# The fertigation unit: a tank, two pumps and one number > 400 litres of solution, dosing at 250 ml per minute and a target EC of 1.8. Why the unit looks the way it does, and what fails in it first. Source: https://naasson.com/en/blog/fertigation-tank-two-pumps-one-number/ Published: 2026-04-02 Language: en Direction: Agro by Naasson (https://naasson.com/en/agro/) Publisher: Naasson — https://naasson.com --- Fertigation is irrigation where the water carries the fertiliser. It sounds like a minor agronomic detail. In practice it is the most dangerous unit in a greenhouse: the one place where automation can ruin the whole crop in a minute, not just leave it short of water. So the unit is built to be plain and to trust nothing. ## What it consists of The pH and EC probes sit in the reservoir, not at the outlet to the plants, but in the tank where the solution is mixed. The tank holds 400 litres. Two dosing pumps deliver concentrate at 250 millilitres per minute. An ultrasonic sensor measures the level. The target electrical conductivity of the solution is 1.8 millisiemens per centimetre. The whole unit revolves around this one number. It tells you how much salt is dissolved, that is, how rich the solution is. ## Why the probe sits in the tank The temptation to put the sensor closer to the root is understandable: you want to measure what the plant actually receives. But a probe in the supply line reads an instantaneous value in the flow. It jumps when the pump starts, when an air pocket passes, when the concentrate has not mixed in yet. Regulating on that signal means chasing noise. A probe in the tank reads the state of the volume. The signal is slow, and that is exactly why you can make decisions on it. 400 litres do not change their properties in a second, so the operator has time to react before anything becomes irreversible. ## What blocks dosing The rules are short, and all of them push towards stopping, not continuing. If the pH of the solution drops below 5.0 or rises above 7.0, dosing is blocked. If EC leaves the 0.5 to 3.5 mS/cm range, dosing is blocked. If the tank level falls below 15 percent, the system advises topping up, because dosing concentrate into a nearly empty tank gives a concentration spike instead of nutrition. If the climate controller goes offline, scheduled irrigation is paused: watering without knowing temperature and humidity is a gamble, not agronomy. None of these rules tries to "fix" the situation by adding the opposite. Automatic pH correction with a counter-dose of acid or alkali is the classic way to get oscillation. The system swings the solution between two extremes and damages the plants faster than the original deviation would. ## Dosing rate as a safety fuse 250 millilitres per minute into a 400-litre tank is slow, and slow is intentional. At this rate an erroneous command leaves time to react: to shift EC significantly in 400 litres, the pump needs minutes, not seconds. A powerful pump would save those minutes in normal operation. It would also create a failure scenario in which one wrong command is irreversible. We chose the boring option. ## What fails first In our experience, not the electronics but the probes. pH and EC sensors drift, foul and need calibration, and they do it quietly: the instrument keeps returning a plausible number. That is why, in the full configuration, dosing is held by default under the digital twin engine's safety quarantine. Live chemistry does not run until the quarantine is explicitly lifted. This is inconvenient exactly once, at commissioning. It is cheaper than one dump of concentrate into the tank on the readings of a dead probe.