PRAIRIE PLANTS OF SOUTHEASTERN WASHINGTON 229 



light showers of July and August seldom have much influence on 

 the water content of the soil. We may compare the soils of this 

 region to a gigantic reservoir replenished mostly during the non- 

 growing season, and (as will be shown) rather thoroughly emptied 

 of its water during the summer. It is not the absolute rain- 

 fall figures alone which furnish a criterion of climate, for the 

 maximum duration of the drought period constitutes a limiting 

 factor of the greatest importance. The great problem is the 

 extent to which soil water derived from the winter precipitation 

 is conserved through the weeks of drought. The rains in south- 

 eastern Washington are so gentle that there is practically no 

 run-off; and the silt-loam soils have a wonderfully retentive 



TABLE 1 



Precipitation at Pullman (in inches) 



October 1 .70 April 1 .50 



November 3.41 May 1 .84 



December 2.66 June 1.20 



January 2 .55 July .57 



February 2.18 August 0.68 



March 2.02 September 1.29 



Total 14.52 Total 7.08 



n 



power of holding the moisture. As pointed out by Shreve 

 the influence of rainfall upon the distributional and seasonal 

 activities of plants is obviously exerted chiefly through its power 

 to replenish soil moisture, and while rainfall is only mediate in 

 its relations to plants, soil moisture is immediate. 



SOILS AND SOIL MOISTURE 



The prairie soils of the region have originated from the de- 

 composed underlying basalt. By the action of water, and 

 especially by the prevailing southwest wind, the prairie topography 

 has been moulded into rounded hills which reach a height of 100 

 to 360 feet, and resemble sand dunes. The soil is usually many 

 feet deep, and only along the canyons of streams is the lava-rock 

 exposed. The wind has drifted much surface soil and humus 



2 Shreve, Forrest, Rainfall as a Determinant of Soil Moisture. Plant World, 

 17:9-26, 1914. 



