EXPERIMENT STATION BULLETINS. 595 



tion under all circumstances, would be of the greatest service. From 

 theoretical considerations, however, probably no method could ever be 

 expected to ascertain directly the composition of the natural soil solu- 

 tion in the soil, but it might, the concentration. 



A method now in vogue, which would seem at the outset of being 

 capable of measuring the concentration of the natural soil solution in 

 the soil, is the electrical bridge, as developed by the U. S. Bureau of 

 Soils, (8), for the determination of soluble salts in soils. The principle 

 of this method is dependent upon the fact that the electric current is 

 conducted by the salt solution or its ions, and the conduction of the 

 solution or conversely, its resistance to the passage of the current, being 

 determined largely by its concentration. 



Unfortunately, the general utility of this method in soil work is 

 very much limited owing to the large number of factors involved in the 

 soil medium which influence the conductivity of the electric current. 

 It has been found, for instance, that the fine particles of the soil in- 

 crease the resistance, due undoubtedly to the lengthening of the path 

 between the electrodes by the increase in curvature of the grains. The 

 resistance is also influenced by the thinness of the water films. These 

 water films lose the properties of liquid in mass when they become too 

 thin, and together with the capillary spaces in the soil, offer a greater 

 resistance to the electric current. Furthermore, it has been found that 

 the presence of carbonates and organic matter also materially increase 

 the resistance of the soil solution in the soil. 



From these facts, it is evident, then, that the electrical bridge can- 

 not be used to ascertain the concentration of the soil solution with 

 much reliance; this is not only true with soil short of saturation, but 

 also with saturated soils. Hence, some other method is needed to 

 measure the concentration of the soil solution in the soil. 



THE FREEZING POINT METHOD AS A NEW MEANS. 



In conducting a thorough investigation on the general subject of soil 

 temperature at this Station, the influence of soluble salts on the lower- 

 ing of the freezing point of soils was also studied. It was observed 

 that the phenomena of supercooling and freezing behave in moist soils 

 exactly the same way as in solutions. These facts suggested at once the 

 idea that the freezing point method might be employed to measure the 

 concentration of the natural soil solution in the soil itself. In order 

 to ascertain whether or not this could possibly be done, a series of pre- 

 liminary experiments was instituted, using different classes of soil with 

 various moisture contents or known concentrations of ?iolut"'"j^i^. The 

 procedure consisted of placing a short column of soil in a narrow glass 

 tube, inserting the bulb of the thermometer in this column of soil, allow- 

 ing the soil mass to supercool and moving the thermometer in the soil 

 to start solidification. It was discovered that the freezing point lower- 

 ing of the soil solution in the soil, could be determined with great 

 ease. Solidification could be started when the isoil mass was super- 

 cooled to only about 0.3 °C by simply moving the thermometer slightly 

 in the soil. The lowering of the freezing point could be determined in 

 soils from any maximum moisture content down to or below the wilt- 



(•) Bute. 8 and 16, Bur. of Soils, U. S. Dept. of Agr., 1897 and 1910 respectivdy. 



