Runoff EC Too High or Too Low? How to Interpret It Without Guessing

What Is Runoff EC?

The runoff EC (also called drain EC) is the electrical conductivity of the nutrient solution that exits the bottom of the pot after watering. It reveals how much salt has accumulated in the substrate and whether the plant is actually absorbing the nutrients being supplied.

The key metric is the delta EC — the difference between the input EC (what goes in) and the runoff EC (what comes out):

Why runoff EC matters more than input EC alone

The input EC only shows what you are offering the plant. The runoff EC shows what is actually happening in the substrate. Two identical input EC values can produce entirely different root zone conditions — depending on drain fraction, transpiration rate, substrate type, and plant age.

Runoff EC Target Values by Growth Phase

The following values apply to coco/perlite substrates. In rockwool or pure hydro, tolerances may be tighter.

Runoff EC Too High — Causes and Corrective Steps

A runoff EC that sits significantly above the input (delta > 0.5 mS/cm) signals salt accumulation in the substrate. The most common causes:

Causes

• Insufficient drain: If less than 10-15% of the watering volume exits as drain, salts are not being flushed out adequately.
• Input EC too high: The plant cannot absorb all the nutrients being supplied — the remainder accumulates.
• Evaporation at the substrate surface: Especially in warm, dry environments, nutrient solution wicks upward through capillary action and leaves salt crusts behind.
• Declining plant activity: Under stress (root issues, heat stress, insufficient light), nutrient uptake drops — salts remain in the substrate.
• Frequent watering without drain: Watering often with small volumes and negligible drain is a classic recipe for salt buildup.

Corrective Steps

1. Measure current status Measure the runoff EC during the next regular watering. Record the input EC and runoff EC, and calculate the delta. Also measure the runoff pH.
2. Increase drain fraction Raise the drain to 25-30% for the next 2-3 waterings. This passively flushes out excess salts without stressing the plant.
3. Targeted flush if needed If the delta is still above 0.5 mS/cm after 2 days: flush with pH-adjusted water (pH 5.8-6.0) at EC 0.4-0.6 until the runoff EC drops below the target value.
4. Adjust input EC After the flush, resume the regular nutrient solution at a slightly reduced EC (0.2 below the previous value). Monitor runoff EC daily.

Runoff EC Too Low — Causes and Corrective Steps

A runoff EC below the input EC (negative delta) means the plant is pulling more nutrients from the substrate than are being supplied. This is not a problem in the short term, but over time it points to underfeeding.

Causes

• Input EC too low: The plant has a bigger appetite than you might expect — especially during the stretch phase and full flower.
• High transpiration rate: Under high VPD and strong light, nutrient uptake increases disproportionately.
• Overwatering: Excess water dilutes the nutrients in the substrate without the plant absorbing more.
• Fresh coco: Unbuffered coco binds calcium and magnesium from the nutrient solution through cation exchange, which lowers the runoff EC.

Corrective Steps

• Increase input EC gradually — by 0.1-0.2 mS/cm per watering cycle, no more.
• Maintain drain fraction at 15-20% — do not reduce it even if the runoff EC is low.
• For fresh coco: buffer the substrate before planting with a CalMag solution (EC 0.8-1.0).
• Check VPD — at values above 1.5 kPa, transpiration rises sharply and can amplify nutrient stress.

Flush Strategy: When, How, and How Much

A flush is not a cure-all but a targeted intervention. When done correctly, it lowers the salt concentration in the root zone without stressing the plant.

When to flush

• Delta EC persistently above 0.5 mS/cm (across 2-3 measurements)
• Visible symptoms: leaf tip burn, chlorosis, stunted growth
• Before harvest (final 7-14 days, depending on genetics and substrate)
• After a nutrient lockout (pH-related)

Flush Protocol

1. Prepare the flush solution pH-adjusted water (5.8-6.0) with a light base feed (EC 0.3-0.6). Never use pure RO water.
2. Determine volume Use 2-3 times the pot volume. For a 10-litre pot, that means 20-30 litres. Pour slowly — do not flood.
3. Measure runoff during the flush Check the EC every 5 litres. Stop once the runoff EC drops below your target value (typically below 1.0 in veg, below 1.5 in flower).
4. Resume nutrient solution After the flush, wait 4-6 hours, then continue with the regular nutrient solution at a slightly reduced EC. Check the runoff EC the following day.

Tracking runoff EC over multiple days: trend analysis

A single runoff EC reading is often misleading. Much more revealing is the trend over 3–5 days. This shows whether salts are accumulating in the substrate, the plant is undersupplied, or the system is in balance.

What the trends mean

Practical trend tracking

Ideal: measure runoff EC daily at the same time and record values (e.g., in a table). After 3–5 measurements, the trend becomes clear:

• Example (rising salt concentration):
  Day 1: Input 1.2, Runoff 1.3 (Delta +0.1)
  Day 2: Input 1.2, Runoff 1.5 (Delta +0.3)
  Day 3: Input 1.2, Runoff 1.7 (Delta +0.5) → Action: increase drain
• Example (emerging deficit):
  Day 1: Input 1.2, Runoff 1.2 (Delta 0)
  Day 2: Input 1.2, Runoff 1.0 (Delta –0.2)
  Day 3: Input 1.2, Runoff 0.8 (Delta –0.4) → Action: increase input EC

Runoff volume: how much drainage is optimal?

Often overlooked: not only the EC value matters, but also the amount of drain per watering. The rule of thumb: 15–25% runoff is the target in coco systems.

What runoff volume means

Practical calculation

Example with a 10-litre pot:

• Target runoff (20%): 10L × 0.20 = 2L runoff
  → You need to pour roughly 10–12L nutrient solution to generate 2L drain.
• Current runoff only 5%: 10L × 0.05 = 0.5L
  → Increase volume to 11–12L to reach 20%.
• Current runoff 30%: 10L × 0.30 = 3L
  → Reduce volume to 7–8L to reach 20%.