Measurement results

Soil density

Definition

It is important to note that there are different definitions of density. The density (or more precisely, the volumetric mass density ρ), of a substance is its mass m per unit volume V:

The difference between matrix density, field bulk density, and dry bulk density is caused by different definitions of either the mass or the volume. The key to these differences is the porosity φ, which is the volumetric void space in the rock, or the space not occupied by solid materials.

Specific density is defined as the density of the single soil particles. The volume used to calculate the specific density excludes pore space. The mass does not include the fluids in the pores.

Field bulk density is the density of the mixture of soil and water. Both mass and volume include fluid. This is the density as provided by the RhoC5 soil density meter.

Dry bulk density is defined as the density with the water removed. The mass does not include the water, but the volume does include the pore space.

More information on the definitions of density can be found on our wiki: https://the.medusa.institute/wiki/matrix-bulk-and-dry-bulk-density

Soil moisture

As with density, there are different ways to define soil moisture. The soil moisture content provided by the RhoC5 is expressed as the volumetric percentage of soil volume occupied by water.

The RhoC5 moisture sensor measures moisture in a soil layer approximately 50 mm thick, with the center of this layer located 100 mm below the center of the density measurement.

The moisture measurement is based on a capacitive measurement that relies on the dielectric constant (also known as relative permittivity). The dielectric constant measures a material’s ability to store electrical energy in an electric field, relative to a vacuum. It indicates how much electric charge the material can hold when exposed to such a field. The dielectric constant is different for different materials:

• Air: ~1

• Water: ~80

• Dry Soil: Typically between 2 and 5, depending on the soil type

By measuring the combined dielectric constant of the soil and its moisture, the RhoC5 sensor estimates the volumetric moisture content. However, the dielectric constant of dry soil depends on the soil composition, such as whether the soil is sandy, clayey, or rich in organic material. This means the soil type influences the moisture measurement.

The RhoC5 moisture sensor is factory-calibrated for typical Dutch sandy soil, which provides a good approximation for many soils worldwide. However, variations in local soil composition may cause deviations in measurements.

To improve the soil moisture content for a local situation, it is possible to define a user-defined moisture content θ_user by implementing a user function on the device, that translates the moisture content from a Dutch soil into a value that better resembles the local situation using:

The constants in this formula can be changed by going to the RhoC5 section on the Settings page in the mDOS web interface. For more information on mDOS and how to access the web interface, please visit the mDOS manual.

Dry bulk density

Conversion of the field bulk density ρ_fb, as measured by the RhoC5, to dry bulk density ρ_db requires the soil moisture content θ:

If a user-defined moisture content θ_user is available, this value will be used instead.

The soil moisture content and soil density are measured at different depths because the moisture sensor is positioned 100 mm below the density sensor. To calculate the dry bulk density, the moisture content must correspond to the same location as the field bulk density. This is achieved by interpolating the moisture content values based on the available measurements.

For the topmost density measurements, where no moisture data is recorded, the nearest moisture value is used. To ensure moisture measurements cover the entire range of density measurements, it is recommended to start measuring 100 mm above the desired depth, if possible.

Example

The figures below illustrate measurements taken every 8 cm in soil with a uniform dry bulk density of 1.3 kg/l. Up to a depth of 40 cm, the moisture content is 5%. Beyond this depth, the water content in the soil increases, reaching a maximum moisture content of 35% at 50 cm (blue line in figure 1a). This saturation causes the field bulk density to increase between 40 and 50 cm, from 1.35 kg/l to 1.65 kg/l (orange line and dots figure 1a).

However, the measured moisture content changes between 30 and 40 cm. This discrepancy occurs because the moisture measurement was actually taken 10 cm lower due to the sensor offset. Figure 1a shows these uncorrected measurements as blue dots. After applying interpolation to correct for the offset, the moisture content depth aligns with the depth of the density measurement (blue dots in figure 1b), allowing for accurate determination of the dry bulk density (orange dots in figure 1b).

Figure 1a: Measured moisture content (blue dots) before correction of the depth offset and measured field bulk density (orange dots). the solid lines shows the actual value.

Figure 1b: Corrected moisture content (blue dots) and calculated dry bulk density (orange dots).

Note that moisture and density measurements will align when the measurements are taken at regular intervals that are exact fractions of 10 cm (10 cm, 5 cm, 3.33 cm (1/3 of 10 cm), 2.5 cm etc). In these cases, the depths of the moisture and density measurements will overlap, ensuring that data from both sensors corresponds directly to the same depths.

Depth

The depth sensor uses radar technology to locate the top of the plastic baseplate. The position of the density sensor is calculated with respect to the bottom of the baseplate. Therefore, it is essential to always use the baseplate.

The depth provided is the center of the density measurement. The density measurement measures an average value over 50 mm layer of soil, so this is 25 mm below and 25 mm above the provided depth.