368 
Journal of Agricultural Research 
Vol. I, No. 5 
in the Bureau of Soils . 1 The method is simple and rapid and the meas¬ 
urements can be readily made in the field, which is a great advantage in 
studying the distribution of vegetation in relation to the salt content 
of the soil. The method is, however, necessarily an approximate one, 
owing to the variation in the composition of the soil solution and to the 
fact that the salts found in soils differ greatly with respect to their molec¬ 
ular weight and ionic migration velocity. To interpret the observed 
resistance, a calibration curve was prepared, based upon the observed 
relationship between the electrical resistance and the salt content, gravi- 
metrically determined, of a number of soils from different parts of the 
valley. (See fig. i.) 
In making the gravimetric determinations, the usual practice was followed of digest¬ 
ing ioo grams of dry soil with 500 c. c. of water, filtering, and evaporating an aliquot 
Fig. i. —Curve showing the relation between the salt content (in percentages of the dry weight of the 
soil) and the specific electrical resistance (in ohms) of the soil when saturated with water. 
portion of the filtrate to dryness. A number of the samples examined were rich in 
gypsum, and in digesting such soils with an excess of water the total quantity of gyp¬ 
sum which goes into solution is greatly in excess of the quantity dissolved when the 
soil is simply saturated with water. The gravimetric determination of the salt content 
of soils which are rich in gypsum is consequently too high, and this accounts in part at 
least for the outlying points above the calibration curve. (Fig. 1.) 
By means of a suitable centrifugal apparatus it is possible to remove and collect a 
portion of the soil solution in an unsaturated soil. From the concentration of this 
solution and the initial moisture content of the soil, the salt content of the soil can be 
calculated. This method gave results more nearly in accord with those indicated by 
1 Whitney, Milton, and Means, T. H. An electrical method of determining the soluble salt content of 
soils. U. S. Dept. Agr., Div. Soils Bui. 8,30 p., 6 fig. 1897. 
Briggs, E. J. Electrical instruments for determining the moisture, temperature, and soluble salt content 
of soils. U. S. Dept. Agr., Div. Soils Bui. 15, 35 P-» 12 fig. 1899. 
Davis, R. O. E., and Bryan, H. The electrical bridge for the determination of soluble salts in soils. 
U. S. Dept. Agr., Bur. Soils Bui. 6i, 36 p., 7 fig. 2 pi. 1910. 
