12 BULLETIIsr 191, U. S. DEPAETMENT OF AGRICULTUEE. 



manently established and in vigorous condition (fig. 4). True 

 buncli grasses, on the other hand, reproduce entirely from seed, two 

 to three years being required to establish fully a seedling plant. 

 Thus, while a shoot originating from the rootstock of a turf- forming 

 species is largely nourished through the medium of the deep-rooted 

 parent plant, the establishment of a bunch-grass seedling is dependent 

 upon its own development for moisture and nutriment. Accordingly, 

 turfed wheat grasses gain dominion over the soil in the drier situations 

 where their rate of occupation may from time to time be more or less 

 seriously interrupted through drought, but where the well-established 

 plants are seldom killed. In such habitats the bunch grasses are 

 usually killed out in the seedling stage, or the established plants, in 

 competition for water, are crowded out by the shallow-rooted turfed 

 species. On the other hand, habitats which receive considerable pre- 

 cipitation and are characterized by soils which permit of ready per- 

 colation of water are capable of supporting the deep-rooted plants. 

 Such habitats are seldom if ever congenial to the domination, or in- 

 deed the conspicuous presence, of the turfed wheat grasses. 



SOIL WATER CONTENT. 



A comparison of the soil moisture conditions on a typical turfed 

 (small wheat grass) area and on a typical bunch grass (blue bunch 

 wheat grass) area in close proximity to each other may be made by 

 observing the graphs in figure 5. 



Section A of the graph, representing the moisture conditions of the 

 soil supporting a typical stand of small wheat grass during the grow- 

 ing season of 1915, shows a rather sharp decline in the moisture con- 

 tent in the three soil strata studied (0-6, 6-12, and 12-24 inch depths) 

 from July 1, which marks the beginning of vigorous spring groAvth, 

 to September 20, the end of the growing period. On July 1 the high- 

 est per cent of moisture was recorded in the 0-6 inch layer of soil, 

 11.8 per cent of the water content being available for the use of the 

 plant. In the 6-12 inch layer of soil for the same period there was 

 approximately 1 per cent less moisture than in the upper layer, while 

 in the 12-24 inch depth there was 4 per cent less. During the second 

 period, however, there was a sharp decline in the moisture of the 

 surface layer, and during the third period a rather striking increase. 

 In the 6-12 and 12-24 inch depths for the same periods the decline 

 was gradual, which is typical of all subsequent periods at the two 

 lower depths. 



The most significant facts brought out in section A, however, are 

 (1) the rather striking fluctuations in the water content in the 0-6 

 inch layer, and (2) the fact that the water content in the 0-6 inch 

 depth of soil is reduced to a point at which it becomes unavailable 



