How do researchers measure volumetric soil water content (VWC)?
Volumetric soil water content measurement requires in-field destructive testing – meaning you have to carefully dig up multiple soil samples and take them to a lab for analysis. Fundamentally, measuring soil moisture in inches / mm cannot be done in any reliable, convenient way. To calculate VWC effectively you would need to collect multiple soil samples from the field about the size of a coffee cup, weigh that soil, then dry it in an oven for several days (1 – 3). You would first calculate the gravimetric content by subtracting the weight of the original sample from the weight of the dried one and divide that result by the weight of the dry sample. Once you have that number you need to adjust it according to the density of your soil. Of course, in real life, that’s a very time and labor consuming way to figure out what’s going on in the field, and in essence you’d only be measuring just the top layer or two of soil, when in practice, knowing what’s happening at the active root zone would be more useful.
What about other soil moisture analysis technology?
There are other technologies besides volumetric soil water content that are used for soil moisture measurement, but most are impractical for on-farm irrigation scheduling; they are either too inaccurate (capacitance sensors) or too costly and difficult to use (TDR and NMM). Here’s a breakdown on those.
Neutron moisture meters (NMM)
Neutron moisture meters (NMM) are also called neutron probes. These typically cost thousands of dollars and may require a special permit to use. As the name suggests, neutron probes use neutrons to analyze the soil moisture content and therefore carry the risk of radiation exposure to the user. Neutron probes are permanently buried underground and covered with soil which must be removed when measurements are to be made. This means it’s practical where only annual readings are needed, such as during an annual pre-plant soil analysis. Due to these cautions and the complexity of using a neutron probe, they are more commonly used by researchers than individual farmers.
Time domain reflectometry (TDR)
Time-domain reflectometry (TDR) sensors provide a non-destructive way to measure moisture content indirectly based on the correlation to electric and dielectric properties of materials, such as soil, agrarian products, snow, wood or concrete. Most TDR moisture sensors are handheld and thus require a visit to the field and inspection at multiple sites and some studies have shown loss of accuracy when used in saline soils.
Electromagnetic sensors known as capacitance sensors exhibit much more variability in the field than either the NMM or direct soil water measurements like VWC. Capacitance sensors are not recommended for soil water balance studies due to the impractically of a large numbers of access tubes and sensors that are required and because they are rendered inaccurate by changes in soil bulk electrical conductivity (including temperature effects) that often occur in irrigated soils, particularly those containing appreciable amounts of clays with high ion exchange capacities, even when using soil specific calibrations.
Why Doesn’t AquaSpy measure volumetric soil water content (VWC)?
We know farmers don’t have a lot of extra time in their day to visit their fields to check sensors or dig up soil samples. We have developed a simple, hands-free layer by layer approach to understanding how much moisture a crop needs at each stage of the growing season. Our approach not only accurately, and without calibration, indicates the relative amount of moisture available in the active root zone, it also indicates as how well the crop is utilizing that moisture.
The AquaSpy Vertical Approach to Root Zone Analysis
AquaSpy uses a vertical approach to assessing soil moisture by sending signals out at 4” (12cm) depth intervals in the root zone. The applied technology in the probe is RF Spectroscopy. Essentially, it functions as a complex permittivity sensor. We send out a signal into the soil and we are looking at the reflected signal. We can discern the phase and amplitude which in turn is an expression for the moisture and electrical conductivity. We also have a temperature sensor with each of the 12 sensors.
Measuring permittivity rather than volumetric water amounts allows us to give a view deeper into the soil and deliver that information to the grower in just a matter of minutes, without the grower having to visit the field or do any programming. Our wireless soil moisture sensors continuously gather data layer by layer, and then we apply intelligent algorithms that process all of that data and deliver it via the Cloud right to a desktop or mobile device in understandable charts that tell exactly what’s happening in the soil and plant roots hour by hour.
What is permittivity?
Permittivity is the ability of a substance to store electrical energy in an electric field. Air, water, and soil all have different permittivity.
Think of a capacitor that stores energy in an electric field. There are the two conductors (plates) and there’s an insulator in between them which is a non-metallic substance. We are measuring the air, water and soil. The permittivity of air = 1 (very low), water = 81 (very high), dry sand = about 7, clay = about 20 to 25. We look at these changes layer by layer so we have an understanding of what’s happening in the active root zone and also below it since our probes have multiple sensors spaced 4 inch (12 cm) apart.
How does AquaSpy measure using permittivity?
We are measuring how effectively we can transmit a signal and what’s interfering with it, that is, the water / moisture content. The amount of water will impact the shift of the signal.
The reflected signal is dampened by soil and it is phase shifted a few degrees. We can separate those two, so we can also see the electrical conductivity (EC) component which informs us about the salinity of the soil.
The measurement is affected by three variables: moisture, soil type and temperature. The largest component is moisture. Some manufacturers will ask the grower to input the soil type in an attempt to calibrate the soil variable out of the equation. The problem with this approach is that there is not a consistent soil profile all the way through the layers of soil.
We have adopted a different approach where we are using the combined moisture value as a relative value. We automatically look for drainage after irrigations and establish a field capacity for each layer (we call that 100). Based on that field capacity, we use an extensive crop specific statistical method to estimate the “empty” point (we call that 0). In this case it is not truly empty, but the plant’s daily consumption has reduced to about 50%. Now we have the operating range of each sensor and we can report that to the grower.
Of note, AquaSpy Wireless Soil Moisture Meters don’t require any calibration, work in any soil type, and give accurate and reliable data, all while being durable and cost-effective. Don’t believe us? Try it out for yourself. Contact your regional sales rep or AquaSpy partner today and see for yourself!