A tensiometer is one of the most wonderful devices for figuring out when to water plants. It works equally well in orchard and garden situations. It follows the soil moisture content as it is depleted by the plant and will tell you when it is time to rewet the root zone. If, you monitor it faithfully. If you don't, it can give erroneous results. If you are not watching it, it can go dry and give a false reading. That should only tell you the reader to pay more attention to the plant's water needs.
It's actually very reliable if it is installed correctly. And once it is and maintained it can give good results for years.
Here is a video showing Gary Bender installing a tensiometer correctly.
http://ucanr.edu/sites/alternativefruits/?story=1743
Of course there are many ways to monitor soil moisture and the water needs of plants. A shovel, auger or trowel are incredibly accurate devices if you get down and dirty. The tensiometer allows you to stay clean, at least for soil moisture monitoring.
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