UC Cooperative Extension Ventura County
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Ventura CA 93003
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Portable Soil Moisture Meters - 1993
There are many ways of measuring soil moisture status - neutron probe, tensiometers, electrical resistance blocks, etc. - all of which have some limitations. The problems associated with these instruments may be cost of the device, inconvenience or timeliness of reading or need to measure in only one place. Water management in droughty, urban areas would benefit from a portable, fast and relatively inexpensive soil moisture measuring device.
Many garden centers sell a hand-held meter that works on the principle of electrical potential developed across a bimetallic tip. As soil moisture content increases, so does the flux of energy. The flow of energy is registered on a gauge which has a range of values indicating how dry or wet the soil is.
To evaluate their performance three soil textures - loamy sand, clay loam and gravely loam - were wetted to field capacity or kept air-dry. Two intermediate moisture regimes were also created which were equivalent to 40 and 80 cbar, which is in the range that most plants grow well. In each soil container a gypsum block was implanted and a portable tensiometer was used to get a standard reading of soil moisture. Two brands of inexpensive, homeowner type of moisture meters (Instamatic Moisture Meter and Plant Care Moisture Meter) were used to make soil moisture readings. In all three soils , the homeowner devices were able to discriminate amongst the four different moisture contents.
A possible limitation to the hand-held meters examined is the shortness of the probe that is inserted into the ground. In the two models tested, the lengths were 6 and 8 inches long. However, this is in the active root zone of most plants for water uptake. The meters are also affected by soil salinity. They give artifically high readings in saline soils. Also, costing $5 - 15, these are not the sturdiest devices and a rocky soil might be a problem for them. As an attractive alternative to other soil moisture measuring devices, these devices can make quick, multiple measurements and are inexpensive.