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  • Sara Laura Wilson

Radio Star | MADMEC Materials Science Solutions for Sustainability Competition 2019

Updated: May 15, 2021

Award: 3rd Place (featured on MIT News)

Position: Team Member

Summer/Fall 2019

More than 500 million smallholder farms exist worldwide, feeding about 2 billion people mainly in the developing world. In the age of data-driven agriculture, these farms need cheap, reliable soil sensors to collect data on soil health. Numerous sensors already exist for soil monitoring, but all have limitations, and most were designed with industrial agriculture in mind. The motivation of this project was the need for a dirt cheap, brainlessly simple sensing device to be used where other options are prohibitively expensive. Our solution? A soil-responsive retroreflector.

A retroreflector is an optical element that sends directional light back towards its source over a range of inlet angles. If a chemical stimulus from soil causes a change in the optical properties of a retroreflector in a field, that change would be easily resolvable with the use of any directional light emitter and detector – like a farmer with a flashlight, or a drone with a camera. A suite of retroreflective sensors could be deployed simultaneously to test soil conditions over several chemicals and parameters at once. This mechanism promises a long range, high-resolution, unpowered, low-cost solution to agricultural data collection.

As a proof-of-concept for our sensing mechanism, we designed a pH-responsive retroreflective gel from hydrated gelatin and universal indicator. We demonstrated a full color change in both acidic and basic environments without a loss of retroreflection, calibrated the color response of the gel to the range of interest for agriculture, and added a wicking string interface to probe subsurface soil conditions. Future work will springboard off of this proof of concept into new sensors for other soil parameters, such as nitrogen and phosphorous levels, moisture, and the presence of heavy metals. Field testing of our pH-active prototype is also on the horizon.

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