THE WATER OF SOILS. 199 



undersaturated air. Since air thus undersaturated occurs not uncom- 

 monly in the arid regions of the world, the fact that the soil cannot be 

 farther dried by such air of the same temperature, is of some practical 

 significance. 



In view of the highly variable composition of soils and of 

 the doubtless varying hygroscopic properties of their several 

 physical constituents, it is not to be expected that any one nu- 

 merical law will hold good exactly for all kinds of lands. 

 Mineral powders, colloidal clay, ferric hydrate, aluminic hy- 

 drate, the zeolites, humus, and other hydrates known to occur, 

 doubtless each follow a different law in the absorption of mois- 

 ture and gases ; so as to modify the hygroscopic properties of 

 the soil in accordance with their relative predominance in each 

 case. (See table of absorption of gases, chapter 14). 



Utility of Hygroscopic Moisture to Plant-growth. The 

 early experimenters considered the hygroscopic moisture of 

 the soil to be of very great importance to the welfare of crops. 

 Within the last twenty-five years much doubt has been cast 

 upon this claim, even to the extent of stating that " the 

 hygroscopic efficacy of soils must be definitely eliminated from 

 among the useful properties " (Mayer's Agriculturchemie, vol. 

 2, p. 131). Yet Mayer himself concedes the cogency of the 

 experiments made by Sachs, which proved that dry soil im- 



1 E. A. Mitscherlich (Hodenkunde fiir Land-und Forstwirthe, p. 156 et al.) 

 claims that all determinations of soil hygroscopicity thus far made are grossly 

 incorrect on account of the dew liable to be condensed on the soil layer from 

 fully saturated air, as the result of slight changes of temperature. lie therefore 

 would have all such determination made either in an air-vacuum, or over a io' 

 solution of sulfuric acid. 



Such dew-formation, however, cannot happen to any appreciable extent under 

 the conditions maintained in the writer's work, viz, absorption within a thick- 

 walled (two-inch) wooden box of the dimensions given above, and sunk in the 

 ground in a cellar in which the temperature varies only a few tenths of a degree 

 during 24 hours. The soil layer of one millimeter thickness being put down in 

 the morning, the 7 hour absorption period falls at the time of slightly rising tem- 

 perature, as an additional precaution against dew-deposition. Mitscherlich fails, 

 moreover, to show that this source of error produces any wide or serious dis- 

 crepancies except under such long absorption periods as he finds it necessary to 

 use because of the great thickness of his soil layers. It is doubtful whether the 

 limits of errors in soil sampling do not greatly exceed any of those involved in 

 the writer's method, and whether such accuracy as is attempted by Mitscherlich is 

 of any practical significance. 



