Irrigation Management with Soil Sensors: Considerations for Choosing the Best Method
December 9, 2019 There are two main types of soil sensors used in agriculture to measure the availability of water to plants.
- Sensors that measure soil moisture.
- Sensors that measure the water tension in the soil (tensiometers).
What’s the difference between these two methods and is one better than the other? In order to understand that, let’s examine what determines the ability of plants to absorb water from the soil. The water in soil is held in soil pores – the spaces between soil particles. This is very similar to the way water is held inside of a sponge.
Smaller pores can retain water more tightly than bigger ones, due to stronger capillary forces. In order to absorb the water, plants must overcome the force by which water is held within the pores.
Sensors that measure the tension in the soil measure the actual force that must be applied in order to remove water from soil, while sensors that measure soil moisture measure the volumetric water content (θ) in the soil – the percentage of the soil volume that is occupied by water.
The tension and the moisture are two related parameters. Under the same tension, a soil with larger pores will hold less water than a soil with smaller pores. This means that the volumetric water moisture of a soil can be relatively high, while a large portion of this water is not available to the plant, because it is held too tightly in the small soil pores.
Soil moisture sensors include TDR sensors (Time domain reflectometry), ADR sensors (amplitude domain reflectometry), and neutron probes. The last ones are considered the most reliable method for measuring soil moisture. However, their cost is high, and they require a certified operator.
Soil Moisture Status
Three important terms are used associated with the water status in the soil: field capacity, wilting point, and available water. Field capacity is the water content of the soil after excess water has stopped draining after irrigation or rain. It is estimated to be the water content in the soil at a matric potential (Ψ) of -1/3 bars.
Permanent wilting point is the soil water content below which plants wilt and cannot recover. It is estimated to be the water content in the soil at a matric potential of -15 bars.
The available water is defined as the amount of water in the soil between wilting point and field capacity.
The relationship between the moisture content in the soil and the matric potential is described in the water retention curve. An example for such curves is given in the following figure:
From the above curve we can see, for example, that at field capacity, the volumetric water content of the clay soil is about 44%, while for the sandy soil it is only 6%.
Which sensor is best for you?
It is clear from the above that in order to properly interpret the readings of a soil sensor that measures the volumetric water content of the soil, the reading must be calibrated to the soil type where the sensor is installed.
Tensiometers determine matric potential directly, while volumetric soil moisture sensors provide an indirect measurement.
However, when the soil type is known, soil moisture sensors can provide an instant reading that can be transmitted in real-time to a software or an app, while this is more difficult to achieve with tensiometers.
For example, CropX Technologies has developed an ADR soil moisture sensor that auto-calibrates to pre-integrated soil data and reads soil moisture at three different depths. The sensor sends the data to a mobile app, which also provides irrigation prescriptions.
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