JOURNAL OF AGRICULTURAL RESEARCH 



DEPARTMENT OF AGRICULTURE 



Vol. VI Washington, D. C, August 28, 1916 No. 22 



USE OF THE MOISTURE EQUIVALENT FOR THE INDI- 

 RECT DETERMINATION OF THE HYGROSCOPIC 

 COEFFICIENT 



By Frederick J. Alway, Chief, Division of Soils, Agricultural Experiment Station of 

 the University of Minnesota, and JouETTE C. RusSEL, Professor of Chemistry and 

 Physics, McPherson College, Kansas 



INTRODUCTION 



The maximum amount of soil water available for growth and for the 

 maintenance of life in the case of ordinary crop plants appears to be 

 approximately equal to the free water — the difference between the total 

 amount of water and the hygroscopic coefficient — in those portions of the 

 soil and the subsoil occupied by the roots (1, p. 121). 1 The hygroscopic 

 coefficient (8, p. x; 10, p. 243) expresses the percentage of moisture con- 

 tained in a soil which, in an air-dry condition, has been brought into a 

 saturated atmosphere, kept at a constant temperature, and allowed to 

 remain until in approximate equilibrium with this atmosphere. 



Hilgard's method for the direct determination (10, p. 243; 11, p. 17) of 

 the hygroscopic coefficient requires provision for the maintenance of a 

 constant temperature in the room in which the absorption boxes are 

 placed and also presents difficulties in insuring the actual saturation of 

 the atmosphere in these boxes. Accordingly, any indirect method which 

 gives results in satisfactory accord with those obtained by direct deter- 

 mination and at the same time requires only apparatus which is less 

 inconvenient, either of installation or of operation, will prove useful. 



Briggs and Shantz (7, p. 73) have recently proposed several indirect 

 methods, and to the consideration of the reliability of one of these the 

 present paper is devoted. These authors derived formulas for the indi- 

 rect determination of what they designate the "wilting coefficient," 

 denned as the moisture remaining in the soil in immediate contact with 

 the roots when the permanent wilting of a plant occurs, from the moisture 

 equivalent (6, p. 140; 4, p. 276), from the maximum water capacity as 



1 Reference is made by number to "Literature cited," p. 845. 



t£T Journal of Agricultural Research, Vol. VI, No. 22 



Dept. of Agriculture, Washington, D. C. (833) Aug. 28, 1916 



,_ fc Minn.— n 



