Successful precision agriculture decision making requires characterizing soil heterogeneity at high spatiotemporal resolution in real-time in order to optimize input (such as water and nutrient) amounts and location. In order to achieve this goal, a printed soil moisture sensor fabricated from biodegradable materials is demonstrated. These devices are intended to function during the growing season and then harmlessly degrade afterward, enabling high-density deployment, eliminating the need for sensor retrieval, and enabling the use of simple device structures and low-cost materials and fabrication techniques. A capacitive structure is used with a water-soluble zinc electrode printed onto a biodegradable substrate. Rapidly degrading substrate and electrode are encapsulated in a slowly degrading wax blend that protects the device, reduces drift, and controls degradation time. A linear capacitance response is observed for soil samples with a volumetric water content from 0 to 72%. Accelerated degradation testing demonstrates that the sensor responds predictably and stably until the encapsulation is breached, at which point the sensor fails rapidly, providing a clear distinction between the functional and nonfunctional lifetimes of the sensor. These results demonstrate the potential of biodegradable sensors to allow maintenance-free, affordable, and real-time soil moisture measurement at high spatial density for precision irrigation control.