💧 Mastering Irrigation in Controlled Environments: Substrates, Water, EC, and Crop Steering Strategies

In controlled environment agriculture (CEA), managing water and nutrients is far more complex than it seems. It’s not just about watering at the right time — it’s a science rooted in fundamental physical principles and precise measurement tools. This article breaks down the key concepts you need to optimize your grow, whether you’re a beginner or a seasoned expert.

🌱 What is the real purpose of a substrate?

In soilless systems, the substrate is not meant to feed the plant directly. It has three main roles:

1. Support the plant structure

2. Provide water and nutrients (via the nutrient solution)

3. Ensure oxygen supply to the roots

That third point is often overlooked — yet oxygen deficiency severely limits nutrient uptake and encourages pathogens.

🔁 SPAC: Understanding water movement

Water moves through an energy gradient — from the substrate, through the plant, and into the atmosphere. This is known as the Soil-Plant-Atmosphere Continuum (SPAC).

Water flow is driven by water potential, measured in kilopascals (kPa). The lower (more negative) the water potential, the harder it is for the plant to access the water.

Water potential is made up of three forces:

• Matric potential – how tightly water is held in the substrate (capillary action)

• Osmotic potential – affected by salt concentration (more salts = less available water)

• Gravity – especially relevant in coarse substrates

Total water potential = Matric + Osmotic + Gravitational

🧪 How substrate properties affect irrigation

Each substrate behaves differently:

• Silt loam holds water tightly but has poor aeration

• Coco offers a balance between retention and air space

• Rockwool releases water easily, offers high aeration, but is very sensitive to irrigation patterns

In rockwool, nearly all the water is instantly available to the plant — so even slight drying can be critical. By contrast, in soil, plants may show stress even with 25% water content still present.

👉 In porous substrates like rockwool or coco, visible signs of water stress come too late — if you can see it, it’s already a problem.

🌊 The role of dryback

Dryback — controlled drying of the substrate — is a key technique for crop steering.

The principle: let the plant consume water, causing salt concentration to rise, which lowers osmotic potential.

➡️ This stress pushes the plant toward generative growth (flowering) instead of vegetative growth (leaves, stems).

Important: It’s not the amount of water that stresses the plant — it’s the concentration of salts, measured via electrical conductivity (EC).

🔌 EC: the key metric to monitor

• EC measures the salt content in the substrate water

• Higher EC = lower osmotic potential = increased water stress

• Simple conversion: –40 kPa per dS/m

Example: 2.5 dS/m ≈ –100 kPa

It’s critical to distinguish:

• Bulk EC – overall EC in the substrate

• Pore water EC – the EC “seen” by the roots (usually higher)

To estimate nutrient availability (in kg/m³), multiply pore water EC by the substrate water content.

📉 Channelling: when water escapes

In coarse substrates like rockwool, water flows mostly downward due to gravity — skipping upper roots.

👉 Solution: apply small irrigation shots with pauses, allowing water to spread evenly.

Recommended shot sizes:

• 3.3% of pot volume during vegetative stage

• Less than 10% during flowering

• Ideal pause between shots: 15 minutes

🔍 Field capacity and drain management

Field capacity = how much water the substrate holds after gravity drainage.

👉 Water applied beyond this point drains straight out.

By monitoring when runoff starts, you can calculate how much the plant has consumed.

Leaching fraction = drained volume ÷ irrigated volume

→ Helps manage root zone EC and prevent salt buildup.

🌬️ Oxygen: invisible but essential

Oxygen diffuses more efficiently in aerated substrates (rockwool >>> silt loam).

But the wetter the substrate, the slower the oxygen diffusion.

💡 Keeping the root zone well-aerated is critical for preventing disease and promoting growth.

🧠 Conclusion: steering means anticipating

Irrigation in CEA is a matter of precision, anticipation, and strategy.

It’s not “more water = better” or “less water = more flowers”.

It’s about balancing water, nutrients, oxygen, and controlled stress.

And it requires reliable sensors, accurate calculations, and a real understanding of substrate behavior.

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📍Want to learn how to implement an effective dryback strategy, steer your crops like a pro, or finally understand the real signals to watch for?

👉 Join Botafarm’s personalized coaching — private access, in-depth training, and strategic one-on-one guidance.