334 Journal of Agricultural Research voi. xv, no. 6 



For accomplishing the latter the tube is placed directly in the ice mix- 

 ture having a temperature of about -2.5° C, and the soil is stirred con- 

 stantly with the Beckmann thermometer until the temperature falls to 

 about I degree above the zero point of the thermometer. Then it is 

 allowed to remain undisturbed until the temperature falls to about 0.5 

 degree below the zero point, when the soil is again stirred with the 

 thermometer in order to cause solidification to take place. As soon as 

 solidification begins, the tube is at once taken out of the ice mixture and 

 placed in the air jacket in the same bath. The soil is gently stirred and 

 the thermometer gently tapped and the freezing point read by means 

 of a lens. By this procedure it takes only about 10 minutes to make a 

 freezing-point determination. 



The proportion of 10 cc. of water to 15 gm. of air-dry soil has been 

 found to be the best, as it gives a sufficient amount of excess water to 

 practically all classes of soil, except peat, muck, and some soils con- 

 taining an exceedingly high content of organic matter. Where a com- 

 parison of the salt content of all kinds of soil (with a few exceptions) is 

 desired, therefore, the above proportion is the best. On the other hand, 

 where a comparison of the salt content of light soils is desired, the pro- 

 portion of 10 cc. of water and 20 gm. of soil is more advisable. The 

 best ratio is that which gives a sufficient amount of excess water and 

 at the same time a comparatively high concentration. In the case of 

 alkali soils a ratio of one of soil to five of water may be used. 



The salt content of soils can be expressed both in degrees of depression 

 and in parts per million of solution. The latter can be easily and con- 

 veniently calculated by following the formula that a depression of 0.004° 

 C. is equivalent to 100 p. p. m. of solution (4). 



In determining the salt content of natural soils from the field the 

 following factors should always be taken into consideration in inter- 

 preting the results: (1) Season of the year in which the soil is collected; 

 (2) amount of rainfall and length of period elapsed after the rainfall 

 before sample is collected; (3) temperature and rate of evaporation; 



(4) cultural conditions of the field, whether cropped or uncropped; and 



(5) depth of collecting sample, etc. All these factors play a very great 

 part, if not the controlling role, in the amount of salts found in soils. 

 Thus, for instance, in the early spring, when the soils are thoroughly 

 washed by the melted snow and the spring rains and when the rate of 

 solubility and nitrification are slow on account of the low temperature, 

 the salt content of all soils, including the richest soils, is exceedingly 

 small, amounting, as a rule, to a depression of only about 0.010° C, or 

 250 p. p. m. , when the ratio of water to soil is about i to 0.7. In summer 

 time after a long drouth the salt content of bare soils at the surface is 

 quite high, amounting in some soils to a depression of 0.200° C, or 4,878 

 p. p. m. of solution, when the ratio of water to soils is i to 0.7. Imme- 

 diately after a heavy and prolonged rain, however, all these salts are 



