Electrical conductivity

SOIL ELECTRICAL CONDUCTIVITY

Electrical conductivity (EC) gives the total amount of soluble salts present in the soil and is expressed as dSm-1.

         It is known that solutions offer some resistance to the passage of electric current through them, depending upon the concentration of salts present. Hence EC is measured in terms of electrical resistance between parallel electrodes immersed in the soil suspension of water. In such a system, the solution between the electrodes becomes the electrical conductor to which the physical laws relating the resistance are applicable. The electrical resistance “R” is directly proportional to the distance “L” between the electrodes and inversely proportional to the cross sectional area “A” of the conductor.

                     Hence R µL/A (or) R = r x L/A

                     Where r = proportionality constant known as electrical resistivity

                       If L = 1 cm and A =1 cm2 then R =r where r is called specific resistivity

         Hence specific resistance is the resistance of conductor of 1 cm in length and 1 cm2 in area. Higher the salt content, higher the passage of current and lesser is the resistance to the flow of the current. Hence the reciprocal of specific resistivity is called specific conductivity.

         Specific conductivity is defied as the conductivity of a solution enclosed in a cell whose electrodes are exactly 1 cm and posses a surface area of 1 cm2. The resistance is expressed as ohms / cm and the conductivity is expressed in reciprocal ohms or mhos / cm. It is not possible to make a conductivity bridge having electrodes of 1 cm2 area placed exactly 1 cm apart. Hence the factor called cell constant is determined for the given cell.

Principle

         As the amount of soluble salts in a solution increases the electrical conductivity also increases. The electrical conductivity is measured in terms of the resistance offered to the flow of current using a conductivity bridge.

Procedure

·  Switch on the electrical conductivity bridge and wait for 10 minutes

·  Check the instrument with saturated CaSO4 and 0.01 N KCl solutions. EC should be 2.2 and 1.41 dSm-1 respectively.

·  Use the same soil water suspension used for measuring pH for the estimation of EC.

·  Stir the content and allow the soil to settle for 10 minutes.

·  Wash the electrodes carefully and immerse them into soil solution

·  Adjust the temperature condition. Adjust the meter knob until the magic eye of the null indicator is at the widest width. The readings on the scale at this position indicate the electrical conductivity. Multiply this by the cell constant (noted on the cell itself) to get specific conductivity.

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