Fine Fraction Limit describes the maximum percentage of very small particles (fines) allowed in a grow mix before airflow and drainage begin to suffer. These fine particles include peat dust, compost fines, degraded bark, and other materials small enough to pack tightly together.
Fines matter because they settle into the gaps between larger particles. As their proportion increases, they block air pathways, slow drainage, and reduce oxygen diffusion to roots, even if the mix initially appeared well-aerated.
How to interpret fine fraction values:
- Lower fine fraction means more open pore space, faster re-oxygenation, and a more predictable and reliable dry-down rate.
- Higher fine fraction increases water retention but narrows the margin for error by slowing oxygen return and extending saturation time.
The fine fraction limit is not about how the mix feels on day one. Even stable mixes gradually generate fines as materials break down, which is why starting with a controlled fine fraction matters for long-term root health.
In short, fine fraction limit defines how much “dust” a mix can tolerate before root-zone oxygen becomes unreliable, helping plant parents understand why some mixes fail months after repotting despite unchanged care habits.
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