11
silt loam 5
Silt
4.1
clay loam
8.4
silty clay loam
silty clay
9.7
Clay
12.6

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

20. 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

21. 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