Irrigation by Drain EC and pH: Feedback from the Substrate

Drainage is not waste. It is feedback from the substrate — showing what is happening inside where you cannot see.

Key Definitions

• Drain EC — electrical conductivity of the liquid leaving the substrate after irrigation; indicates the salt concentration remaining in the root zone
• Drain pH — acidity of the drain liquid; differs from input pH due to chemical reactions between nutrients and the substrate
• Irrigation feedback — using drain EC and pH readings to adjust irrigation schedules and nutrient recipes

What Drain EC Tells You and What to Do

Drain EC lower than feed EC (e.g. feed 2.0, drain 1.4):

• Plants are absorbing more salts than are being supplied
• Nutrients are being depleted
• Action: raise feed EC by 0.2–0.3 mS/cm; re-measure after 2–3 days

Drain EC close to feed EC (±0.3–0.5 mS/cm):

• Normal balance

Drain EC significantly above feed EC (feed 2.0, drain 3.5–5.0):

• Salt accumulation in the substrate
• Causes: insufficient drain percentage, excessive feed EC, plant stress, or overcast weather
• Action: increase irrigation volume to achieve 20–30% drainage; lower feed EC; or flush the substrate

Safe drain EC range: no more than 0.5–1.0 mS/cm above feed EC under stable conditions.

What Drain pH Tells You

Drain pH higher than feed pH (e.g. feed 5.8, drain 6.5–7.0):

• Substrate is becoming alkaline
• Common in new coco substrate or with hard water
• If drain pH exceeds 7.0: iron and manganese become locked out
• Action: lower input pH to 5.5–5.6 until drain stabilises to 5.8–6.5, or flush the substrate

Drain pH lower than feed pH (e.g. feed 6.0, drain 5.2–5.4):

• Substrate is becoming acidic
• Caused by accumulation of organic acids from root exudates
• If drain pH drops below 5.2: calcium and phosphorus become locked out
• Action: raise input pH to 6.2–6.5 until drain reaches the target range

Target drain pH: 5.8–6.5 regardless of substrate type.

When and How to Measure Drainage

Frequency: at least once per week during stable production; daily for 3–5 days when making changes.

Timing: not immediately after the first morning irrigation; collect samples after the 2nd or 3rd irrigation of the day.

Sampling points: for up to 20 plants — 3–5 sample slabs; for larger areas — 10% of total slabs.

Three Mistakes That Cost the Most

1. Not measuring drainage at all — trusting a "set and forget" approach
2. Measuring immediately after the first morning irrigation and treating the result as accurate
3. Adjusting feed EC or pH daily in response to every drain deviation — the substrate responds slowly

Signs of Correct Drain-Based Irrigation Management

Drain EC remains consistently within ±0.5–1.0 mS/cm of feed EC over the course of a week. Drain pH stays in the 5.8–6.5 range without large fluctuations. Irrigation adjustments are made based on data, not instinct.