Quick Glossary
- Perlite — thermally expanded volcanic glass; chemically inert, improves aeration and drainage, holds no nutrients
- Vermiculite — a mica-group mineral; retains water and cations (CEC 10–20 meq/100g)
Perlite: Its Real Role in a Mix
Perlite acts as a physical spacer between organic substrate particles. Without it, peat and coco compact under irrigation.
| % Perlite | Effect |
|---|---|
| 10–15% | Good moisture retention, moderate aeration |
| 30–50% | Fast drainage, more frequent watering required |
Particle size:
- Fine (1–3 mm) — better capillary rise
- Coarse (4–6 mm) — better drainage, less even distribution
Vermiculite: When It Helps and When It Doesn't
Useful: when substrates dry out quickly, in seedling mixes, and for slow-growing crops.
Not needed: when overwatering is already an issue, or in drip-irrigated systems with already high substrate moisture.
Mix Formulation Logic
| Application | Composition |
|---|---|
| Seedlings / germination | Peat 60–70%, perlite 20–30%, vermiculite 10–15% |
| Herbs, basil | Coco 50–60%, perlite 30–40%, vermiculite 0–10% |
| Tomato, cucumber (long cycle) | Coco 50%, perlite 30%, vermiculite 20% |
Three Mistakes That Cost the Most
- Adding perlite without increasing watering frequency — at 40% perlite, the substrate will dry out too quickly
- Substituting perlite with vermiculite — they serve opposite functions; sensitive crops risk anaerobic conditions
- Coarse perlite in small seedling trays — causes uneven moisture distribution for seedlings
Signs of a Correct Mix
- Drainage appears 30–60 seconds after watering
- 1–2 hours after watering, the substrate is moist but not wet
- Roots are evenly distributed throughout the entire substrate volume