Using Backscatter Radio for a Soil Sensor Network

With almost 8 billion souls to feed and a changing climate to deal with, there’s never been a better time to field a meaningful “Internet of Agriculture.” But the expansive fields that make industrial-scale agriculture feasible work against the deployment of sensors and actuators because of a lack of infrastructure to power and connect everything. So a low-power radio network for soil moisture sensors is certainly a welcome development.

We can think of a lot of ways that sensors could be powered in the field. Solar comes to mind, since good exposure to the sun is usually a prerequisite for any cropland. But in practice, solar has issues, the prime one being that the plants need the sun more, and will quickly shade out low-profile soil-based sensors.

That’s why [Spyros Daskalakis] eschewed PV for his capacitive soil moisture sensors in favor of a backscatter technique very similar to that used in both the Great Seal Bug and mundane RFID tags alike. The soil sensor switches half of an etched PCB bowtie antenna in and out of a circuit at a frequency proportional to soil moisture. A carrier signal from a separate transmitter is reflected off the alternately loaded and unloaded antenna, picking up subcarriers with a frequency proportional to soil moisture. [Spyros] explains more about the sensor design and his technique for handling multiple sensors in his paper.

We really like the principles [Spyros] leveraged here, and the simplicity of the system. We can’t help but wonder what sort of synergies there are between this project and the 2015 Hackaday Prize-winning Vinduino project.


22 thoughts on “Using Backscatter Radio for a Soil Sensor Network

  1. So, RF retroreflectors? Am I understanding the text correctly by interpreting that eliminating PV (power source) was possible because the RF retroreflector transmission technique is much less power-hungry and powering a transmitter? So the RF isn’t supplementing power at all, rather reducing the load such that a battery is sufficient?

        1. The dielectric constant of the medium determines the capacitance of the electrodes. The dielectric constant of water is about 80x that of air or vacuum. Capacitive sensing is exquisitely sensitive to water content, and permits sealing the electrodes.

    1. Neither of these measurements are as good as using a neutron probe if you want absolute volumetric measurements. It is very difficult to get a good reading with other probes due to the interface with soil and that you can’t get significant penetration around the probe. If you take one of the capacitive probes and place the palms of your hands on it while measuring, you will notice that the reading dramatically change the harder you press. Now imagine installing this in the soil without disturbing it in order to make representative soil water content measurements. At best, the capacitive probes will give you a good relative measurement for knowing when to water, assuming you have them placed at multiple depths.

  2. Capacitance for moisture content (dielectric permittivity) … Conductance for ion measurement (salts and the like). Would be great to measure nitrates (hard) and ph (very hard insitu).

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