Change of Electrical Conductivity With Temperature and the Relation of Osmotic Pressure to Electrical Conductivity and Ion Concentration for Soil Extracts

T1TEASUREMENTS of electrical conductivity of soil ex*•'•!• tracts are now widely used to appraise soil salinity. To simplify the interpretation of conductivity measurements, it is necessary to convert the results to a standard temperature. Whitney and Briggs' (4) published temperature coefficient data for moist soils, using nine soil types. Making use of these data, Whitney and Means (5) published a table which has been used for nearly 50 years for converting soil paste resistance measurements to a standard temperature. Data are available in the International Critical Tables and elsewhere on the electrical conductivity of single salt solutions at various temperatures. Richards and Campbell (3) measured the temperature coefficient of electrical conductivity for saturation extracts from 10 soils and prepared a table of temperature correction factors. In view of the practical importance for salinity work of the effect of temperature on conductivity, it was thought desirable to measure the temperature coefficient of electrical conductivity for an additional group of saturation extracts of known chemical composition. To obtain the measurements, the soil extracts were contained in glass conductivity cells having platinum electrodes that were coated with platinum black in the conventional manner. The cells were mounted in Visking cellulose sausage casing bags and were immersed in a kerosene constant temperature bath controlled to ±•015° C. Standard KC1 solutions were used for determining the conductivity cell constants. Care was taken to minimize electrical energy losses external to the sample being measured. The alternating current resistance of the soil extracts was measured at five different temperatures. Tests showed that negligible errors were introduced by inequality of the 1000-ohm resistors used for the arms of the Wheatstone bridge. A new six-dial Leeds and Northrup No. 4,750 resistor was used to balance the bridge. A 1000-cycle voltage source was used on the bridge and witti the use of an amplifier the bridge could be read to a ±.02 ohm at 1000 ohms. The conductivity measurements are shown in Table 1. The symbol EC is used as an abbreviation for electrical conductivity in the table and also may be used for designating the unit for expressing electrical conductivity. It is assumed that the mho/cm is the standard unit. EC X 10, therefore, represents electrical conductivity expressed in millimhos/cm and the conductivity values for the various soil extracts at 25° C are listed in the first column of data in the table.