[EN] Soil humidity and salinity. Help documentation

1. What do soil moisture sensors actually measure?

Currently there are no sensors that directly measure how much moisture, or water, is contained in the soil. Instead, the measurement technology estimates soil moisture based on the electrical properties of the water, solids, and air that make up the substrate or medium in which it is being measured.

Typically these sensors measure the dielectric permittivity of the soil. This property indicates how much load a soil can hold. A soil with a higher water content could potentially store more carrying capacity.

The dielectric permittivity has a range of values ​​from 1 (air) to 80 (pure water). For example, an air-dried soil has a value around 5, while a saturated soil has a value around 50. That is, between pure water and air, for different cases of soil composition (mixture of mineral elements , water, salts and others) there is a gradient in the dielectric permittivity values ​​obtained.

Once a value of the dielectric permittivity of the medium being evaluated is obtained, a% volumetric water content (% VWC) is obtained by means of a calibration equation. In the case of CERES sensors, the Topp equation is used.

This calibration is carried out in the laboratory and it is not necessary to recalibrate the sensors later regardless of the type of soil in which they are installed. The Topp equation shows excellent results in most mineral soils.

2. How do we measure the dielectric permittivity of the soil?

There are different principles of operation or technology for measuring dielectric permittivity. The main differences lie in the accuracy of the measurement and in the cost of manufacture, although they are also related to the durability of the sensor. The following are shown as a summary table:

All this type of sensor uses the permittivity and the Topp equation, in some way, to get the relationship of soil moisture with respect to it.

3. Salinity and Electrical Conductivity (EC)

Electrical conductivity (EC) is the most common measure of soil salinity and is indicative of the ability of an aqueous medium or solution to conduct an electrical current. Soil salinity refers to dissolved salts such as sodium chloride, calcium chloride, and magnesium chloride. The salts can not only be chlorides, but also carbonates. Fertilizers such as nitrates do not have strong conductivities, therefore the EC measured in a soil is mainly attributed to sodium.

In fertilization, many of the nutrients used are salts, a source of salinity. Aspects such as nutrient build-up, poor drainage, and / or saltwater intrusion can lead to the unwanted effects of salinity build-up in the soil. On the other hand, a high EC can affect humidity readings and create errors with sensors based on capacitances (charge times and frequency).

Salt, or specifically the ions dissolved in solution, is the main component of the soil matrix that conducts electricity. While EC is highly dependent on the salinity level of the soil, it will also rise and fall with soil moisture.

The effect of EC on the moisture available to the roots of a plant is very large. The accumulation of salinity in the soil is not beneficial for the crops, grasses or the microbial community in the soil. Soil salinity also affects soil hydrology. Phyto diseases and pathogens, reduced crop yields or even failures can occur due to excessive soil salinity, therefore proper control of soil salinity will help ensure the health of your crops.

Soil EC can change dramatically with water content and can be affected by water quality for irrigation, fertilization, drainage, and other natural processes. Compaction, clay content and organic matter can influence humidity, maintaining trends over time, which also affects the capacities of the EC in soil.

4. EC of the medium VS EC of the Water Pore (CPW)

EC in soil is more complex than in a water sample. and it can be difficult and confusing to interpret. The electrical conductivity of the medium (or bulk soil) (σb) is the EC of the soil / water / air matrix and is the parameter measured by the soil sensors. It is important not to confuse the EC of the medium with the EC of the soil pore water (σp). The water pore EC is the electrical conductivity of the water in the air pore contained in the soil. Because the EC of pore water can be difficult to measure directly, a soil slurry can be prepared by taking one part dry soil and two parts distilled water and measuring the EC of the water extract from the suspension. The EC of the extract (ECe or σe) is the parameter traditionally found in soil science or agricultural literature because it is relatively easier to measure and provides an "apples to apples" comparison of soil salinity conditions.

Understanding the EC of the CERES medium (EC pathways in soil)

As mentioned, the soil is a matrix that is basically made up of solid material, water in the pore spaces and air. In situ soil sensors measure the electrical conductivity of the medium (σb) which is the electrical conductivity of the combined soil / water / air matrix.

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The image above shows the three ways that electrical conductivity can spread through the soil. Density, porosity, tortuosity, water content and ion concentration working in concert with the different pathways, drastically influence the EC of a soil.

Track 1 traverses different profiles of water, land, water, and land again. The contribution of the electrical conductivity of track 1 is a function of the conductivity of the water and the soil. As the water increases, the electrical conduit of track 1 increases which can increase the electrical conductivity of the soil as a whole.

Dissolved salts will increase the conductivity of lane 2; However, like path 1, increases in soil water content will also increase the size of the path, therefore they end up increasing the electrical conductivity of the medium. That is, there are two factors that influence the conductivity of pathway 2: the concentration of dissolved salt and the size of the pathway attributed to the amount of water in the soil.

Path 3 is the electrical conductivity of the soil particles. Like the other pathways, the contribution of pathway 3 is influenced by a number of factors including density, soil type, oxidation / reduction reactions, and ion translocation.

The EC measurements of the medium provided by soil sensors are the electrical conductivity of the dynamic matrix of the soil as a whole, which is the sum of the electrical conductivities of all the different pathways. No soil sensor can directly distinguish the difference between the different paths nor can it distinguish the difference between sodium chloride and any other kinds of ions in the solution which all have some influence on the electrical conductivity of the soil / water / air matrix.

5. Applications of the EC measurement of the medium

While it is difficult to make “apples to apples” comparisons with the middle CE, we can identify certain benchmarks. For example, if soil moisture reaches a threshold such as field capacity, the EC of the medium can be recorded at that threshold for successive comparisons. This would be useful in situations where soil salinity is an issue and monitoring is necessary.

In some circumstances, the water pore EC can be estimated from knowledge of the soil EC (Hilhorst 1999). This equation allows us to make comparable estimates of the water pore EC from the measurement of the medium EC in most soils.

In this case, only sensors based on the coaxial impedance reflectometry method allow this estimation to be made.

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