Popular Methods Used for Measurement of Soil Moisture and Tension are: 1. Gravimetric Method 2. Measurement of Resistance 3. Neutron Scattering 4. Tensiometer!
Soil moisture can be measured directly or by observing soil moisture potential (tension).
1. Gravimetric Method:
It is the measurement of soil moisture content in a given mass of soil. Moist soil sample is taken from the orchard and weighed.
The sample is dried at 100°C in an oven for 24 hours. The dry soil sample is again weighed. The difference of weights represents the soil moisture in the sample. It can be expressed on mass or volume basis. Normally water supplied to orchard is measured on acre inch or cm. This is the amount of water required to irrigate one acre of an orchard to a depth of four inches or ten centimeters.
2. Measurement of Resistance:
Porous material as gypsum, nylon or fiberglass is used to measure electrical resistance. Gypsum blocks with electrodes are placed in the moist soil. These blocks absorb water from the soil in proportion to the moisture content of the soil. The electrical resistance of these blocks is related to the amount of water absorbed by the blocks.
The calibration is done in the laboratory, after this these blocks are placed in orchard soil from where blocks absorb water. Electrical resistance is noted when equilibrium has reached. This electrical resistance measures the amount of water absorbed by porous blocks in turn of water content of the orchard soil. These blocks measures content in the range of -1 to -15 bars potential.
The method is based on the ability of hydrogen atoms in water to check the speed of fast moving neutron and scattering them. There is one drawback in this method. That is most of the hydrogen in soil is present in organic compounds in addition to water.
Working of a Neutron Moisture Meter:
The aluminum access tube having a source fast neutron and a slow neutron tube placed into the soil in the plant root zone.
Neutrons emitted by the source at a high speed collide with atom of hydrogen in soil water. The direction of movement of neutrons is changed and some energy is lost. The detector tube and scale measures the slowed down neutrons. This reading on the scale relates to the soil moisture content of the given orchard soil.
The tensiometer determines the moisture tension/stress in the field. Tensiometers have a porous ceramic cup at the end of the tube and through which water can move out to the soil in vicinity. On the other end of the tube a vacuum gauge is fitted at the top. Water filled tube reads zero in vaccum gauge when cup is placed in soil depth with low moisture content, the water from porous cup is drawn causing a decline in water pressure in the tube till equilibrium is reached.
The gauge measures this pressure called tension. The length of the tube varies in different tensiometers. The porous bulb should reach the depth of the soil where most of the fruit tree roots are present and draw water. Tensiometers are successfully used to irrigate orchards. Moisture requirements for different fruit plants have been worked out and growers can easily read the soil moisture tension of his orchard at different locations and depths with the help of a tensiometer and can adjust frequency of irrigation accordingly.