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How Does a Soil Moisture Meter Work?

You can monitor soil moisture by digging into it and observing the moisture levels. However, this method is time consuming and is not accurate. For this reason, many devices for measuring the moisture of soil have been developed and most use the electrical conductivity of water to help gauge soil moisture.

Tensiometer

Tensiometers check the water tension within the soil to determine how moist the soil is. These instruments have a porous cup at the bottom that is saturated with water. The instrument measures whether water is drawn out of the cup (meaning the surrounding soil is dryer) or whether water flows into the cup (meaning the surrounding soil is moister). The rate at which water flows through this device give the tension readings, usually in a water tension reading called a centibar. These instruments must be calibrated to a specific soil type to give meaningful readings.

Electrical Resistance Blocks

Electrical resistance blocks measure the electrical conductivity of the soil. As soil moisture increases around the block, water flows into the block and spreads the conducted electricity over a wider surface, decreasing the electric tension. As moisture decreases, water flows away from the block and increases tension, which is translated onto the instrument display in centibars, as in the tensiometer. These blocks are usually embedded in a gypsum medium to keep soil and water salinity from affecting the conductivity and skewing the readings.

Hand-Held Meters

Many commonly used moisture meters for measuring around-the home or indoor potted plant soil moisture content are hand-held. They give generalized moisture readings in the range of high moisture, medium, low and dry, or some equivalent thereof. The tips of these devices consist of two metals over which electrical currents can run if there is a conductor such as moisture. The higher the current, the more moisture in the soil. However, if the soil or the water in the soil is saline (especially when fertilizers with salts are frequently used), then the moisture meter displays incorrectly high moisture readings.

Considerations

Moisture meters come in a very wide range of complexities and costs. Small, hand-held varieties gain preference for their low price so long as they are in simple home operations. They tend not to last very long. Moisture sensors such as electrical resistance blocks and tensiometers are especially useful for keeping long-term track of soil moisture in many areas of a large garden, orchard, crop field or pasture.

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