Goal: Estimate how much of the water held by a propagation substrate is available for plant uptake.
Not all of the water held by a propagation substrate (the 'water holding capacity') is available for plant growth because the substrate holds tightly onto the last amount of water in very small pores and inside substrate particles. The 'plant available water' is the amount of water the plant can extract before reaching the permanent wilting point (severe drought stress).
Step 1. Requirements.
You will need:
Step 2. Measure tray weight at container capacity
Regardless of which method you are using, you will need to irrigate the tray to container capacity using clear water by either top irrigation or sub]irrigation. Make sure the substrate is thoroughly wet and drained, with moisture level 4. Now weigh the whole tray. We will call this measurement Wt1 (in grams, g).
Method 1 (on a tray without plants)
Plant available water estimated as 89% of water holding capacity, providing a rough estimate for propagation substrates.
Air dry the substrate for several days to plug tray moisture level 1 (or oven dry at 70F) until a stable weight is reached. Weigh the tray again (weight Wt0 in g). Available water (mL/tray) can be estimated by:
(Wt1 - Wt0) x 0.89
This is a rough estimate, but is based on our experience with testing 38 commercial propagation substrates using bedding plant impatiens (see chart above).
Method 2 (with plants)
Measure difference between water level at saturation versus at the plant wilting point
Stop irrigation and wait until all the plants in the tray wilt, with the stems bent over and the leaves drooping, but the plant not so stressed that it will not recover. Then weigh the tray weight Wt2 (g).
Plant available water in this substrate can be estimated as:
Wt1 - Wt2 (mL/tray)
When comparing substrates, you might also want to measure days to wilt as well as plant available water, but make sure that plants are the same size in different substrates (small plants dry more slowly).
Plant available water increases in a propagation substrate as water holding capacity increases.
A dry-down test using 38 propagation substrates indicated that approximately 89% container capacity water in a 50-count tray can be taken up by impatiens seedlings.
If you want to compare the 'wetness' or 'dryness' of substrates, you can do a porosity test and estimate plant available water. You can also refine the results using established plants, and testing the amount of water and days until wilt.
For more information: Contact authors Jinsheng Huang and Paul Fisher. Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Fafard et Frères Ltd (Canada), Greencare Fertilizers, Pindstrup, Premier Tech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers.
Adapted from University of Florida IFAS Bulletin FRA S6. April 11, 2013.