Particle size testing

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: why is it important?

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. 

How to measure it?

Step 1. Requirements

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

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. 

Weight 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

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%

How to interpret it

For propagation mixes, we define five particle size fractions:

  1. 'very coarse' (greater than 4.8 mm, held on 4-mesh),
  2. 'coarse' (2-4.8 mm, held on 10-mesh),
  3. 'medium' (2 to 0.5 mm, held on 35-mesh),
  4. 'fine' (less than 0.5 mm, held on 100 mesh) and
  5. 'dust' (less than 0.15 mm, passing 100-mesh).

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.

Rules of thumb

  • Aim for a fairly narrow band of particle sizes (coarse to medium for liner propagation, medium to fine for plugs) for better aeration and less settling.
  • Limit the dust fraction to less than 10%.
  • Very coarse particles are normally only used in propagation of large woody cuttings and are likely to clog Ellepot equipment or lead to uneven filling of small cells.
  • Observe the color of each size class: each component should have a similar distribution to other components (e.g., not all coarse particles made up of perlite, and fine particles from peat).
  • Test each new shipment of substrate.  You can develop your own standards based on experience for your  substrate and crop types.

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.