Fundamentals of Soil Science (1+1)

Measurement of Soil Moisture (Cont..)

4. Neutron scattering method:

• The most rapid and indirect method for measuring soil water content is probably that of neutron scattering. In this method number of hydrogen nuclei present per unit volume of soil is measured.

• Fast moving neutrons emitted from a radioactive source (usually Radium- Beryllium or Americium-Beryllium) when collide with particles having mass nearly equal to their own, like hydrogen atom in the soil, release their energy and are thermalised or slowed down.
• The slowed down neutrons are detected by a detector and recorded on a scaler. Commonly used detector of slowed down neutron is a tube containing BF3 gas.
• More the neutrons are slowed down, higher will be the water content of the soil. The zone of influence is generally 15-20 cm around the detector.

 

5. Pressure plate apparatus:

• Pressure plates are used to apply matric potentials from -10 to -1500 kPa (-0.1 to -15 bar). An external source of compressed air is used to push water out of an initially saturated soil sample kept in pressure plate/membrane apparatus.
• The pressure applied within the container decreases the matric potential of water in the plate.

• Under the influence of the applied pressure, water held by the soil is forced out until the equilibrium is attained between the applied pressure and the force binding the water to the soil (matric suction).
• Field capacity (FC) and permanent wilting point (PWP) can also be determined in the laboratory with this equipment using applied pressure of 0.33 bar (0.33 bar for clayey and 0.1-0.2 bar for sandy soils) and 15 bars, respectively.
• As far as possible, undisturbed cores should be used for laboratory determination of field capacity and permanent wilting point.
• For disturbed samples, soil should be air dried, pulverized and passed through 2 mm sieve.

6. Time domain reflectometry method:

• Time domain reflectometry (TDR) is the latest mehod of measuring soil water contents. It makes use of the unique electrical properties of water molecule to determine the water content of soil.
• The speed with which an electromagnetic pulse of energy travels down a parallel transmission line depends on the dielectric constant, (Ka), of the material in contact with and surrounding the transmission line.
• Higher the dielectric constant, slower is the speed.
• Soil is composed, in general, of air, mineral and organic particles and water. The dielectric constants (Ka) for these materials are 1, 2-4 and 80, respectively. Because of the great difference in the dielectric constant of water from the other constituents in the soil, the speed of travel of a microwave pulse of energy in a parallel transmission line buried in the soil is very much dependent on the water content of the soil.
• When soil is completely dry, Ka will be 2 to 4.
• If volumetric wetness is 25 per cent, Ka will be approximately 11-12.
• For agricultural soils the value of Ka depends primarily on the volumetric water content of the soil and is largely independent of the type of soil.

A. Mini trase system B. Wave guide C. Wave Guides installed vertically

• When a microwave pulse travels down a transmission line it behaves in many ways like a beam of light.
• At radio frequencies, the dielectric constant of water is about 80. Most of the other solid components of soil have dielectric constants in the range 2 to 7, and that of air is effectively 1.
• Thus, a measure of the dielectric constant of soil is a good measure of the water content of the soil. The TDR technique measures the transit time of microwave pulse in frequency range of 1 MHZ to I GHZ launched along a parallel wave guide of known distance.
• The apparent dielectric constant, Ka, of the air-soil-water complex can then be determined by the formula:

Ka = (ct/2L)²

Where ‘L’ is the length of the waveguides in centimeters,‘t’ is the transit time in nanoseconds (billionths of a second), and ‘c’ is the speed of light in centimeters per nanosecond.

• The transit time is defined as the time required for the pulse to travel in one direction from the start of the waveguide to the end of the waveguide.