Measuring for Crop Steering: VWC, EC and Runoff
Crop steering is only as good as the measurements behind it. A misplaced sensor or an unmeasured emitter will steer the room in the wrong direction with complete confidence.
What to measure
| Measurement | Why it matters | How |
|---|---|---|
| Substrate VWC | The core steering signal: dry-back, shot response, field capacity | Substrate sensor logging every few minutes |
| Substrate EC | Shows salt building up or being flushed | Substrate sensor or pore-water extraction |
| Input and runoff volume | Confirms how much water went in and how much came out | Measure delivered volume per emitter; collect runoff from a test pot or tray |
| Runoff EC and pH | Shows what the root zone is holding | Handheld meter, same sampling routine every time |
| Lights-on and lights-off times | Every phase is timed from them | Controller schedule, checked against a light sensor |
| Air temperature, RH, VPD | Sets how fast plants use water | Climate sensor at canopy height |
| Plants | The only proof steering worked | Weekly height, internode length, stem thickness, dry weight at harvest |
Sensor placement
- Place sensors in rooted, representative substrate. Avoid plug holes, the wet spot directly under an emitter, slab seams and drainage gaps.
- Push probes in straight and do not wiggle them. Air gaps around the rods give false low readings.
- Use more than one sensor per zone if you can: near the start and far end of the irrigation line, at the edge and in the middle of the room.
- Keep each sensor's data separate. An average can hide one pot drowning and another drying out.
- Cross-check against pot weight and a handheld meter every few weeks.
Find your field capacity
Field capacity is the VWC after the substrate has been fully saturated and has stopped draining. Every steering target is set relative to it, and it differs by substrate, pot size and how tightly the substrate is packed.
- Saturate the substrate evenly.
- Let it drain freely until runoff stops.
- Record the VWC reading and the pot weight. That is your field capacity.
- Repeat after roots have established. Roots, settling and salt change how the substrate holds water.
Calculate shot size
Shot size is written as a percentage of substrate volume. A 3% shot into a 4 L pot is 120 mL.
shot time (s) = substrate volume (mL) × shot % ÷ 100 ÷ emitter flow (mL/s) ÷ emitters per pot
Example: a 4 L pot with two 2 L/h emitters (0.556 mL/s each), 3% shot: 4000 × 0.03 ÷ 0.556 ÷ 2 = about 108 seconds.
Use measured emitter flow, not the number on the packet. Then check the real VWC rise after a few shots. If a 3% shot gives a 1.5-point rise, the substrate volume, the flow or the sensor placement is wrong.
Emitter uniformity
- Catch the output of several emitters for a fixed time and compare. Emitters at the end of a line often deliver less.
- Pressure-compensating emitters help when lines are long or not level.
- Check for clogged stakes, leaks and siphoning after the pump stops.
- Measure the shortest valve opening that still gives a steady flow. Very short shots may never reach full pressure.
Four kinds of EC
EC numbers in crop steering guides can refer to different things. They are not interchangeable.
| Type | What it measures |
|---|---|
| Input EC | The nutrient solution at the dripper |
| Runoff EC | The water draining out; lowered by dilution after large shots |
| Bulk EC | What a substrate sensor reads directly; depends on water content |
| Pore-water EC | The EC of the water around the roots, calculated or extracted |
Pore-water EC is usually higher than bulk EC for the same root zone, and both change with temperature and water content. When comparing numbers from a guide, check which one it means.
Reading runoff
- High runoff while VWC barely rises points to channeling or dry, hard-to-wet substrate.
- Low runoff with rising substrate EC means salt is accumulating.
- A lower runoff EC after a large shot does not mean fewer nutrients were lost. More water carried them out at a lower concentration.
- Sample the same way every time. The first runoff of the day and a pooled daily sample give different numbers.
Coco coir vs rockwool
Both steer well because they drain freely and give clear sensor readings. Rockwool has a consistent field capacity and responds quickly to shots. Coco is more forgiving of small mistakes. Dry-back speed depends on substrate, pot size and plant size, so set targets from your own measurements rather than copying numbers between the two. Mixes with large chunks or air pockets make sensor readings harder to trust. Smaller pots allow faster dry-backs and finer steering but leave less room for error if a pump or valve fails.