Page 38 - soil-plant-water relationship and water requirement

SOIL-PLANT-WATER RELATIONSHIP AND WATER REQUIREMENT

3.4.3 Tensiometer

Tensiometer operates on the principle that a partial vacuum is developed in a
closed chamber when water moves out through the porous ceramic tip to the
surrounding.

A vacuum gauge or a water or mercury manometer can measure the
tension. The gauge is usually calibrated in centibars or millibars.

The tensiometer is filled with water after the porous cup is placed in the soil.
Water drains from the porous tip into the soil around it (as suction is more in
the soil). When the water in the tensiometer is in balance with the soil water,
a point is attained. The gauge's reading is then compared to the moisture
content using a calibration curve.

Figure 4.4.4-1: Tensiometer

3.4.4 Neutron Probe:

It consists of a probe lowered down a hole in the soil. A box (rate meter or rate
scalar) is at the top.

Within the probe is a radioactive source e.g. beryllium (435 years life span).
Close to the source is a detector. The source emits fast neutrons, some of
which are slowed down when they collide with water molecules (due to
hydrogen molecules).

A cloud of slow neutrons (thermal neutrons) build up near the probe and
are registered by the rate meter or rate scalar which measures the number of
slowed down neutrons.

The method is quick but very expensive. It is also dangerous since it is
radioactive and must be used with care.

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