Irrigation Scheduling of Grapevines
Gypsum Blocks
Gypsum blocks, commonly called electrical resistance blocks, consist of two parallel electrodes embedded in blocks of gypsum. Their shape varies from rectangular to cylindrical, and they are about matchbox size. They have a pair of wires connected to the electrodes extruding from the block that are about 2 yards (2 m) long.
Principles of Operation
Since the blocks are porous, water moves in and out of the block in equilibrium with the soil moisture. Meter resistance readings change as moisture in the block changes which, in turn, is an indication of changes in the amount of water in the soil.
Placement of Monitoring Sites
The sensors should be installed under the drip line to protect them from traffic but not directly under the emitter. Enough sensors should be installed to give a representative picture of what is happening through the root zone and to match the soil profile.
Installation
Before installation, soak blocks overnight in distilled water or rain water to remove all the air from within the blocks. Label the end of the wire that will be exposed with the depth at which the block is to be placed.
Readings
Soil water tension readings indicate the timing of irrigations but does not tell the grower the amount of irrigation water needed for the vines. Soil water tension is usually measured in centibars (cB), where a centibar is 1/100th of a bar, and a bar is roughly equivalent to 1 atmosphere of pressure.
Maintenance
Gypsum blocks, like tensiometers, require regular monitoring and occasional maintenance or replacement if they are to continue to give an accurate reflection of the soil water tension during the growing season.
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