Toxic algae blooms can be nasty, killing fish, other wildlife and sometimes even people. One of the main causes of such blooms is excessive amounts of fertilizer running off of fields and into waterways. A new type of buried sensor, however, could help address that situation.

Fertilizers can cause algae blooms by increasing populations of phytoplankton (algae is a type of phytoplankton), which produce biotoxins. These toxins deplete oxygen from the water, plus the plankton block sunlight from reaching fish and aquatic plants, causing them to die. Additionally, fish and animals living adjacent to the water can ingest the biotoxins, which humans will also occasionally inhale.

Unfortunately, when farmers are applying nitrogen-based fertilizer to their crops, they have no quick method of knowing how much is enough. They can take soil samples, but these have to be sent away to a lab for analysis – it can take a long time to get results, plus the process isn't cheap. As a result, farmers often forgo the soil tests and end up applying more fertilizer than is actually needed, some of that ultimately running into a nearby body of water.

With that in mind, scientists from Iowa State University and the University of Florida are developing disposable "bury-and-forget" sensors that can be shallowly-buried at various locations around a farmer's fields.

Utilizing inkjet-printed, laser-treated graphene circuits, these inexpensive devices will continuously measure nitrogen concentrations in the soil, wirelessly transmitting data to an internet-based hub that can be accessed by the farmer on a computer or even a smartphone. Based on that data, the farmer can then decide if more fertilizer really is required, and if so, in which areas. If no more is needed, not only are the chances of algae blooms lessened, but the farmer is also spared the cost and effort of needlessly applying additional fertilizer.

Once the prototype sensors are built, the researchers plan on testing how deep they can be buried while still maintaining a wireless network connection. They will subsequently be trialled in a testbed facility, in which a crop of tomato plants is being grown. It is hoped that once perfected, the sensors will last for an entire growing season.

"If we had a better predictive model, we could have better remedies for farmers," says Iowa State's assistant professor Jonathan Claussen, leader of the study. "A better model could tell them they can use less fertilizer."

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