Abstract

Do you or your family have a lawn, garden, or potted plants that you water regularly? Irrigation—or the artificial application of water to plants and landscaping—accounts for over two-thirds of the world's freshwater consumption (U.S. Geological Survey, 2016)! While that total includes farms, in the United States landscape irrigation still accounts for almost one-third of residential water use. As much as half of that water is wasted due to inefficient watering methods (WaterSense, 2016) like watering when the soil is already wet.

The project Green Technology: Build an Electronic Soil Moisture Sensor to Conserve Water shows you how to build a soil moisture sensing circuit. The circuit has two probes that you insert into soil (Figure 1) and a small LED that turns on when the soil is dry. If the LED is off, that means the soil is already wet. This allows you to use the circuit as an indicator for when you need to run a sprinkler system, water a garden, or water indoor potted plants.

Can you build the circuit following the directions in the original project, and design an experiment to see how using it affects the growth of plants and overall water consumption? Since this is an abbreviated project idea, we will not give you an exact procedure to follow, but here is a general outline of what you could do.

1. Go to a garden center and decide what type of plant you want to use for your experiment. Talk to an employee to find out what conditions the plants require to grow: type of soil, amount of sunlight, and especially how much and how often they should be watered.
2. Build the soil moisture sensor and test it on several of that type of plant. The LED should turn off when you insert the probes into damp or wet soil. Depending on the type of plants and soil you use, you may need to adjust the default circuit so the LED comes on at an appropriate soil moisture level.
   1. There are several ways to adjust the circuit. See the Variations section of the original project for some ideas.
   2. Also keep in mind that you only have one circuit, but will be monitoring multiple plants. It is important that you are able to easily remove and re-insert the soil probes at a fixed distance and depth, which is part of the engineering challenge of the original project. You could also leave a set of probes in each pot and use alligator clips to disconnect and reconnect them from the circuit.
3. Separate your plants into at least two groups (make sure you have at least three plants in each group). Water one group with a specific amount of water according to a regular schedule (for example, every day or twice per week). Water one group with the same specific amount of water each time, but only according to the sensor (only when the LED turns on). You could also add other groups; for example, an over-watered group (always add enough water to keep the soil saturated) or an under-watered group (wait to apply water a few days after the LED comes on).
4. Water the groups of plants according to the schedules you defined. Keep track of the total water consumption for each plant, plant growth, and plant health. Does using the soil moisture sensor allow you to conserve water while maintaining or increasing plant health and growth?

See the Bibliography for some resources that may be helpful when doing this project, including how to measure plant growth and how to measure soil moisture content.

Materials and Equipment

The soil moisture sensor can be built using parts from the Electronic Sensors Kit, available from our partner Home Science Tools.

Frequently Asked Questions (FAQ)

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