Are your trays filling unevenly or are too compacted?

How you fill propagation trays with growing substrate can make a big difference in germination and rooting. Packing too much substrate into the cell squeezes out air and increases substrate cost. Under-filling trays leads to settling, reduced root zone volume, and uneven drying and plant size. Consider the following factors for quality control when filling trays.

Settling and compaction test

Over-compacted trays have little or no settling. Under-filling results in settling and uneven filling. Fill trays, irrigate, and drop onto a flat hard surface from 8-in (20-cm) to observe how they are likely to settle out in the greenhouse (see Settling and compaction in loose-filled trays (in BackPocketGrower.com, “BPG” for detailed procedures). Test substrate compaction whenever you adjust the tray-filling line.

Tray with compost

Particle sizes

Fine particle size of peat and other components are more prone to settling in the tray and loss of air porosity. Test particles when changing substrate components or with each major shipment of substrate. Our Testing particle sizes protocol in BPG provides guidelines.

Testing particle size

Moisture level

  • If the substrate is too dry, under-filling or uneven filling are likely. Growing mix will easily fall out of the bottom of the tray. Dry substrates are also hard to rewet. Adding water to the mix is necessary before tray filling.
  • If the substrate is very wet, it can cause inconsistent filling. For loose-filled trays, a target moisture level is 50-60%. Our Look and feel method in BPG provides guidelines on how to test moisture.

Hand demonstrating look and feel method

Adding water to hopper

Once substrate is placed into the hopper, water should only be added if the moisture level is less than 50%. When adding water, preferably spray onto the substrate on a mixing line, or if necessary in the hopper itself. Use a spray nozzle rather than a coarse hose. Mix in water as evenly as possible, but without breaking up the particles. Excessive handling of substrate in hoppers can break down particle size, thereby increasing settling.

Tray filling machine

The tray filling machine should be adjusted to avoid under or over-filling the tray. If after adjustment it is not possible to evenly fill the tray, check wear of brushes and equipment maintenance.

Machine filling trays with compost

Storage and handling after tray filling

High irrigation force from booms and hand water can increase compaction in plug trays. Growers who pre-fill and stack their flats and plug trays should include a layer of cardboard between the flats to prevent compaction, or offset trays to avoid compaction on the tray below. Reduce vibration and movement of the stacked trays to avoid compaction.

For more information: Contact authors Jinsheng Huang and Paul Fisher of University of Florida IFAS Extension, and Dr. Bill Argo of Blackmore Co. Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Everris, Fine Americas, Greencare Fertilizers, Klasmann-Deilmann, Pindstrup, Premier Tech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers. The University of Florida does not endorse any product, and our research focuses on quality testing on these and competing products to assist grower success. June 30 2015.