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% Unavailable Water 13% Permanent wilt point or lower limit Solid 50%

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 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)

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 Add water, mix and equilibrate Measure water potential and water content Compute -1.5 MPa water content Convert to volumetric water content

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

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 Insert sample Seal chamber Wait 3-5 min. and read the result

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

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 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 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

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 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