Quick Glossary
- ORP — oxidation-reduction potential (mV); a measure of the solution's ability to suppress pathogens
- Biofilm — an organised colony of microorganisms on system surfaces
- Pathogens — Pythium, Fusarium, Phytophthora
Who Lives in the System
Nitrifying bacteria — critical in bioponics; minimal role in mineral hydroponic systems.
Rhizosphere bacteria — produce antibiotics and enzymes against pathogens. These are the organisms added as microbial inoculants.
Pathogens — under optimal conditions (DO, temperature, competition) they do not cause disease.
Conditions That Determine the Balance
DO — the most important parameter. Pythium thrives without oxygen; below DO 4 mg/L the balance shifts in favour of pathogens.
Temperature — at 26°C and above, Pythium multiplies 3–4 times faster than at 20°C.
ORP >650 mV — most pathogens are physically suppressed.
Organic load — regular partial solution replacement reduces the nutrient supply available to pathogens.
When Disinfection Does More Harm Than Good
Shock disinfection destroys everything. The system becomes microbiologically "empty," and pathogens recover faster due to their greater resilience. A more effective approach: maintain conditions (DO, temperature, ORP) and apply disinfection surgically when needed.
Three Mistakes That Cost the Most
- Regular preventive disinfection — destroys the root's microbiological defence
- Ignoring DO and temperature — fungicides without addressing the underlying cause produce only a temporary effect
- Treating any cloudiness as infection — real warning signals are: change in smell, dropping ORP, and symptoms on the roots
Control Indicators
- DO ≥ 6 mg/L
- Temperature ≤ 22°C
- ORP >600 mV, stable
- Neutral smell
- Roots white or cream-coloured with no slime