Goal: To rapidly measure dry bulk density, water holding capacity and air porosity of substrate in greenhouse and nursery containers. You can use the 'Container porosity' app under 'Tools' to help with calculations.
The space in a pot is filled with solid substrate particles, air, or water. It is essential to have balanced air and water in a substrate for healthy root growth, and high quality plants.
Step 1. Requirements
Step 2. Seal the empty pot with a plastic bag & measure the pot volume.
Step 3. Bring substrate to saturation & measure pot weight at saturation
Slowly and gently add water to the top of the pot so that the top of the substrate is glistening
Allow the substrate to equilibrate for 5 minutes.
Set the scale to zero, then put the filled pot on the scale and record the weight (W1 in grams).
Step 4. Measure pot weight after drainage (near container capacity)
Step 5. Weigh the dry substrate, or estimate its weight
If you want a very rough and quick estimate before the substrate is air-dried, divide the pot volume V (in mL) by 1000 then multiply by 100 grams/liter for an approximate bulk density of substrates made up of peat, perlite, vermiculite, and coconut coir.
For example with a 2L pot filled with a peat/perlite substrate
W3 = V 2000 mL /1000 x 100 = 200 grams.
Step 6. Calculate air porosity, water holding capacity and dry bulk density
You can use the 'Container porosity' app under 'Tools' for calculations, or
Water holding capacity (milliliters per pot) = (W2 - W3 - W0)
Air porosity (%) = 100 x (W1 - W2)/V
Water porosity (%) = 100 x (water holding capacity)/V
Solid % by volume = 100% - air porosity - water porosity
Dry bulk density (g/L) = 1000 x W3/V
Example for a coir substrate in a tall thin pot (as shown in pictures):
Calculations would give:
For a 6-inch (15-cm) diameter, 1-liter pot by volume in peat-based substrates:
The large pores (holes or spaces) in growing substrate hold air when the pot is drained. Small pores in a growing substrate hold water.
Substrates with coarse particles therefore tends to have a high air porosity and low water porosity (a 'dry mix'). Substrates with fine particles have lower air porosity and high water porosity (a 'wet mix').
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, PremierTech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers. Adapted from: University of Florida IFAS Bulletin FRA S12. July 31, 2016. See the North Carolina State University protocol for a more detailed and precise laboratory method.