Porosity testing for propagation trays

Goal: To measure dry bulk density, water holding capacity and porosity for a consistent growing substrate used in seed or cutting propagation.

Why is it important?

The space in a propagation tray cell is filled with solid substrate particles, air, or water. It is essential to have balanced air and water in a substrate for high quality and healthy plants.

How to measure it

Step 1. Requirements

You'll need:

  • Two sections cut from a plug tray (128-count or less). One section should be empty. The second section should be filled with substrate with the same volume and compaction as would occur with commercial flat filling equipment.
  • Also need a water basin, tape, scale, saucer, blade.

Before filling with substrate, seal the bottom drainage holes of the section with tape, then cut through the sealed tape with a surgical blade, in order to not lose substrate through the drainage holes, but to allow free water to drain.

Step 2. Measure cell volume

Seal the bottom drainage holes of a section cut from an empty plug tray (no substrate) with tape (with n=16 cells). Fill cells with distilled water to the same level as would normally be filled with substrate, and then weigh the trays.

The density of water is 1 gram (g) per milliliter (mL), and record as plug cell volume (V). V (mL/cell) = total volume of water (mL)/n cells filled with water.

Step 3. Measure saturated weight by sub-irrigation

Measure saturated weight

Bring the trays filled with loose substrate to saturation by subirrigation. Wet the substrate in a container from underneath by gradually submerging the container into the water basin, until the water level is about 1 cm below the top of the substrate.

Once fully saturated (the substrate surface glistens), remove the section from the sub-irrigation tank and quickly transfer it into a saucer for weight measurement (W1).

Step 4. Measure tray weight after drainage at container capacity

Put aside the section of tray and wet substrate for complete drainage (15 min), then weigh the tray again at container capacity (W2).

Step 5. Weigh the dry tray and substrate weights

Air dry the substrate in a warm dry environment until weight does not change. Then weigh the tray and substrate again (W3).

Tip out the substrate from the tray and record the empty tray weight (W0) in grams.

Step 6. Calculate air porosity, water holding capacity and dry bulk density

Air porosity (%) =100 * (W1-W2)/(n*V)

Water holding capacity (mL/cell) = (W2-W3-W0)/n

Water porosity (%) = 100 x (water holding capacity)/V

Bulk density (g/L) =1000*(W3-W0)/(n*V)

How to interpret it

For a 128-count plug tray at a typical level of compaction, based on our average survey data:

  • Dry bulk density was around 108 g/L, range 73 to 157 g/L.
  • Air porosity was 6.9% (4.8 to 9.7%).
  • Water porosity was 78% (57 to 86%), and
  • Water holding capacity was 19.5 mL/cell (780 mL per L of substrate), range 14.4 to 21.5 mL/cell (or 576 to 860 mL/L).

These survey ranges provide a benchmark to compare against whether your substrate is an open, 'dry' mix or a 'wet' mix.

For example, if your substrate tends to stay wetter longer than desired with your climate and irrigation conditions and your mix has high water holding capacity relative to our survey levels, consider more coarse components.

For more information: Contact authors Jinsheng Huang and Paul Fisher of University of Florida IFAS Extension. Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Everris, Fafard et Frères Ltd (Canada), Fine Americas, Greencare Fertilizers, Pindstrup, Premier Tech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers. August 23 2014.