How much do you know about soil moisture monitoring?
Soil moisture sensors contribute to good irrigation management. Good irrigation management produces better crops, uses fewer inputs, and increases profitability. Soil moisture sensors help irrigators understand what’s happening in the root zone of their crops.
To be used effectively, a soil moisture sensor must:
Irrigation shifts for even water supply
Properly installed and placed in an area representative of the crop being grown
Used in conjunction with other irrigation management information (soil moisture sensors measure only a small portion of the irrigation shift):
Evaporation Based Scheduling
Soil Moisture Monitoring
Basically there are two sets of sensors:
Water potential sensors such as tensiometers and particle matrix sensors
Soil moisture sensor, which gives the percentage or relative amount of soil moisture.
water potential sensor
These sensors measure how hard it is to remove water from the soil, giving plants the best indication of available water. Soil type and moisture content will affect the suction pressure required to remove water from the soil, but a monitoring sensor that records and graphs it will show a sharp drop, indicating that water has become difficult for plants to enter.
Tension Gauge Sensor
Tension gauges are the most sensitive water potential sensors and they require the most care and maintenance. There are two types of tensiometer tips: one for sand and one for clay and loam. Use the appropriate prompts to see quick reactions to changes in water status. The soil water potential sensor is a practical method to study soil water movement from the energy point of view. The soil water potential sensor is a practical instrument that reflects soil moisture and guides irrigation. The soil water tension measured by the soil water is potential the suction force of the soil to water. The wetter the soil, the lower the suction power; otherwise, the greater the suction power. When the soil moisture increases until all voids are filled with water, the soil water tension will drop to zero.
How much do you know about moisture sensors?
These sensors use the timing or frequency of pulses traveling between electrodes or back to the electrodes to measure the moisture content of the soil. The most common types are capacitance and time or frequency domain. Most sensors are accurate to within 2-3% of actual soil moisture.
Capacitive sensors typically measure multiple depths at 10 to 20 cm intervals and are available in lengths from 40 to 180 cm. Multiple depth measurements provide useful information about the movement of water through the soil profile and the relative moisture content of the soil at different depths.
A limitation of most capacitive sensors is that they can only measure very small amounts of soil outside the pipe or wall where the sensor is inserted. Proper installation must maximize soil contact and ensure that water is not allowed to migrate preferentially down to the outside of the sensor. If this happens, the measurements will not reflect conditions in undisturbed soil far from the sensor. The measurement method means that these sensors are affected by the salt in the soil: Salt increases conductivity, which means the sensor will inaccurately overestimate soil moisture.
Time and Frequency Domain Sensors
True Time Domain Reflectometer (TDR) sensors are very accurate but require fairly complex and expensive measurement equipment. A similar, less expensive alternative is a sensor that uses a water cut reflectometer (WCR) and a time domain transmissometer (TDT) for measurement. This type of sensor usually consists of two or three metal prongs, between 5 and 30 cm in length, that are pushed into the side of a soil pit to measure undisturbed soil. The measuring range extends to approximately 3 to 6 cm around the sensor and measures larger soil volumes (0.3 to 8 liters). With proper installation into undisturbed soil and measuring larger volumes of soil, the data from these sensors will be more representative of the entire area than capacitive sensors. They are also less affected by the salt in the soil. Accurate estimation of water availability using time or frequency and capacitive sensors can only be achieved through calibration of soil tension measurements. If not calibrated, estimates of water availability rely on the interpretation of changes in the curves produced by periodic measurements and graphs.