Goal: Test the distribution of particle sizes of a growing susbstrate for a consisten, uniform container substrate, with desirable air and water holding characteristics. Guidelines are provided for seedling plugs or vegetative cutting liners, but the protocol can be adapted to larger containers.
Particle size influences substrate air and water porosity. Very coarse ix dries too quikly and will not hold the root ball together, increasing production time to produce a transplantable crop. Very fine particles cause anaerobic conditions, and settling of substrate in the container.
Step 1. Requirements
You'll need: Sieves (for propagation and greenhouse substrates, these will include pan, 4-mesh, 10-mesh, 35-mesh, 100-mesh), a weight scale, 1 L beaker, paper bags, and a plate. For outdoor nursery substrates and large containers, sieves with larger holes will be needed.
Step 2. Air dry mix
Air dry samples down to about 20% moisture (loose with little dust, not held together, light tan color).
Step 3. Weigh the air-dried sample.
Use the beaker to fill a 1 L sample and weigh the same (weigh 'a').
Step 4. Sieve the air dried sample.
Place the weighed sample in the top sieve of 4 stacked sieves in the order of US 4 mesh (4.8mm), 10 mesh (2mm), 35 mesh (0.5mm), 100 mesh and collection pan. Shake for 5 minutes.
Step 5. Weigh the fractions.
Weigh each fraction that remains on the screen or in the pan. Label weights ('b') as 'Very Coarse' (held on 4-mesh), 'Coarse' (held on 10-mesh), 'Medium' (on 35-mesh), 'Fine' (on 100-mesh) or 'Dust' (pan). The interpretation of these size classes is based on our experience with propagation and greenhouse container substrates. With outdoor nursery substrates that contain very large particles, the interpretation of size ranges will differ.
Step 6. Complete the calculations.
Divide the weight of each 'Very Coarse', 'Coarse', 'Medium', 'Fine', and 'Dust' fraction by total dry weight, using the formula:
(b/a) x 100%
For propagation mixes, we define five particle size fractions:
Based on our survey of commercial substrates, the distribution for mixes for liners and bedding plant flats averaged coarse 35%, medium 33%, fine 23%, and dust 9% particles.
For plug mixes, distribution averaged coarse 17%, medium 44%, fine 28%, and dust 11%, respectively.
For more information: Contact authors Jinsheng Huang and Paul Fisher from 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 22 2014.