Soil Moisture, EC and pH Sensors: What Each One Measures and When You Need All Three

Growers buy soil moisture sensors first, EC sensors second, and pH sensors when something has already gone wrong.

That order makes sense, but it means most people learn what EC and pH actually tell them after a season they'd rather have back. The three measure completely different things, and only one of them tells you when to irrigate.

This guide explains what each sensor reads, what it can't tell you, and how to decide whether you need one, two or all three.

The three properties, side by side

The right-hand column matters more than people expect. Moisture changes hour to hour, so it needs continuous logging. pH moves slowly, so a reading every few hours is plenty. That difference shapes how you deploy them.

Soil moisture: the one everybody needs

A soil moisture probe measures how much water is held in the soil around it, usually expressed as volumetric water content.

It answers the only irrigation question that matters: is there enough water in the root zone right now? Scheduling by calendar or by feel both overwater, and overwatering costs twice, once in water and again in leached fertiliser.

Most soil moisture probes log temperature alongside moisture, which is more useful than it sounds. Root zone temperature drives germination timing and nutrient uptake rate, and it lags air temperature by enough that guessing from a weather station gets it wrong.

If you buy one soil sensor, buy this one.

EC: what it tells you and what it doesn't

Electrical conductivity measures how well the soil solution conducts current, which comes down to how many dissolved salts are in it.

What EC tells you: whether the total salt load in your root zone is rising, falling or holding steady. That covers fertiliser accumulation, salt buildup from irrigation water, and leaching effectiveness after a flush.

What EC does not tell you: which salts. This is the misconception worth clearing up, because it costs people money.

An EC sensor cannot report nitrogen, phosphorus or potassium separately. It reports the total. A soil at 2.0 mS/cm might be perfectly fertilised or badly salted, and EC alone can't distinguish them. If a product promises NPK readings from a conductivity probe, treat the claim carefully.

Used correctly, EC is excellent at trends. Rising EC with stable irrigation means salts are accumulating and a flush is due. Falling EC after fertigation means you're leaching nutrients past the root zone. Neither of those is visible in a moisture reading.

A combined soil EC probe that also reads moisture and temperature is usually the better buy over a standalone EC sensor, because EC is meaningless without knowing the moisture content it was measured at. Wet soil conducts better than dry soil at identical salt levels.

pH: the one that decides whether nutrients work at all

Soil pH doesn't feed the plant. It decides which nutrients the plant can reach.

Iron, manganese and phosphorus all become progressively unavailable as pH climbs. Push past 7.5 and a crop can show iron deficiency in soil that contains plenty of iron. Drop below 5.5 and aluminium toxicity becomes the problem instead.

This is why pH problems look like nutrient problems. The fertiliser is there. The plant can't get to it. Growers who add more fertiliser in response make the EC problem worse without fixing anything.

A soil pH probe covering roughly 3 to 9 pH at ±0.3 accuracy is enough resolution for agricultural decisions. You're looking for drift across weeks, not third-decimal precision.

Two practical notes. pH sensors need periodic calibration, unlike moisture probes which mostly don't. And amendments act slowly, so a lime application shows up over months rather than days. Log it continuously and you'll see the correction working long before a lab test would confirm it.

Do you need all three?

The pattern: the more control you have over inputs, the more the other two matter. A rain-fed field grower can't adjust much beyond timing. A greenhouse operator controls every input, which means every input can drift.

A reasonable sequence for most growers is moisture first, add EC when you start fertigating, add pH when you're either seeing symptoms you can't explain or working in a substrate where pH moves quickly.

Placement and depth

Sensor position affects the reading more than sensor accuracy does.

Depth. Put the probe in the active root zone, not at the surface. For most row crops that's 10 to 30cm. If you're running two depths, 10cm shows irrigation response and 30cm shows whether water is reaching the deeper roots.

Horizontal position. Between plants in the row, not in the wheel track and not directly against a stem. Compacted soil and root-dense soil both read differently from the field average.

Contact. Backfill firmly. An air gap around the probe gives you a reading of the air gap.

Replication. One sensor tells you about one spot. Soil varies enormously across a field, so if you're making decisions worth real money, install at two or three representative positions rather than trusting a single point.

One constraint worth knowing before you plan the layout: a logger reading multiple probes on one bus needs each probe to be a different type. Three identical moisture probes across a field means three loggers, not one splitter.

Start with moisture, add the rest as the questions get sharper

Most growers get the biggest return from the first sensor and the sharpest insight from the third. Moisture tells you when to irrigate. EC tells you whether your fertiliser is accumulating or leaching. pH tells you whether any of it can be absorbed.

Browse the full range in external sensors to compare probes, or start from a matched configuration in bundles if you'd rather not spec the logger separately.

Not sure which of the three your situation calls for? Tell us what you're growing and how you're irrigating at UbiBot USA, and we'll tell you which sensor answers your question, including when the answer is that you only need one.

FAQs

Can an EC sensor measure NPK? 

No. EC measures total dissolved salts, not individual nutrients. It shows whether the salt load is rising or falling, which is genuinely useful, but it can't separate nitrogen from potassium. Nutrient-specific analysis still needs a lab.

How deep should soil sensors be installed? 

In the active root zone, typically 10 to 30cm for row crops. Two depths give you more than one: a shallow probe shows irrigation response, a deeper one shows whether water is reaching the lower roots.

How often do soil pH sensors need calibrating? 

Periodically, and more often than moisture probes, which mostly need none. Check the manufacturer's interval and calibrate before any season where pH decisions matter.

Why does my EC reading change when the soil dries out? 

Because conductivity depends on water content as well as salt content. Wet soil conducts better than dry soil at the same salt level, which is why EC readings are far more useful alongside a moisture reading from the same point.

Can I use one probe for moisture, EC and temperature? 

Yes. Combined probes read all three from one insertion point, which is usually better than separate sensors because the readings come from identical soil conditions.

How many soil sensors does one field need? 

At least two or three at representative positions if the data drives spending decisions. Soil varies across a field, and a single probe describes one spot rather than the block.