THE PRAIRIES OF THE PACIFIC NORTHWEST. 



41 



feet deep. The wind has drifted much surface soil and humus material 

 from the exposed south and southwest slopes and deposited it upon 

 the steeper north and northeast leeward slopes. 



It may be seen in tables 8 and 9 that the soil in the spring shows a 

 maximum water-content, the autumn and winter precipitation having 

 replenished the water lost during the long period of summer drought. 

 Table 8 gives the available water-content of the prairie soil to a depth 

 of 10 inches during 1913 and 1914. Table 9 shows the available water- 

 content of prairie soils to a depth of 5 feet during 1913-14. The minus 

 sign indicates water non-available for plant growth. 



Table 10. 



The greater water-content on northerly exposed slopes is the result 

 of several causes. More precipitation actually occurs here as far as 

 effective moisture is concerned, much of the 46 inches of normal snow- 

 fall being blown over to the sheltered north hillsides. The soil on 

 north slopes is deeper, has more humus, and a greater water-holding 

 capacity. It has been seen that this reflects itself both in plant dis- 

 tribution and root development. An examination of table 9 shows 

 not only a gradual depletion of soil moisture as the season progresses, 

 but also that to a depth of 2 or 3 feet little or no moisture is available 

 in late summer. 



This seasonal march of soil-moisture, together with its greater 

 scarcity on south slopes, is further illustrated in table 10, which shows 

 the water-content of soil samples taken from another set of stations 

 during 1914. 



To evade the drought conditions thus imposed upon them, a few 

 plants, notably the shallow-rooted grasses, complete their period of 

 growth and produce seed early in the season, and then lie dormant 

 until revived by the autumn rains. Most prairie species, however, 

 extend their roots far down into the deeper moist soil. As the moisture 

 supply decreases with the progress of summer, the evaporating power 

 of the air becomes more and more intensive. This is shown in table 1 1 . 



Thus, high evaporation and low available soil moisture combine 

 to make conditions rather unfavorable for plant growth, especially 



