To determine the electrical conductivity and soluble salt content of soil. Soil salinity, which is caused due to the total soluble salt content, is determined by measuring the electrical conductivity of soil. EC is a measure of the ability of soil solution to carry electric current by the migration of ions under the influence of an electric field. Like metallic conductors, solutions also obey ohm’s law.
R in ‘ohms’
E in ‘‘volts’
J in ‘amperes’
The reciprocal of resistance is called conductance
Conductance (C) = 1/R; the units are ‘mhos’.
Specific conductance or electrical conductivity is the conductance 1 cc of solution measure between two electrodes placed 1 cm apart. It is expressed in mho cm-1 or mmho cm-1 or dS m-1.
1 dSm-1 = 1mS cm-1
Principle of Electrical conductivity
Pure water offers maximum resistance to the passage of electricity. If any soluble salts are present in water then they dissociate thus increasing the flow of current. The soil salinity in relation to plant growth is generally measured in terms of conductivity of the soil water suspension or saturation extract of the soil. The percentage of soluble salts in soils can then be computed from the following relationship Percentage of soluble salts in soil = 0.064 x EC x SP/100 Where SP the saturation percentage of soil and EC is in dS m-1 Principle of conductivity meter The electrical conductivity of a solution is measured with a conductivity meter known as ‘salt bridge’ or ‘solubridge’. It is based on the principle of Wheatstone bridge in which alternate current is used. Two fixed resistances R1 and R2; a variable resistance Rv are connected in a branched circuit with the conductance cell having resistance Rx (Fig.). The variable resistance Rv is adjusted till no current passes and the reading is recorded. Rv is calibrated in terms of conductance.
Apparatus
Balance, beaker (100 mL), measuring cylinder, a glass rod, a conductivity meter.
Reagents
Potassium chloride (0.01 N): Weigh 0.7456 g of potassium chloride and make the volume to 1 L with distilled water. This solution gives an electrical conductivity of 1.41 dS m-1 at 25 oC.
Procedure
- Weigh 25 g of air-dried soil into 100 mL beaker and add 50 mL of distilled water.
- Carry out intermittent stirring with a glass rod for 30 minutes and then allow to stand for obtaining clear supernatant solution.
- Connect the salt bridge to the power supply, switch on the bridge and adjust it to room temperature with the help of temperature setting knob.
- Calibrate the conductivity bridge with the help of standard KCl solution.
- Dip the conductivity cell in the supernatant solution so that platinum electrodes are completely immersed in solution.
- Note down the EC value.
Result
The Electrical Conductivity (1:2) of the given soil is —————- dS m-l.
