Irrigation Scheduling of Grapevines
Neutron Probes
Neutron soil moisture meters (neutron probe) use radioactive material for measuring total soil water content by volume (See Figure 12.1). The soil probe is connected by cable to a control unit which remains on the surface. The control unit includes electronics for time control, a counter, memory, and other electronics for processing readings. Access tubes are installed in the ground throughout a vineyard, and the probe is lowered into the tubes to the desired depths of measurement.
Principles of Operation
The neutron probe indicates soil moisture by detecting hydrogen nuclei present in the soil water. A counter reads the number of neutrons that are reflected by the hydrogen in the soil. This number is then used to calculate the moisture content.
Placement of Monitoring Sites
At least two sites for each 10 acres are recommended, but this really depends on soil variability in the vineyard.
Installation
Two-inch diameter aluminum pipe, Class 125 PVC pipe, or electrical, metal tubing (EMT) is commonly used for access tubes. Because meter readings are affected by access tubing material, the same material must be used for each vineyard site.
Readings
Neutron probe measurements are taken by lowering the probe cylinder into the access tube to the desired depth. The neutron probe measures a large (volleyball-size) sample.
Maintenance
Neutron meters require little maintenance beyond checking to ensure proper operation. Access tubes should be checked occasionally for water or foreign materials.
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