Measuring soil moisture is easier than you think

February 23, 2012

If you Google ‘soil moisture’ and click on the Wikipedia entry for ‘water content’ you get a very complex answer that explains that the quantity of water contained in soil is defined mathematically as:

where V_w is the volume of water and V_T = V_s + V_v = V_s + V_w + V_a is the total volume (that is soil volume + water volume + air space).

The article goes on to explain that the measurement of water content can be done by using a known volume of the material, and a drying oven. Volumetric water content, θ, is calculated using:

where

m_wet and m_dry are the masses of the sample before and after drying in the oven;

ρ_w is the density of water; and

V_b is the volume of the sample before drying the sample.

Then it goes on to talk about laboratory methods of measuring soil moisture which involves an equally complex formula.  Under this explanation is the definition of the Geophysical methods that approximate in situ soil water content. These methods include: time-domain reflectometry (TDR), neutron probe, frequency domain sensor, capacitance probe, electrical resistivity tomography, ground penetrating radar (GPR), and others that are sensitive to the physical properties of water. Geophysical sensors are often used to monitor soil moisture continuously in agricultural and scientific applications.

At Van Walt we recognise that nowadays it is very easy and safe to measure soil moisture with TDR technology. Our award winning TRIME TDR devices generate high-frequency-pulses (up to 1GHz) which propagate along the wave guides generating an electromagnetic field around the TRIME-probes. At the end of the wave guides, the pulse is reflected back to its source. The resulting transit time and dielectric constant are dependent on the moisture content of the material.

This highly complex but extremely easy to use and safe technology should not be confused with capacitance or FDR techniques which, although acceptable for general relative measurements cannot be used for serious research. In particular accuracies can be seriously compromised in clay rich or very organic soils.

TDR, and in particular TRIME TDR on the other hand returns results in line with the accuracy of the now discontinued neutron probe. TRIME TDR can be single point, multipoint, profiled or networked. A huge additional advantage is the availability of data from these systems to report a signal which can interpret the soil conductivity. For more information on this equipment visit: https://www.vanwalt.com/soil_tdr-Auger-Soil-Sample-Contamination.htm.