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. Very similar to the way water is held inside 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 in which water is held within the pores.
Sensors that measure the tension in the soil, measure the actual force that has to be applied in order to move water in the 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.
These 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 tight in the small soil pores.
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.
For a sandy soil, the water retention of which is given above:
Wilting point: 0.04 (4%)
Field capacity: 0.06 (6%)
Available water: 0.06 – 0.04 = 0.02 (2%).
For the silt loam soil:
Wilting point: 0.07 (7%)
Field capacity: 0.35 (35%)
Available water: 0.35 – 0.07 = 0.28 (28%)
And for the clay soil:
Wilting point: 0.18 (18%)
Field capacity: 0.45 (45%)
Available water: 0.45 – 0.18 = 0.27 (27%)
SO WHICH IS BETTER?
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 has to be calibrated to the soil type or, even better, specifically to the water retention curve of the soil. We can therefore conclude that measuring the tension in the soil is a direct measurement of the matric potential and can be used for determining the irrigation schedule, while measuring the volumetric soil moisture is an indirect measurement.
The common practice is to start irrigation when up to 50% of the available water was depleted.