Measurement of soil moisture is necessary in several soil and hydrological investigations. Soil moisture measurements are needed for scheduling irrigations, in experimental studies on crop water requirements and evaluating surface irrigation systems. Soil moisture measurements are also needed in areas of less rainfall for evaluating different moisture conservation practices.
There are many methods available for measuring soil moisture. Most of these methods are confined to laboratories and research stations and the average farmer still relies on visual observations.
The different methods for soil moisture determination are:
1. Appearance and feel of the soil,
2. Gravimetric method, and
3. Use of instruments.
1. Appearance and Feel of the Soil:
One of the oldest methods for determining the soil moisture is visual observation and feel of the soil. While the accuracy of judgment improves with experience, Table 5.2 will serve as a guide for estimating the available soil moisture in soil samples obtained from the root zone of the crops.
2. Gravimetric Method:
Soil samples are taken from the desired depths at several locations in each soil type. The samples are weighed, dried in an oven at 105°C for 24 hours and then weighed again. The difference in weight is the amount of moisture in the soil usually expressed as percentage on dry weight basis.
This method is the most accurate, but is not generally practical for farm use. Its accuracy depends on the number of samples taken and in their mixing. It is used primarily in experimental work and is a standard against which other methods of moisture determination are compared.
3. Measuring Instruments:
Several instruments are available commercially for measuring soil moisture. These devices may not be as accurate as the gravimetric method, but they are quick and less cumbersome.
The instruments now available are:
(2) Electrical conductivity measuring devices; and
(3) Neutron moisture meters.
The tensiometers consist of a porous cup filled with water and connected by a continuous water column to a vacuum measuring device, either a dial type gauge or a mercury manometer. The cup is placed in the soil at the desired depth. Because of the porous nature of the cup, a balance is established between the water inside the cup and the water in the soil outside.
The water moves out or in the cup depending upon the tension of the soil water increases or decreases. Fluctuations of the soil moisture tension are read above ground on the vacuum indicator. A moisture characteristic curve for each soil is needed to convert the moisture-tension measurements into moisture percentages.
Tensiometers are suitable up to tensions of 0.8 atmospheres. Beyond this tension, air enters the closed system through the pores of the cup and the instrument is no longer accurate. Since tensiometers operate satisfactorily only up to tensions of 0.8 atmosphere, they are useful in sandy soils, where this represents a major portion of the available water, or for crops that must be irrigated frequently. They are not so well adapted to clay soils where tensions normally are high.
(2) Electrical Conductivity Measuring Devices:
Electrical instruments use the principle that a change in moisture content produces a change in some electrical property of the soil or of an instrument in the soil. Electrical properties of resistance (or conductance), capacitance, and dielectric strength can be used to indicate moisture content, as all these properties are affected by a change in the moisture content. While only limited success has been achieved using capacitance or dielectric, the resistance or the conductance principle has been successfully utilized.
Electrodes permanently mounted in conductivity units made of plaster of paris, nylon, fibre-glass, or gypsum are buried at the depths to be studied. Resistance or conductance meters measure the changes in electrical resistance in the blocks associated with changes in the moisture content of the soil.
The blocks used should be calibrated for each soil and a calibration curve indicating the moisture content and resistance is prepared. Salts in the soil are found to affect the working of the conductivity units.
Gypsum is found to be less sensitive to soil salts than nylon and fibre glass because of the concentration of soluble gypsum within the water of a gypsum block. In general nylon and fibre glass units are more sensitive than gypsum blocks at high moisture content and low tension conditions. Gypsum blocks operate best at tensions between 1 to 15 atmospheres.
(3) Neutron Moisture Meters:
The neutron moisture meter has become popular for soil moisture determination because of its convenience in using and accuracy of measurements. The instrument consists of two main parts viz. – the probe and a scaler or rate meter. In the area where soil moisture measurements are desired an access tube suitable to the probe is installed. The probe contains a fast- neutron source which emits γ-radiation as well as a detector of slow neutrons.
The fast neutrons are emitted radially into the soil. They collide elastically with various nuclei present in the soil and are slowed down. Some of these return to the probe where a detector of slow neutrons counts them. The counts are transmitted to the scaler or indicated by a rate meter. The count rate depends upon the volumetric water content of the soil.
To use the instrument in a particular soil, it should first be calibrated i.e. the relation between the, count rate and the volumetric water content must be established. In most soils it is linear. The probe essentially monitors the moisture in a spherical volume around it.
The volume monitored depends also on the moisture content. Because of the low spatial resolution, the instrument is not suitable to detect water-content discontinuities like wetting fronts or boundaries between two layers. The instrument is also not suitable for measuring moisture very near the soil surface.
Two tensiometers have been installed in a root profile at a depth of 30 cm and 60 cm. The top one shows a suction of 60 cm of water and the bottom one shows a suction of 40 cm of water. In what direction the water is moving?
Considering the lower tensiometer as the datum, the gravitational potential for the upper tensiometer = 30 cm and the hydraulic potential = 30 – 60 = – 30 cm. For the lower tensiometer, the hydraulic potential = 0 – 40 = – 40 cm.
The direction of flow is from higher to lower potential or it is downward.