Southeastern Washington and Adjacent Idaho. 47 



than on the exposed slopes, as was shown by six sets of humus 

 determinations made for each foot of soil on the two slopes, re- 

 spectively. This combination of more clay and more humus on 

 the north and northeast slopes reflects itself especially in the 

 increased water holding capacity of the soil. An average of six 

 determinations gave a mean water holding capacity of 56 per 

 cent, (based on dry weight at 104 C.) for the first foot of soil 

 on north slopes, as compared with 48 per cent, on south slopes. 

 This margin of 8 per cent, is rather an important difference in 

 favor of the soils on the protected slopes, since the wilting coeffi- 

 cients of the two soils differ but little. These substratum differ- 



FIG. 12. Graphs showing the march of soil water to a depth of 10 

 inches from April to September, 1913 (heavy lines), and from May to 

 August, 1914, on a northeast slope (solid lines), and a southwest slope, 

 respectively. j7 



