Penn Alexander


Philadelphia Water teamed up with students at the Penn Alexander School to install a web-connected, solar-powered soil moisture sensor in the school’s garden. The electronics are housed in a special compartment of a birdhouse whose roof angles the solar panel southward for optimal exposure to sunlight. The data below will let gardeners know when the plants need watering.

VH400 soil moisture sensor is inserted approximately 4 inches deep into the garden’s soil and takes a measurement every 2 hours. The sensor returns a voltage that corresponds to how wet or dry the soil is—this is based on the dielectric constant of the soil. (The dielectric constant of water is about 80; the dielectric constant of air is about 1. Read more about soil moisture, how it is measured, and what types of tools are used here.)


As we collect data and observe the conditions of the soil, we will be able to relate our voltage readings to the soil’s volumetric water content (VWC). This is the ratio of water volume to soil volume. The VWC values will help to determine when to water the garden. The sensor device—we call it the Soil Cell, because it uses the cellular network to communicate—can even text or email us to let us know the soil is too dry. We can also use weather forecasts in order to be efficient about watering:

Want to build your own low-cost, high-tech soil sensor? Check out the Soil Cell 2G or Soil Cell 3G DIY pages.

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