"LECA is inert — just rinse it and plant." But fresh LECA from the shop has an alkaline pH of 7.5–8.5 and production dust that immediately raises solution EC. After a few growth cycles, biofilms, salt deposits, and root debris accumulate on the surface of the pellets — and the "inert" substrate starts actively affecting solution chemistry. Preparation and between-cycle sanitation of inert substrates is not an optional step; it is a condition for stable operation.
Quick Glossary
- LECA (Lightweight Expanded Clay Aggregate) — fired clay pellets with a porous structure; chemically near-inert with minimal CEC, but fresh material has an alkaline pH and production dust
- Inert substrate — a substrate that does not react chemically with the nutrient solution and has no significant CEC; includes LECA, gravel, and crushed stone
- Substrate recirculation — reusing LECA or gravel between growth cycles after cleaning and disinfection
What Makes an "Inert" Substrate Not Quite Inert
Fresh LECA from a shop is not a neutral material. Three problems appear before the first irrigation:
Alkaline surface pH. Firing clay at 1,000–1,200°C leaves alkaline oxides on the pellet surface. On first contact with water they dissolve and push pH to 7.5–8.5 even when irrigating at pH 5.8. Running the first cycle without preparation means a constant struggle with pH.
Production dust. Fine clay particles and transport dust enter the solution and clog pumps, nozzles, and filters. Solution EC rises from suspended dust then falls once it settles — unexplained instability with no obvious cause.
Physical non-uniformity. Pellets of varying size create uneven moisture and root distribution in the container. Larger fractions drain better but give less root-to-moisture contact. Smaller fractions retain moisture better but risk compaction and reduced aeration.
How to Prepare New LECA
Rinsing. Rinse until the water runs clear — this removes dust and fine particles. For large volumes: soak in water for 12–24 hours, drain, and repeat twice.
Acidification. Soak in a solution at pH 5.0–5.5 for 24–48 hours to neutralise the alkaline pellet surface. After acidification, rinse with clean water until drainage pH reaches 6.0–6.5.
Pre-launch check. Irrigate several times with working solution and measure pH and EC of the drainage. If drainage pH is consistently in the 5.8–6.3 range and EC is no more than 0.2 above the input solution EC — the substrate is ready.
The same procedure applies to gravel and crushed stone — alkalinity and dust are present in all mineral inert materials regardless of source.
Recirculation: Returning Substrate to Service
Inert substrates differ fundamentally from organic ones in that they can be fully cleaned and reused. But a simple rinse between cycles is not enough — after a growth cycle the pellet surface carries biofilms, salt deposits, and root exudate residue.
Mechanical cleaning. Remove large root debris by hand or by rinsing with agitation. Soak in water for a few hours — organic residue softens and releases more easily.
Chemical disinfection. Soak in a solution of H₂O₂ (3–5%) or PAA (peracetic acid, 0.1–0.2%) for 30–60 minutes. These oxidisers break down the organic matrix of biofilms and sanitise the surface. Chlorine is less effective — it kills bacteria but does not destroy the organic matrix as well.
Rinsing and verification. After disinfection, rinse with clean water until the smell is neutral and EC ≤ 0.1. Check pH: if after several cycles drainage pH starts rising again, the pellet surface has degraded or carbonate deposits have built up — repeat the acidification step.
Three Mistakes That Cost the Most
Starting a new cycle after a "simple rinse" with no disinfection. Biofilms from the previous cycle remain on the pellet surface. The next cycle starts with established microbial colonies — and at the first favourable conditions (falling DO, rising temperature) a Root Rot or bacterial infection outbreak is virtually guaranteed.
Not checking drainage pH and EC after preparing new substrate. Different batches of LECA from different manufacturers have different baseline alkalinity. The only way to know the substrate is ready is to measure it.
Not removing carbonate deposits between cycles. With extended use, a white film — calcium and magnesium carbonates — builds up on pellet surfaces. It raises pH and reduces aeration in areas where pellets compact together. Removed by soaking in a mildly acidic solution (pH 4.0–4.5) for a few hours.
How to Know the Substrate Is Ready to Use
- After preparation or recirculation: drainage pH is consistently 5.8–6.3 across three test irrigations
- Drainage EC is no more than 0.2 above the input solution EC
- Smell is neutral — no sour or putrid odour
- Pellets are clean with no white film and no visible organic residue