Quick Methods for Determining Plant Available Water

Gaylon S. Campbell, Ph. D.Decagon Devices, Inc.

Pullman, WAwww.decagon.com

Volume Fractions: a “Typical” Soil at Field Capacity

Air 24%

Unavailable Water 13%

Solid 50%

Plant Available Water 13%

Field capacity orDrained upper limit

Permanent wilt pointor lower limit

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

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

Infrared SensorMirror

Optical Sensor

Fan

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)

Steps for Quick -1.5 MPa Water Content

Air dry, crush and sieve the soil sample

Determine the soil textureFind the approximate -1.5 MPa and air dry water content for that texture

Calculate the amount of water to add

Steps Continued

Add water, mix and equilibrateMeasure water potential and water content

Compute -1.5 MPa water contentConvert to volumetric water content

Texture w-1.5 (g/g)

wad (g/g)

Sand 0.008 0.003

loamy sand 0.015 0.005

sandy loam 0.045 0.015

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 0.204 0.068

Clay 0.234 0.078

How Much Water to AddSilt loam soil: w-1.5 = 0.098 g/g

wad = 0.033 g/g

50 g. sample

ggg

ggggg

w

wwMM

ad

adadw 15.3

/033.00.1

/033.0/098.050

1

Add 3.15 ml. of water to 50 g of air dry soil

Reading the Water Potential

Insert sampleSeal chamber

Wait 3-5 min. andread the result

Computing the -1.5 MPa Water Content

m

mww

1000

ln

5.11000

ln

5.1

ggw /103.0898.6

502.6109.0

01.11000

ln

5.11000

ln109.05.1

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

1 10 100 1000

Water Potential (-MPa)

Wat

er C

on

ten

t (g

/g)

Wm = 0.109m = -1.01

Convert to Volumetric Water Content

Probes measure volumetric wcSoil stores in terms of volumetric wc

If bulk density were 1.4 g/cm3

333

3

5.15.1

/144.0/0.1

/4.1/103.0 cmcm

cmg

cmggg

ww

b

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

Remove cap from tensiometer

Insert the probe and read

Water Content with EC-5 and Echo Check

Extrapolating to get the -33 kPa water content

b

mm

/1

33 33

0.1

1

1 10 100 1000

Water Potential (-kPa)

Wat

er C

on

ten

t (g

/g)

Palouse Silt Loam

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 7.7

silty clay 9.7

Clay 12.6

b values for Soils

Calculating the -33 kPa water content

Assume Sandy loamWater potential reading is -18 kPa

Water content reading is 0.25 m3 m-3

337.3/1

33/1

33 21.033

1825.0

33

mmmmb

mm

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

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