402 Soil, Soil Solution and Freezing -Point Determinations 



investigating the soil solution which depend on its isolation from the 

 soili. 



None of these methods can give, as a final product, the soil solution 

 in the state in which it exists in the soil. Only a fraction of the total 

 moisture content can be obtained by direct methods such as centrifuging, 

 while indirect methods such as mixing the soil with water and filtering, 

 give a solution bearing an unknown, but probably qualitative, relation 

 to the original soil solution. In other words, any modification of the 

 moisture content causes a change in the complicated colloidal phenomena 

 existing in the soil, and hence the portion of soil solution extracted will 

 bear no simple relation to either the portion unextracted, or the original 

 amount. 



Further progress in our knowledge of the soil solution must depend 

 on the use of fresh methods. Bouyoucos has attacked the problem in 

 two ways, of which brief descriptions follow. The essential feature is 

 that in each case the solution is examined in situ. 



DiLATOMETER AND FrEEZING-POIXT MeTHODS OF INVESTIGATING 



THE Soil Solution. 



The dilatometer method is an application to soil of Foote and 

 Saxton's^ experiments on the freezing of inorganic hydrogels. Tiie 

 moist soil is placed in the bulb of the dialatometer and the free space 

 then filled with ligroin. From the reading of the meniscus and the known 

 bore of the tube, the expansion occurring when some of the soil moisture 

 freezes can be calculated, and hence the amount of water frozen. It was 

 found that the water present in soil did not all freeze at one given tem- 

 perature ( — 1-5° C.) and the amount which failed to freeze varied con- 

 siderably in different soils. A similar result was obtained at — 1° C. 

 and — 78° C, although the amounts of unfrozen water were smaller, 

 esjiecially in the colloidal types. No definite relationships could be 

 traced between the amounts of unfrozen water at these temperatures. 



On the basis of these results, the soil temperature is classified into 

 three groups: "free," freezing at — 1-5°C. ; "capillary-adsorbed," 

 freezing at — 4°C. down to — 78°C. ; and "combined," not frozen at 

 — 78° C. The divisions of course merge insensibly into one another, but 

 the values obtained at the temperatures chosen are considered as giving 



1 A summary of these methods is given by Bouyoucos {Tech. Bull. No. 24), and also 

 by Stiles and Jorensen (Journ. Ecology, 2 (1914), p. 245). The latter is the more detailed 

 account, but does not include the Morgan oil pressure method (Soil Sci. 3 (1917), p. 531). 



2 Journ. American Chem. Soc. 38 (1916), p. 588; 39 (1917). p. 1103. 



