Soil sensors are the important tools of modern agriculture. Good irrigation management can make crops grow better, use fewer inputs, and increase profits. Soil moisture sensors help with good irrigation management. They are a useful tool to help irrigators understand what is happening in the root zone of crops.
Essentially, a Soil moisture sensor measures the quantity of water contained in media such as soil, on a volumetric or gravimetric basis.
There are two main groups of soil sensors:
soil water potential sensors, such as tensiometers and soil granular matrix sensor.
Soil moisture sensor that give a percentage or relative content of soil moisture.
Instrument Choice focuses on soil sensors that capture soil moisture, rather than water potential.
Water Potential Sensor
These soil sensors measure the difficulty of removing water from the soil, which provides an excellent indication of water availability for plants. While soil type and water content influence the amount of suction required to remove water from the soil, recording and charting measurements will highlight any major changes that indicate that water has become hard for a plant to access.
Soil Granular Matrix Sensor
Soil Granular matrix sensors are reliable, generally low cost but less accurate soil sensors. They work by passing a current across a porous media, such as gypsum, with the resulting changes in electrical resistance indicating the amount of water drawn in and out of the soil.
The main disadvantage of this these soil sensors is their accuracy. They are prone to generate readings that underestimate plant stress, particularly during conditions where soil is rewetted.
Tensiometer
Tensiometers work by measuring suction pressure at the tensiometer’s porous tip. Water is drawn out of or into the tip, depending on water availability. This in suction pressure, which indicates the relative suction force required for a plant to obtain water from the soil. Measurements can be obtained by reading a vacuum gauge, or automatically recorded via a logging pressure transducer.
Soil Water Content Sensor
Soil water content sensor measures the water content of a soil using the time or frequency of a pulse travelling between or returning to electrodes. The most common types are capacitance and time or frequency domain. Most sensors are accurate within 2–3% of the actual soil moisture.
Soil Capacitance Sensor
Soil Capacitance Sensor is usually deployed to measure several depths at intervals of 10 to 20cm and supplied in lengths from 40 to 180cm. Multiple depth measurement produces useful information on water movement through the soil profile and relative moisture content of the soil at different depths.
Soil Time and frequency Domain Sensor
There are two groups of this category of soil sensor; i) true time domain reflectometry (TDR) and ii) soil moisture sensors that measure using water content reflectometry (WCR) and time domain transmissometry (TDT). The former is a complex and more expensive technology compared to the latter.
This type of sensor generally consists of 2 or 3 metal prongs between 5 and 30cm long that are pushed into the side of a soil pit to measure the undisturbed soil. The measurement extends to about 3 to 6cm around the sensor, giving a larger volume of soil measured (0.3 to 8L).
If deployed correctly, soil time and frequency domain sensors provide measurements that are more representative of the whole area of the soil if compared to capacitance soil sensor. They are also less affected by soil salts.
Soil tension calibrations are required in order to derive accurate indications of water availability with time or frequency and capacitance soil sensors. Without calibration, estimation of water availability relies on plotting measurements over time and interpreting any changes in the resulting curve.