574 STATE BOARD OF AGRICULTURE. 



AN HYPOTHESIS TO EXPLAIN THE INCREASE OF THE DEPRESSION WITH THE 



DECREASE IN THE WATER CONTENT. 



No definite and proved theory was offered in the first investigation to 

 explain tlie great increase in concentration of the soil solution with the 

 decrease in moisture content. An hypothesis, however, which appeared 

 to explain the phenomenon quite satisfactorily was tentatively offered. 

 This hypothesis has been carefully considered and critically examined in 

 the second investigation and it still appears to present the best explana- 

 tion for the phenomenon under consideration, and consequently it is re- 

 offered here in practically its original form. 



In this hypothesis it is assumed that the soil solution contains salts or 

 their ions produced by the reaction of the dissolved components of the 

 soil minerals, hydrolysis of the minerals, application of fertilizers, de- 

 composition of the organic matter, etc. These salts have as a rule a high 

 solubility and would require a large amount of them to form a saturated 

 solution. They are formed rather slowly and since the frequent rains, 

 especially in the humid regions, tend to leach them away only a rela- 

 tively small amount of them is present at a time. At a high moisture 

 content these salts are greatly diluted and the lowering of the freezing 

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

 concentrated and the freezing-point depression becomes greater. Since 

 they are present only in relatively small quantities, they do not form a 

 saturated solution, 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 water and other studies, 

 lend considerable support to the above hypothesis. 



A consideration of the soil mass shows that it is a heterogeneous mix- 

 ture 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 animal and plant degre- 

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

 matter are gradually being reduced to the liquid state. The organic 

 matter, for instance, mainly through the activities of the living organ- 

 isms, is rendered soluble, and many organic and inorganic compounds 

 are formed, including the nitrates, humic acids, etc. The nitrates may 

 exist as NaNO';,, Ca(N0:.)2j MgCNOg),, etc., depending upon the pre- 

 ponderance of the base present to unite with the acid radical. These 

 nitrate salts have a very high solubility constant and to form a saturated 

 solution a very large amount of them is required. Thus, to form a satur- 

 ated solution at 20°C would require 83.7 grams of NaNOg, 31.6 grams of 

 Mg (NOo)o, and 54.8 grams of Ca(N03)2 in 100 c. c. of water. It is 

 doubtful, therefore, whether a normal 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 

 present in the soil solution it naturally follows that at a high moisture 

 content it is diluted, and as the percentage of water is reduced it be- 

 comes more concentrated, hence the lowering of the freezing point would 

 tend to increase from the high to the low water content. 



The dissolution of the mineral matter is accomplished by the solvent 

 action of water aided by the foreign substances that it may be charged 

 with — such as acids, bases, salts, etc. The soil minerals, however, are 

 as a rule but slightly soluble and their rate of solution is generally very 



