From soil moisture to irrigation schedule: how sensing becomes a decision

"Smart irrigation" is often described as sensors plus an app. In practice the hard part is the step in the middle: turning a number like "soil moisture 23%" into a decision a farmer can trust. Here is how that chain works.

Start with what the plant can use

Soil holds water at a range of tensions. Two reference points matter:

Field capacity — the amount of water soil holds after excess has drained away.
Permanent wilting point — the point at which plants can no longer extract water.

The difference is the plant-available water. Good scheduling irrigates before plants are stressed, usually when a fraction of that available water — often around 40–50% for many crops — has been used. The right threshold depends on crop, growth stage, and soil, which is why a single fixed number for every field does not work.

Choose the right sensor and place it well

Two common families:

Volumetric sensors (capacitance/FDR, TDR) report the percentage of water by volume. They are convenient but need calibration for the soil type; the same reading means different things in sand and clay.
Tension sensors (tensiometers, granular-matrix sensors) report how hard the plant must pull to get water. This relates more directly to plant stress across soil types.

Placement often matters more than sensor brand:

Install at several depths across the root zone, not one.
Choose representative locations — and replicate, because fields vary.
Ensure good soil contact; an air gap around the probe gives false readings.
Install near the wetted zone of the emitters for drip systems.
Record exactly where each sensor sits, so the data stays interpretable.

Add the atmosphere

Soil data says what the plant has. Weather says what it will lose. The standard approach, from the FAO's Irrigation and Drainage Paper No. 56, estimates crop water use as:

ETc = Kc × ET₀

where ET₀ is reference evapotranspiration calculated from temperature, humidity, wind, and radiation, and Kc is a crop coefficient that changes through the season. Combining it with soil readings gives a schedule that follows both the plant's demand and the field's real condition.

Forecasts add a final check: skip or reduce a cycle when meaningful rain is coming, and avoid watering ahead of a strong wind.

Close the loop with flow and pressure

A schedule only helps if water actually arrives. Flow meters and pressure sensors make irrigation verifiable:

Flow when a valve is closed → leak or stuck valve
Lower flow than expected → clogged emitters or filter
A sudden jump → burst pipe

Comparing planned with delivered water per zone also gives a clean, measurable record of what was used. Savings claims should be measured against a baseline season or a control zone, not assumed.

Use energy when it is cheap and clean

Pumping is energy. When the farm has solar generation, irrigation can be scheduled for the hours the panels produce, so food production runs on the energy the same field generates. That is a scheduling decision the platform can make once agriculture and energy data meet in one place.

Add nutrients with care

Fertigation links irrigation to nutrition. Monitoring EC (a proxy for salinity and total dissolved salts) and pH helps keep nutrient delivery in balance and reduces fertilizer waste. Note that soil EC readings move with moisture, so interpret them alongside moisture, not alone.

Say it in farmer language

What reaches the farmer should not be a chart of raw sensor values. It should be a clear recommendation with a reason:

Zone 3 — irrigate tonight, about 40 minutes. Root-zone moisture is approaching the refill point. No rain expected before Friday.

…with an easy way to confirm, adjust, or override. People trust systems that explain themselves.

What good looks like

Sensors verified against hand checks when installed
Data-quality rules that flag a stuck or drifting probe instead of acting on it
Automatic irrigation within safe limits (maximum runtime, minimum rest)
A record of what was planned, applied, and the field's response

Every drop becomes measurable. Every cycle becomes intentional.

Our planned smart irrigation network puts this chain into practice on a working farm.