Brand: Fertile Fibre MPC | Supplier: Via Amazon | Price: £0.60/litre
Personal observations
Fertile Fibre is in a different category to everything else in this test. It is clearly coir-based: light brown, very uniform, with visible coconut fibre strands and hard granules of shredded coconut shell. The vermiculite addition is obvious immediately — flat, pearlescent, mica-like flakes that catch the light. It is very clean, almost no smell, and quite dry on opening. The numbers are the most extreme in the test group: water retention of 9.5 times dry weight is nearly three times the next highest mainstream product. But — and this is the critical finding — it dried completely in just 10 minutes, the fastest in the test by a large margin. This is the paradox of coir: it can hold a lot of water, but once it starts drying out it releases moisture very quickly and rehydration can be difficult. This is not a compost in any traditional sense. It is a manufactured growing medium designed for controlled environments — greenhouses, propagation, indoor growing — where watering is managed carefully. In an outdoor hanging basket through a hot week, it would be challenging.
Quick-test data (April 2026)
| Metric | Band | Value | Context |
|---|---|---|---|
| Bulk density (dry) | L | 0.08 g/ml | Very light — lightest in test group; coir air-space structure |
| Water held per g dry weight (WHC) | H | 9.5× | Exceptional — highest in test group by large margin |
| Dry-down time | — | 10 min | Very fast — fastest in test group |
| Sieve: <2mm fraction | — | 15.0% | Low — coarse coir structure |
| Sieve: 2–10mm fraction | — | 85.0% | Very high — highest in test group |
| Sieve: >10mm fraction | — | 0.0% | Negligible |
| Price per litre | Premium/Specialist | £0.60 | Online via Amazon — P&P included in price shown |
The very high WHC and very fast dry-down are not contradictory — both are explained by coir’s capillary/free air-space water retention mechanism. Water is held in structural pores rather than by humic or Van der Waals forces, so it is released rapidly once the surface starts to dry. This is fundamentally different from the moisture-holding behaviour of a mature compost or biochar-enhanced product at a similar WHC figure. Best suited to controlled-environment growing where irrigation is consistent.
Price includes P&P — online only. The premium reflects both specialist ingredients (coir, vermiculite) and distribution costs rather than direct comparison with mainstream retail compost. For an explanation of why brands offer multiple product types at different price points, see our FAQ
Comparative hands-on testing, April 2026. Consistent method across all 18 products. See methodology note.
At-a-glance rating breakdown
| Category | Weight | Score |
| Ease of use | 40% | 7.0 |
| Composition & quality | 35% | 5.9 |
| Sustainability | 25% | 2.5 |
| Overall (weighted) | 100% | 6.2 / 10 |
What we liked
- Very clean and uniform growing medium.
- Peat-free by design.
Points to consider
- Made from coir – formulated in UK, but coir is imported from Asia.
- Low biological diversity without amendment.
