PRAIRIE PLANTS OF SOUTHEASTERN WASHINGTON 231 



southwest prairie slope, respectively. The ordinates represent 

 percentages of soil moisture in the first 10 inches of soil, on the 

 dates indicated by the abscissae. The rainfall between the 

 intervals of readings is also shown in inches, each ordinate rep- 

 resenting 0.1 inch. The horizontal solid and broken lines show 

 the wilting coefficients of the soils on the northeast and south- 

 west slopes respectively. 3 The greater amount of moisture on 

 the northeast slope (in some cases being twice that of the south- 

 west slope) may be noted at a glance ; while the fact that the soil 

 on the exposed slope reached its wilting coefficient about July 

 15, and more than five weeks before similar conditions obtained 

 on the sheltered slope, is significant. Records for the fall of 

 1913 were discontinued when the rains of late September re- 

 plenished the moisture of the parched soil. In 1914 these 10- 

 inch soil moisture determinations were made only at longer 

 intervals and with the object of determining the time at which 

 the wilting coefficient was reached. The water contents on the 

 dates of these determinations are indicated by the light lines 

 (fig. 1), the solid line representing soil moisture on the northeast 

 slope. The rainfall for June, 1914, being approximately normal 

 (and not 1.6 inches in excess of the mean, as in 1913) the wilting 

 coefficient of these soils was reached much earlier than in the 

 preceding year. An examination of these determinations to- 

 gether with the rainfall records at Pullman, shows that at no 

 time after June 28 and until September 14 was there water avail- 

 able for plant growth in the first 10 inches of soil on the south- 

 west slope. Only 0.13 inch of rain fell in July, none in August, 

 and the light showers of the first 13 days of September gave a 

 total precipitation of only 0.33 inch. 



The autumn and winter rains replenish the water lost during 

 the long period of drought and in the following spring the soils 

 again show a maximum water content. Soil moisture records 

 obtained at 6 inches and 12 inches respectively, at the prairie 

 stations on the two slopes from October 15, 1912, to January 1, 



3 Samples of 100-150 grams of soil were invariably taken in duplicate, dried in 

 an oven at 100-104°C, and the water content calculated in percentages on the 

 basis of the dry weight. 



