Moisture meters are stuck in soil in order to find how much water is present. This information is useful in maintaining plants that require a certain amount of moisture. There are two main kinds of moisture meters. The first uses electrical contact to measure how much water is in the soil by inserting two prongs into the dirt, while the second uses sensors that simply send signals through the surface of the soil. You can make the pronged version at home, given the right materials.
You will need to use electrical current to measure the moisture content of the soil, which means you will need some way to measure the resistance between the metal prongs used in your moisture meter. The most practical method is to use a portable ohmmeter to measure the resistance the prongs encounter.
To make the meter itself, you will need two separate pieces of metal, such as wire from a coat hanger, paper clips, or even galvanised nails. You will also need to get a wide straw large enough to fit the two pieces of metal without them touching each other or the straw (this is important; there must be room between the metal prongs to give accurate readings and only the points of the prongs should touch the soil you insert the probe into). You will also need a plaster of Paris mixture, some scissors, and the necessary wires to attach the prongs to your ohmmeter. Hot glue and a glue gun, or another thick fast-drying glue, will be used as well.
Making a Moisture Meter
Cut the prong wires so that two ends jut out and away from each other, or make hooklike shapes that avoid making contact but give you a place to attach your wires for electricity measurement. Position the wires parallel to each other and about 1/4 inch apart, and secure their position by applying dabs of hot glue or another thick fast-drying glue between the prongs, about 1/2 inch from the top and bottom.
After the glue is dry position the prongs in the straw so that the probe ends protrude slightly beyond the end of the straw, and the other ends protrude sufficiently beyond the other end of the straw to ensure that you will be easily able to attach the wires leading to the ohmmeter. Cut the straw down to the appropriate length. The prongs should not touch the straw--to keep them in correct position use more glue. At the top end (where wires will be attached) use enough glue to seal the opening completely so that the plaster of Paris will not escape when you pour it in the other end. At the bottom end use as little glue as possible to make it easy to pour in the plaster of Paris.
Mix your plaster of Paris following the package instructions--you do not need much, only enough to fill the bottom of a cup. Pour the plaster of Paris into the straw, tapping it down gently as you proceed to make sure all gaps are filled. If you like, you can fashion a point at the probe end by shaping the plaster of Paris, but be careful not to cover the ends of the prongs.
Wait about an hour, then carefully cut away the bottom three-quarters of the straw to reveal most of the dried plaster. Then wait until the moisture meter fully dries (about 24 hours) and is ready to be used.
You can insert the meter in the soil, wait for several seconds, then use your ohmmeter to measure resistance--the higher the resistance, the drier the soil. Experiment with several types of soils and moisture levels to find out what readings your meter gives based on the amount of water present. You will then be ready to use your moisture meter to get an excellent idea of how much moisture your pots or gardens have.