Porosity testing for stabilized substrates

Goal: To measure bulk density, water holding and porosity for a consistent stabilized foam, paper-wrapped, or peat-polymer 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 is this measured?

Step 1. Requirements


You will need:

  • A section cut from a plug tray (128-count or less) filled with stabilized substrate,
  • water basin,
  • beaker (about 1 cup or 250 mL volume),
  • tape,
  • scale,
  • tweezers or similar to grab cells,
  • saucer,
  • blade.

Step 2. Measure cell volume (V in mL/cell)

Measure cell volume

For paper-wrapped Ellepot and Fertiss, which have a cylindrical shape, the cell volume (cm3 or mL) is calculated as (Π x ((cell diameter in mm)/2)2 x (cell length in mm))/1000.

For foam or peat-polymer cells such as Oasis or Preforma, carefully dip the pre-wetted cells in boiling wax being sure to wear heat-resistant clothing, gloves, goggles, and taking safety precautions such as ventilation.

Measure the wax-coated cell volume (mL) by submerging the substrate into a 250 mL beaker filled with water, and measure volume of water displaced by the cell by weighing the beaker before and after submerging the cell (1 g water = 1 mL).

Step 3. Measure saturated weight by sub-irrigation

Measure saturated weight

Bring the trays filled with stabilized substrate pots ('n' number of cells) to saturation by sub-irrigation, gradually submerging in the 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 all substrate cells one at a time from the water basin and quickly transfer them onto a saucer for weight measurement (W1).

Step 4. Measure cell weight after drainage at container capacity

Measure cell weight

Put aside the stabilized pots for complete drainage (15 min), then weigh them again at container capacity (W2).

Step 5. Weigh the dry substrate weight

Measure dry weight

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

Note that all calculation steps above assume that the plastic tray is not included in the weights – if the stabilized media is adhered to the tray, subtract the tray weight from W1, W2, and W3.

Step 5. Calculate air porosity, water holding capacity and dry bulk density

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

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

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

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

How to interpret the results

Based on our survey data of stabilized substrates in 25-mm-diameter cells:

  • average dry bulk density was around 120 g/L, range 18 to 197 g/L;
  • water holding capacity was 687 mL/L (370 to 910 ml/L);
  • air porosity was 4% (1.9 to 5.9%);
  • water porosity was 67% (37 to 91%).

Compare your substrates with these survey levels to evaluate whether your mix is on the dry (high air) or wet (high moisture) side, and adjust components if the mix tends to stay to wet or dry under your conditions.

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.