Soil Moisture Sensors: Advantages and Disadvantages

This page explores the advantages and disadvantages of using moisture sensors in soil.

Definition: A moisture sensor measures the water content within the soil. Often, a probe will contain multiple sensors. These sensors are valuable for managing irrigation systems effectively, helping farmers conserve water, increase crop yields, and improve crop quality.

Figure 1 shows the SM150 moisture sensor, commonly used for soil moisture measurement.

Image alt: Moisture Sensor

As shown, the sensor consists of two sensing rods attached to a plastic body. These rods are directly inserted into the soil for testing and measurement, producing a differential analog DC voltage output.

When power is supplied to the probe, a waveform is generated and applied to the rods. These rods then transmit an electromagnetic field into the soil. The soil’s permittivity significantly affects the applied field, which the probe detects, resulting in a stable voltage output.

Types of Soil Moisture Sensors

Here’s a list of common types of soil moisture sensors:

• Gravimetric
• Watermark sensors
• Capacitance sensors
• Tensiometers

Each of these methods has its own strengths and weaknesses.

Advantages of Moisture Sensors for Soil

Here are some key benefits of using moisture sensors in soil:

• Simple Measurement: The measurement process is relatively straightforward.
• Immediate Results: Sensors deliver results quickly.
• Low Cost (Watermark & Tensiometers): Watermark sensors and tensiometers are generally inexpensive.
• Accurate Results: Sensors provide accurate readings of moisture levels.
• Large Reading Range (Watermark Sensors): Watermark sensors offer a wide moisture reading range, typically from 0 to 200 cb or kpa.

Disadvantages of Moisture Sensors for Soil

Here are the drawbacks to consider when using soil moisture sensors:

• Site-Specific Evaluation Required: Initial evaluation of site-specific conditions is necessary before choosing the appropriate sensor.
• Probe Insertion Required: The probe needs to be physically inserted into the soil. This requires labor to collect data and maintain the measurement process.
• Material Dependency: Measured values depend on the properties of the soil. Correct interpretation and use of the moisture data are crucial.
• Reduced Accuracy in Sandy Soils (Watermark Sensors): Watermark sensors are less accurate in sandy soils due to the large particle size.
• Calibration Required (Watermark Sensors): Watermark sensors need to be calibrated for each soil type. Tensiometers also require periodic servicing.