The sensors continuously measure moisture, temperature and nutrient data and transmit the information wirelessly back to a central computer. "There are some wireless systems out there," Birrell says, "but most of them need an antenna, so they're prone to being knocked down by farm equipment." Birrell and Kumar's sensors are planted in the field, a foot deep, in a grid 80 to 160 feet apart. They beam the data through the dirt using low-frequency radio signals. The iPod-size sensors will help farmers reduce fertilizer usage, because water and temperature provide clues to how nitrogen and carbon are cycling through soil. Planting four to six sensors per acre would cost farmers $20 or $30, Kumar says, but the savings on fertilizer, water and other resources could add up to $150.