Growing media is sold in Kenya largely on description. “Fine vermiculite.” “Propagation peat.” Those phrases mean whatever the person saying them wants them to mean, and the difference between two deliveries described identically can be the difference between an even batch and a ragged one.
Here is what the numbers on a proper specification mean, and what each one does at the base of a cell.
Particle fraction
The size range of the particles, in millimetres. It is the single most consequential number on the sheet.
Finer material holds more water and less air. Coarser material drains faster and holds more air. A propagation medium has to hold enough water that the plant does not dry out between mistings, and enough air that the roots forming at the base can breathe.
Our vermiculite grades run 0.5–1.0 mm fine, 1.0–2.0 mm medium and 2.0–4.0 mm coarse. Fine for seed and soft cuttings, medium for general propagation, coarse where drainage or aeration is the constraint. If a supplier will not state a range, they are not grading — they are sorting by eye.
Bulk density
Mass per unit volume, in kilograms per cubic metre. It tells you two things.
Whether you have been shorted. Media is often sold by volume and delivered by weight, or the reverse. Without a density you cannot reconcile the two. Our vermiculite runs 100–130 kg/m³ fine, 90–120 medium and 70–100 coarse.
How it will behave in a tray. A denser medium settles more, which changes fill depth between the day you fill and the day you graft.
pH
How acid or alkaline the medium is. It governs which nutrients a root can actually take up, regardless of how much of them are present.
Vermiculite is naturally slightly alkaline — ours runs pH 7.0 to 8.5 depending on grade. Peat is acidic and is usually limed to a target; our formulations run 5.3 to 6.2 depending on the use. A peat delivered at pH 4.5 will lock up nutrients no matter what you feed.
Conductivity
Electrical conductivity, in millisiemens per centimetre, measured here on a 1:5 volume extract. It is a proxy for how much dissolved salt the medium carries — from starter fertiliser, from the raw material, or from your own irrigation water.
It matters in propagation more than in production, because young roots are salt-sensitive and because repeated misting concentrates salts at the top of the cell as water evaporates. Starting high leaves you no headroom. Our seed formulation runs 0.6–0.8 mS/cm, the propagation formulation 0.4–0.6, and the grafting formulation 0.5–0.7.
Air-filled porosity
The proportion of the volume that is air at container capacity. Seed formulations sit low at 8–12 % because germination wants consistent moisture; grafting formulations sit high at 20–25 % because roots forming under mist need oxygen at the base.
This is the number that quietly explains most “the cuttings rotted” conversations.
Moisture content and contamination
Vermiculite is supplied at 8 % moisture or below. Higher than that and you are paying freight on water, and the material compacts. Contamination — weed seed, foreign material, dust — should be specified as absent rather than assumed.
Cation exchange capacity
How much nutrient the material can hold and release, in milliequivalents per 100 g. Vermiculite is unusually good at this, at 80–150 meq/100 g, which is why it buffers feeding errors that a purely inert medium would not.
What to insist on before you accept a delivery
Six things, all of which a serious supplier already has:
- A stated particle fraction, as a range.
- Bulk density, as a range.
- pH and conductivity, with the extraction method named.
- Moisture content on despatch.
- A lot or batch code on the bag or the bale.
- A certificate of analysis for that batch.
The sixth is the one that converts the other five from claims into evidence — and it is the one your own auditor will ask for, because the propagation standard requires you to record the batch number of the substrate used.
A closing note on peat
Peat is under pressure, and the propagation standard now requires that a proportion of substrate volume comes from alternatives to it. Vermiculite, coir and perlite are not merely substitutes here; they are part of how the standard is met. If your buying pattern is peat by default, this is worth looking at before it is asked about.