JOURNAL OF Affldim MAM 
Vol. IX Washington, D. C., April 9, 1917 No. 2 
RELATION OF THE WATER-RETAINING CAPACITY OF 
A SOIL TO ITS HYGROSCOPIC COEFFICIENT 1 
By Frederick J. Ad way, Chief of Division of Soils, and Guy R. McDolE, Assistant 
in Soils, Agricultural Experiment Station, University of Minnesota 
INTRODUCTION 
In recent years the importance of the water contained in the deeper 
portions of the subsoil—that below the depth penetrated by the roots 
of crop plants—has been a much-discussed question, and most extreme 
views are entertained, both as to its r 61 e in the moisture supply of annual 
crop plants in dry-land regions and as to its influence upon the indefinite 
maintenance of the mineral nutrients in the surface soil. 
In a recent analysis of the outlook for the reclamation of nonirrigable 
lands in regions of very low rainfall, Hall (12) 2 , while mentioning that some 
work in widely separated regions has cast doubt upon the common sup¬ 
position that— 
the subsoil below the actual range of the roots of the crop may still return water by 
capillarity to the higher levels that are being depleted, the deeper subsoil thus acting 
as a kind of regulating reservoir absorbing rain in tiines of excess and returning it 
when the need arises— 
has pointed out that— 
the evidence on either side is far from being conclusive and more experiment is very 
desirable (12, p. 641). 
The present differences in views appear to be due to the failure in 
laboratory experiments and field studies to take into consideration some 
physical constant that is directly related to both the lower limit of avail¬ 
able moisture and the water-retaining capacity of the soil, if we define 
the latter as the maximum amount which a soil will carry after it has been 
saturated and then, protected from both direct evaporation and the indirect 
effects of this as well as the action of plant roots, allowed to come into ap¬ 
proximate moisture equilibrium by the downward movement of the excess 
of water into the subsoil mass. The lower limit of available moisture as 
1 The work reported in this paper was carried out in 1912-13 at the Nebraska Agricultural Experiment 
Station, where the authors were, respectively, Chemist and Research Assistant in Chemistry. 
2 Reference is made by number to “ Literature cited,” p. 70-71. 
Journal of Agricultural Research, 
Washington, D. C. 
(*7) 
Vol. IX, No. 2 
Apr. 9,1917 
Key No. Minn. — ia 
