Soil wet enough? Gypsum blocks will tell you.
Gypsum blocks, which have been around for about 60 years, are simply marshmallow-like plaster of Paris cylinders that contain two electrodes. The blocks absorb water much the way oil does. As the blocks become wetter or drier, electrical charges between the electrodes, which connect by wire to a simple meter, change accordingly.
By burying the blocks at various depths in a plant's root zone, you can not only gauge the soil's moisture content, but also tell how deep you are watering with each irrigation. One type of gypsum block can now even be hooked up to run an automatic sprinkler system.
Where to install gypsum blocks
As with other moisture sensors, it's best to put gypsum blocks in several different microclimates or soil types around your yard. For instance, you might place one set of two or three (depending on the depth of the roots) in soil on the sunny south side of the house, another set on the shady north side, and so on.
To make a hole in the ground for the blocks, you can use either an auger or core sampler (see picture abovee, or bore a hole with a sharp metal bar. The most important step is to make sure the soil put back in the hole is in good contact with the block. Wait 24 hours to get an accurate reading.
Although most gypsum blocks gradually break down and read zero after several years, they are inexpensive to replace. One newer type has a protective cover that prevents breakdown.
Where to buy gypsum blocks
Three companies are the main suppliers of gypsum blocks and related equipment in the West. Each provides excellent instructions. You need only one meter to read any number of blocks. Blocks cost $6 to $16 each, meters $100 to $270.
Irrometer Company, Inc. (Box 2424, Riverside, Calif. 92516) offers blocks enclosed in a protective cover to prevent degradation. They can also be used to control automatic irrigation systems at a price of about $75 per valve.
Soilmoisture Equipment Corp. (Box 30025, Santa Barbara, Calif. 93105) sells a smaller block for a narrower hole.
Supersensors (140 University Ave., Suite 131, Palo Alto, Calif. 94303) offers basic gypsum blocks and a meter, along with directions for making a simple PVC core sampler for installation.
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|Date:||Aug 1, 1990|
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