Everything you need to know about soil moisture measurement




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Measuring soil moisture is a key issue for monitoring and controlling crop development with IoT technology.

There are two basic parameters of soil moisture measurement that describe the state of water in the soil. One is soil water content or the amount of water per unit of soil, and the other is soil water potential or the energy state of water in the ground.
Although water content is useful when trying to describe the water balance of soil (i.e. how much water is moving in, out, or being stored), water potential is often preferred over water content because it determines how water moves in soil or from the soil to the plant.
In addition, you can use water potential to determine plant availability of water, schedule irrigation, or determine the mechanical stress state of the soil.
Therefore, for the general concept “soil moisture” there are 2 technical parameters:

  1. Volumetric Water Content (VWC)
  2. Soil Water Tension is also called Soil Water Potential (SWP)

Volumetric Water Content (VWC)

This concept is easy to understand. It is the percentage of water in the soil relative to the volume. To calculate it, the sensor measures in a “sphere of influence” around it, let’s assume it is 1 m³. If the result is VWC = 2%, it means that, out of that cubic meter of soil, 2% is pure water. That is, 2% of 1000 liters, which is equivalent to 20 liters. This water is obviously distributed with the soil itself, moistening it. This means that if you could squeeze these 1000 liters of soil, 20 liters of pure water would come out.

Soil Water Tension

This concept is a little more complicated to explain. It also refers to the water that is “dissolved” in the soil, but it not only gives the number of liters but also defines the degree of difficulty a plant has in absorbing this moisture. Indeed, the roots suck up the water, exerting a higher tension and fighting against the force of gravity. And this osmosis action costs them energy.

It is important to consider the tension they have to exert because there may be cases where the soil is overflowing with water… but that soil has a lot of sand. This circumstance produces that the plant can hardly suck water because the soil forces it to absorb with a lot of effort through its root, which works like a straw.

As all this is exerted by the action of pressure, the units in which the Soil Water Tension is measured are in kilo-pascals (kPa) or bars.

The composition of the soil on which crops are grown is a determining factor in the measurement of soil moisture. All soil is a combination of sand, silt and clay. Depending on this combination of materials, the quality is better or worse for certain crops. There are studies that define how the 2 parameters are related, depending on the type of soil:

Each plant species has soil preferences (e.g. 20% sand, 50% silt and 30% clay) and absorption capacities (can exert ‘X’ kPa). Most importantly, agronomists and biologists have studied, for each plant species, its comfort conditions both in terms of soil composition and absorption capacity.

Many people ask themselves the question of which parameter is better to soil moisture measurement:

  • Generally speaking, neither is better than the other. They are complementary.
  • For certain applications, one is of interest; for others, the other.
  • The conversion calculation from one parameter to the other can be done (approximately) if the soil composition is known.
  • The soil water tension parameter is more immediately applicable. However, volumetric water content sensors are more expensive. This is usually because their utility is greater.

To summarize:

Volumetric Water Content in soil
Soil Water Tension
Also known as VWC Soil Water Potential
Water potential in soil
Ground water pressure
Unity % kPa, bars
Measures “Amount” of water in soil
Availability of water in the soil
Liters
“Quality” of water in soil
Difficulty sucking
Effort / energy required for the plant
Takes into account the composition of the soil? No Yes
Requieres calibration Yes Yes
Available in Smart Agriculture PRO? No Yes
Watermark
Available in P&S! Smart Agriculture Xtreme? Yes;
GS3 (en EoL)
TEROS 11
TEROS 12
5TE (en EoL)
5TM (en EoL)
Yes
TEROS 21
(previously named as Decagon MPS-6)
Price ~200 € ~50 €

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Original article: Everything you need to know about soil moisture measurement
Author: libelium libelium