Fundamentals of Soil Science (1+1)

Exercise: 7

Aim: Determination of Electrical Conductivity of Soil

• The soils contains varying amounts of soluble salts in the soil. However, excessive accumulation of these soluble salts in the soil results in harmful effects on plant growth. The measurement of electrical conductivity of soil is important as it renders information about the concentration of soluble salts in the soil.
• Electrical conductivity (EC) is the reverse of resistance. Therefore, electrical conductivity is defined as the reciprocal of the resistance of a conductor 1 cm long and 1 cm³ in cross sectional area. It is expressed as mhos cm-1. as the value of electrical conductance obtained for soil solution are very small, therfore, it is expressed in millimhos/cm(ECx10³) or dS/m.
• The salt content of the soil can be estimated roughly from an electrical conductivity measurement of a saturated soil paste or a more dilute suspension of soil in water.

Principle

• Solutions of electrolyte conduct an electric current by the migration of ions under the influence of an electric field. Like a metallic conductor, they obey Ohm’s law. Thus for an electromotive force E (in volts) maintained constant, the current I (in amperes) flowing between the electrodes immersed in the electrolyte will vary inversely with the resistance R of that portion of the solution between the electrodes.
• The reciprocal of the resistance is called the conductance (C) and is expressed in reciprocal ohms or mhos. The electrical conductivity values of soil are now being expressed as dSm-1.

Viz., I = E/R
R = I/E = C

• Since, salt ions allow the electric current to pass through them, therefore, the electrical conductivity of the soil water suspension rises with increasing content of soluble salts in the soil. Thus, EC is a measure of the concentration of soluble salts in the soil at a particular temperature.

Materials required:
i) Analytical/ Electrical Balance ii) Cylinder graduate of 50 ml iii) Beakers (50 ml)
iv) Thermometer (Centigrade) v) Wash bottle vi) Conductivity Meter

Procedure

• Weight out 25 g soil in a 100 ml beaker and add 50 ml of distilled water.
• Stir it intermittently for 4-5 times and than leave it overnight for getting a clear supernatant solution. Alternatively, the clear extract after the pH measurement can also be used for EC measurements.
• Adjust the temperature compensation knob to the room temperature and connect the cellto the meter.
• Measure the EC of supernatant solution using conductivity meter.

Precautions

• Soil suspension should be allowed to stand for a sufficient time so as to obtain a clear supernatant solution.
• The instrument must be set at the temperature of the test solution.
• The electrodes should be completely immersed in the test solution to get an accurate reading.
• The electrodes must be washed with distilled water and cleaned with filter paper after measuring the conductance of every sample.
• When not in use, the electrodes should be kept dipping in distilled water, otherwise, deposition of salts on them will reduce their efficiency.

Interpretation

EC(1:2 Soil :Water) dS m-1 (mmhos/cm)	Approximate salt concentration(%)	Effect
<0.8 0.8-1.6 1.6-2.5 > 2.5	<0.05 0.05 – 0.15 0.15 – 0.20 0.20 – 0.25	Normal-suitable for all crops Critical for salt-sensitive crops Critical for salt tolerant crops Injurious to all crops