14 BULLETIN 791, U. S. DEPARTMENT OF AGRICULTURE. 



to the plant 10 days earlier than in the two lower depths. During 

 the driest period of the season, usually beginning about August 10 

 in the 0-6 inch depth, and about August 20 in the deeper layers, the 

 main root system often occupies soil whose moisture content is well 

 below that at which vegetation can absorb moisture. The fluctua- 

 tions in the water content observed to occur in the superficial soil 

 layer and the practical absence of such fluctuations in the deeper 

 layers, notably the 12-24 inch depth, are chiefly accounted for by 

 the fact that the supply of moisture in the upper stratum, in which 

 the greater portion of the feeding roots are located, is used up by 

 the vegetation at a relatively rapid rate. Since the upper soil layer 

 is especially rich in organic matter, hence is capable of absorbing 

 a very high percentage of water (45-65 per cent) as it percolates 

 through the matlike layer, the rapid desiccation of the superficial soil 

 is all the more significant. Thus it will be seen that the reduction 

 in the water content in the superficial layer to a point below the 

 amount necessary to m,ake it available to the use of the plant was 

 reached as early as August 10, so that any water absorbed by the 

 plant later in the season had to be obtained at a depth greater than 

 6 inches. The two lower depths of soil, it will be noted, likewise 

 became desiccated after August 20. Therefore at the end of the 

 growing season the 0-6 inch layer of soil was 5 per cent below the 

 point of available moisture, while the 6-12 and 12-24 inch layers 

 were 3.3 and 2.1 per cent below, respectively. In general the growth 

 and seed production are comx^leted in the case of small wheat grass 

 by August 15, when the herbage dries up and remains dormant until 

 the spring. 



Section B of figure 5 represents the moisture conditions on a blue 

 bunch grass area during the growing season 1915, the soil sam^Dles 

 from which the data were obtained being taken simultaneously with 

 those represented in section A. Comparing first the general position 

 of the respective curves, it will be seen that the water content was 

 greater on the blue bunch grass area than on the turfed wheat grass 

 area in each period, with the exception of September 10 to 20 in the 

 12-24 inch depth, prior to which growth had been arrested. In con- 

 trast with the condition on the turfed area, the moisture content in 

 the 0-6 inch layer on the bunch grass type was appreciably in excess 

 of that at greater depths during the first four periods. In the fifth 

 period the moisture content decreased rapidly in the upper soil layer 

 and dropped below that recorded at the two lower depths ; but in the 

 sixth period, as the result of a fairly heavy rainstorm, the moisture 

 content again exceeded that in the lower soil layers. Thus, instead 

 of the 6—12 inch soil depth containing the highest percentage of 

 moisture during the most active period of growth, as in the turfed 

 wheat grass type, the 0-6 inch laj^er contained the maximum amount. 



