266 T. H. Means— r Soluble Mineral Matter in a Soil. 



several laboratory determinations of this quantity. The amount 

 of soil moisture being known, the cell factor (the figure by 

 which the resistance must be multiplied to obtain specific 

 resistance) for this quantity can be found. * 



4. Multiply the resistance found in (2) by the cell factor and 

 obtain the specific resistance of the soil solution. For dilute 

 solutions the per cent of salt present is nearly inversely pro- 

 portional to the resistance. In a study of the alkali soils of 

 the Westf where the salts are of rather uniform chemical com- 

 position, it is of great advantage to have a more accurate 

 method of calculating the results. In such studies, a solution 

 of the average composition of the salts is made up and its 

 resistance-salt curve determined. In this way the per cent of 

 salt by weight can be reckoned for any resistance. Such a pro- 

 ceeding was followed in some studies of alkali soils in Montana.;}: 



This method offers special advantage in the study of alkali 

 soils. The apparatus necessary can readily be carried in the 

 field. A convenient form of Wheatstone's bridge adapted to 

 field use has been designed by Mr. Lyman J. Briggs of this 

 laboratory. 



The auger for sampling the soil is in general use in the study 

 of surface geology.§ The only other apparatus necessary are 

 the cubic cell,] thermometer, mixing dish and bottle of dis- 

 tilled water. 



This method offers a ready means for the detailed study of 

 irrigated districts. Two men can examine from sixty to one 

 hundred samples of soil in a day and in this way cover con- 

 siderable ground, obtaining sufficient data for the construction 

 of salt maps of the district. 



U. S. Department of Agriculture, Division of Soils. 



*Ostwald's Physico-Chemical Measurements, translated by Walker, page 225. 

 Note. — If a cubical cell is used — such a cell 4 cm internal dimensions lias given me 

 the best results — the cell factor will vary inversely as the number of cubic centi- 

 meters of solution present. 



f Whitney and Means, Bulletin 14, Division of Soils, Department of Agriculture. 



% Loc. cit. 



§Darton, Am. Geol , vol. vii, page 117-119. 



|| Whitney and Means, Bulletin 8, Division of Soils, Department of Agriculture, 

 page 11. 



