EXPERIMENT STATION BULLETINS. 611 



There appears to be no other physical factor known to affect the 

 freezing point of \A'ater. A nunil)er of minor possibilities have been 

 suggested, but after a careful examination and consideration they all 

 have been discarded. 



The conclusion seems to be inevitable, therefore, that the great lower- 

 ings of the freezing point noted are caused by and represent actual 

 concentration of solution and therefore contradict the foregoing theories 

 regarding the constancy and magnitude of concentration of the soil 

 solution. 



No definite and proved theory can be offered to explain this apparent 

 great increase in concentration of the soil solution with decrease in 

 nioiBture content. The following hypothesis, hoAvever, would appear to 

 explain the phenomena and is here tentatively offered. 



It is assumed that the soil solution contains salts or their ions, pro- 

 duced by the reaction of the dissolved components of the soil minerals, 

 hydrolysis of the minerals, application of fertilizers, decomposition of 

 organic matter, etc. These salts have as a rule a high solu- 

 bility and would require a large amount of them to form a 

 saturated solution. They are formed rather slowly and since the fre- 

 quent rains, especially in the humid regions, tend to leach them away 

 only a relatively small amount of them is present at a time. At a high 

 moisture content these salts are diluted and the loAvering of the freez- 

 ing is small. As the moisture content, however, is reduced they are 

 concentrated and the lowering of the freezing point becomes greater. 

 Since these salts are present only in relatively small amounts, they 

 do not form a saturated solution, or at least many of them do not. 



A critical consideration of the character of the soil system and a 

 thorough examination of the available general data of the soil solution, 

 as obtained through soil water extracts, drainage Avater, etc., lend con- 

 siderable support to the above hypothesis. 



A consideration of the ^oil mass shows that it is a heterogeneous 

 mixture composed of solids, gases, and a liquid, and inhabited by living 

 organisms. The solid portion, in normal soils, is composed of mineral 

 debris and organic matter, from rock and from plant and animal .degra- 

 dation and decomposition, respectively. Both of these classes of solid 

 matter are guadually reduced to the liquid state. The organic matter, 

 for instance, mainly through the activities of the living organisms, is 

 rendered soluble and many organic and inorganic compounds are form- 

 ed, including the nitrates, humic acids, etc. The nitrates may exist 

 as NaNOg, Ca(N03)o, Mg(N03)o, etc., depending upon the preponder- 

 ance of the base present to unite with the acid. These salts have a 

 very high solubility constant and to form a saturated solution a very 

 great amount of them is required. Thus, to form a saturated solution 

 at 20°C would require 83.7 grams of NaNOg, 31.6 gm. of Mg(N03)2. 

 and 54.8 gm. of Ca(N03)2, in 100 gm. of water. It is very doubtful, there- 

 fore, whether a noriual soil and especially under humid conditions 

 ever possesses a saturated solution in respect to these salts. Assuming, 

 therefore, that there is a certain amount of nitrate salts in the soil 

 solution then it naturally follows that at a high moisture content it is 



