Test compaction in paper-wrapped pots

Goal: To measure compaction of paper-wrapped substrates, including Ellepot® and Fertiss®.

Why is it important?

Substrate compaction reduces air porosity in the mix, which can lead to reduced root growth and slow drying. Moderate compaction helps the pot hold together during growing and shipping, and can improve distribution of water through the substrate.

How do you measure it?

Digital force gauge

Step 1. Requirements

  • A digital force gauge (such as Extech 475040 Digital Force Gauge) available online. 
  • 1-inch adapter disc (e.g. HMA-15)
  • Plug tray filled with stabilized pots.
Irrigate to container capacity

Step 2. Irrigate to container capacity

After manufacturing the pots, bring the substrate to container capacity by passing through a water tunnel for 3 to 4 times or using a fine mist nozzle by top irrigation, or subirrigating.

Allow to drain for 30 seconds.

Measure compaction

Step 3. Set the digital force gauge

  • Attach the 1-inch adapter disc to the base of the force meter
  • Slide the POWER switch to the PEAK position
  • Slide the unit button to g (grams) position
  • Press FAST/SLOW key to select SLOW response
  • ZERO the meter before each measurement.

Step 4. Measure compaction

Measure the stabilized pot by slowly pushing the 1-inch adapter disc downward by 3/8-in (1 cm) from the top of the pot, and take the reading on the LCD display (k).

Step 5. Calculate substrate compaction

Compaction (g/cm2) equals the reading on the meter (k) divided by 5.07.

For example, 1520 g on the meter equals 300 g/cm2. If a different sized adapter disc is used (other than 1-inch diameter), with radius r, divide the meter reading k by πr2.

How do you interpret the measurements?

  • Compaction should be 200 to 300 g/cm2 (or 1015 to 1520 g on the meter).
  • Lower than 200 g/cm2 may lead to loss of substrate from the pot during shipping and handling, and tearing of paper during manufacturing.
  • Above 300 g/cm2 will reduce air porosity.
  • This compaction test does not apply to foam or polymer media which may have a rubbery or brittle texture.

Alternative test

For companies that manufacture a large volume of pots with the same substrate, another approach is to air-dry a tray of pots over a period of days, and then weigh the tray.

This 'bulk density' is another compaction measurement that should be consistent over time.


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.