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.

Matrix density is defined as the density of the rock formation. The volume used to calculate the matrix density excludes pore space. The mass does not include the fluids in the pores.

Field bulk density is the density of the mixture of rock and fluid. 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 fluid removed. The mass does not include the fluid, 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 definitions of soil moisture. The soil moisture content, as provided by the rhoC5, is defined as the volumetric percentage that is occupied by water.

The measurement is a capacitive measurement based on the relationship between the dielectric constant of the soil and its moisture content. As the maximum water storage capacity largely depends on the soil type, the sensor is calibrated with a so-called “air and water calibration”. This means that a measurement value of 0% corresponds with the sensor placed in air, and a value of 100% corresponds with the sensor placed in water. This is a very basic calibration method, not considering any soil-specific properties and please be aware that a local calibration is needed.

The dielectric constant of dry soil is higher than the dielectric constant of air, and consequently, the measurement results are higher than 0% when placed in dry soil. As an example, dry sand will read as 26%.

In a first order estimation, the readings can be corrected by subtracting a value ranging between 10 (for more organic soils) and 25 for sandy material. The software of the Rhoc5 gives that option, translating the recorded soil moisture content θ into a user-defined moisture content θ_user using a formula:

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.

Measurements in the Netherlands learned that for the Dutch soils sampled, the correction value B should be -14%, but please be aware that this value can differ for the soils for a specific region.

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.

Note that the soil moisture content is not calculated at the same depth as the soil density. Especially for environments with a significant difference in moisture content over depth, this may result in an incorrect dry bulk density. See also the next section regarding the depth measurement.

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 takes into account a 50 mm layer of soil, so this is 25 mm below and 25 mm above the provided depth.

The soil moisture sensor is located in the nylon tip of the rhoC5, so it is located underneath the density meter, and there is always an offset of 100 mm between the center of the density measurement and the center of the moisture content measurement. As with the density meter, the moisture sensor considers a soil layer with a thickness of 50 mm. Combining measurements into a value for the same depth is left to the user.