Soil Water Basics

Measuring Soil Water

Soil water measurement falls into two broad categories:

The amount of water in the soil is less important than how difficult it is for the plant to extract it. Soil water tension — or matric potential — is what must be overcome for a plant to move water into its root system. Different soil types will have different tensions at the same volumetric water content, making volumetric data relative to local conditions and often requiring site-specific calibration.

Because IRROMETER sensors measure tension directly, no site calibration is required. And because tensiometers have been used in research since the 1920s and commercially available since 1951, decades of university and extension research have produced a well-established body of reference data for recommended tension levels across common crops and landscapes.

Sensor Options

We offer two methods for reading soil water tension, each suited to different conditions and applications:

IRROMETER Tensiometer

The tensiometer is the only direct measurement instrument available — measuring the physical forces at work in the soil rather than indirectly measuring an electrical property. It acts like an artificial root, interacting with soil through a ceramic tip. As soil dries, it draws water from inside the instrument through the tip, creating a measurable tension read by a mechanical gauge or transducer.

This is the most direct and accurate method available. Periodic maintenance is required to replenish water, and instruments should be removed during winter months to avoid freezing damage.

IRROMETER product details →

WATERMARK Sensor

The WATERMARK is a "Granular Matrix Sensor" that electronically reads moisture absorbed through a precisely composed granular matrix. This material buffers the sensor against salinity effects and provides a much longer service life than traditional gypsum blocks. WATERMARK readings are calibrated to the same values measured by a tensiometer.

Maintenance-free with an expected life of 5+ years, WATERMARK sensors can be left in the ground permanently. They require minimal power and are well-suited for data loggers and IoT devices.

WATERMARK product details →

Selecting the Right Sensor

The following table presents the general characteristics and suitability for each sensor type based on soil conditions and crop sensitivity:

Selection can also be made by referencing the traditional soil texture triangle:

Sensor selection by soil texture — IRROMETER SR and LT for lighter soils, WATERMARK for medium to heavy.

Understanding the Data

When using IRROMETER instruments or WATERMARK sensors, use these readings as general guidelines:

0–10 Centibars Saturated soil

10–30 Centibars Soil is adequately wet (except coarse sands, which are drying)

30–60 Centibars Usual range for initiating irrigation — most soils

60–100 Centibars Usual range for irrigation in heavy clay soils

100–200 Centibars Soil is becoming dangerously dry — proceed with caution

Gauge face interpretation guide — suggested management ranges by crop type

Taking time to interpret your data more deeply will yield more specific irrigation thresholds. A few key concepts:

Installing and Preparing Sensors

If sensors aren't in the wetted area where roots are extracting water, the data will have limited value. Proper placement is critical.

Determining Management Thresholds

Thresholds are reference points identifying the upper and lower boundaries for managing allowable depletion. A typical starting point is to manage between 10% (wet end) and 40–50% (dry end) of available water depletion, though this varies by soil type, crop, plant development stage, and irrigation method.

Soil types vary in their capacity to hold water, so available water corresponds to a different tension range for each soil type. For example, 50% depletion in pure loam equals 84 centibars, while in a sandy loam it equals 40 centibars. The chart at right displays these relationships and provides a visual guide for selecting appropriate thresholds.

How to read the chart:

Draw a vertical line from 10% available water depletion (the blue/green boundary) down to your soil type's curve, then horizontally to the left axis to get the WET threshold value. For loam, this is approximately 23 cb (blue arrow).

Repeat from 50% depletion (the green/brown boundary) to get the DRY threshold value. For loam, this is approximately 84 cb (brown arrow).

Note: Irrigation methods applying smaller volumes per cycle (center pivots, drip) typically favor the 30–40% range rather than 50%.

A threshold calculator is available based on this chart — select your soil type, enter an allowable depletion level, and input sensor values to determine whether irrigation is recommended.

There is no substitute for experience and agronomic knowledge. Consult a crop consultant, farm adviser, NRCS, or extension agent for site-specific soil moisture management guidance.

Available water depletion curves by soil texture — tension readings at a given depletion level vary significantly by soil type

Suggested Placement Depths

Based on deep, well-drained soils. In lighter or shallow soils, place instruments accordingly or at an angle. With drip or trickle irrigation, 12" and 24" depths are recommended, with an added 36" instrument for deeply rooted crops.

Additional Research