How to Build Soil Moisture Sensors

By measuring electrical resistance in soil, you can determine the need to water.
By measuring electrical resistance in soil, you can determine the need to water. (Image: Polka Dot RF/Polka Dot/Getty Images)

A moisture sensor allows you to determine the need to water your lawn or garden. This helps you avoid wasting water or harming plants by watering too often. Moisture sensors work by using two metal probes inserted in the soil. These are connected to a multimeter, which measures electrical resistance between the probes. When soil is wet, electrical conductivity of water lowers the resistance between the probes. As soil dries, this resistance increases again. By monitoring this resistance, you can determine whether your soil is wet or dry.

Things You'll Need

  • Utility knife
  • 1/2-inch rubber tubing
  • Plaster of paris
  • Mixing bowl
  • 2 galvanized nails, 3 1/2 inches
  • Wax paper
  • Copper wire
  • Glue
  • Multimeter


Cut 1/2-inch rubber tubing slightly longer than 3 1/2 inches, using a utility knife. Make the cut as straight as possible. Cut the tubing lengthwise, to make it easier to remove from plaster of paris later.

Mix plaster of paris according to package directions. Fill the cut piece of tubing to the top. Stand the tubing upright, bracing it if necessary.

Push two 3 1/2-inch galvanized nails through a small piece of wax paper to just below the nail heads. This will keep the nails from sinking into the plaster of paris. Insert the nails into the plaster of paris-filled tube until the wax paper rests on top of the tubing. Make sure the nails do not touch and are parallel to each other.

Let the plaster of paris dry for at least an hour. Carefully remove the plastic tubing, peeling back along the split line. Remove the wax paper from nails. Let the sensors dry 24 hours longer.

Connect wire to the two nails by wrapping tightly. Cover the joint with hot glue to insulate and protect it.

Hook the two wires to the two probes of a multimeter and measure the resistance of the sensors while dry. Resistance will be shown on the multimeter's display.

Insert the probe into water and measure resistance on the multimeter. You now have readings -- when the probe is completely dry and when it's soaked in water. These can be used to compare readings taken from soil.

Tips & Warnings

  • Check the probes periodically for damage or wear. The plaster of paris will eventually break down in the soil, and the sensor will need to be replaced.

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