ABC of moisture sensors in irrigation work
There are sensors that determine the amount of water in the soil and support the grower to define the right time for irrigation. Edgar Hincapié, a researcher at Cenicaña, resolves some concerns about this technology.
What are soil moisture sensors for?
Humidity sensors measure an electrical signal that calculates the amount of water in the soil, which allows determining the volume of water stored in the soil after irrigation or rain, calculating the consumption of water by the crop in one day or one week and determine the efficiency of irrigation.
For example, an average humidity value of 28.5% measured between 0 and 20 cm depth indicates that in that 20 cm layer of soil there is a 57 mm (28.5 x 20/10) sheet, but does not indicate if they are available or not for plants.
Humidity sensors are different from those with matric potential, because what determines the latter is the water in the soil available to the plant.
How do humidity sensors operate?
Humidity sensors generally consist of a probe with electrodes at one end and an information capture and excitation system. The electrodes are inserted into the ground and their length ranges from 5 to 30 cm.
Humidity sensors do not directly measure this variable, but rely on techniques such as time domain reflectometry (TDR), capacitance (FDR), and apparent electrical conductivity (CE), which are measurements that must be converted to values of volumetric humidity by means of a calibration. They are commonly known as TDR or FDR humidity sensors.
How does this information serve to schedule irrigation?
When measuring soil moisture to program irrigation, the optimum range of humidity in which the plants are not affected by excess or deficit of water must be known. This information is specific for each type of soil and corresponds to the humidity value at a matric potential between -20 kPa and -75 kPa.
In this sense, for a matric potential of -75 kPa, which is the threshold value for the start of irrigation, the humidity for a sandy soil is 12%; for clay soil, 28%; and for a clay loam, approximately 33%.
How are they installed?
As with the matric potential sensors, the number of humidity sensors to be installed depends on the way of taking the measurements (manually or automated).
The depth of installation of the sensors depends on the length of the electrodes and the direction of placement. For example, a sensor with electrodes 20 cm long installed vertically from the surface measures the average humidity between 0 and 20 cm deep. Similarly, when a sensor is installed vertically at 20 cm depth, the average humidity is measured between 20 and 40 cm.
If the sensors are installed horizontally, the average humidity is measured at the depth they are installed: a 20 cm long sensor installed horizontally at 30 cm, measures the average humidity at that depth of the ground.
Installation is simple and quick: dig a hole with auger or tap to the required depth, insert the sensor vertically or horizontally, plug the hole again, and compact it slightly to avoid preferential water flow on site.
Is calibration necessary?
Yes, because they do not directly measure the humidity variable, but they measure other variables that are correlated with the humidity of the soil. The measurement equipment comes with a generic calibration curve, but if more precise measurements are required, such as those required for irrigation control, it is necessary to build calibration curves for each soil. Calibration is usually done in a soil physics laboratory.
What are the advantages and limitations of using this technology?
• Allows measurements with little alteration of the soil.
• Requires little maintenance.
• Measurements can be made continuously over several crop cycles.
• Does not require special facilities.
• Several measurements are required for good precision due to the variability of the soils.
• High cost of equipment.
The article is available at www.cenicana.org: Keys to use sensors in irrigation work which explains which sensors to use, how to install them and how to interpret the data.
On page 8 of this edition you can see the training program for the second semester of 2018, which includes theoretical and practical courses on: Use of sensors for irrigation scheduling.