ROOT SYSTEMS OF TRUE-PRAIRIE SPECIES. 2 ( J 



Wauneta in southwestern Nebraska, after periods of prolonged drought, 

 that "the concordance of the moisture-content with the hygroscopic 

 coefficient (wilting coefficient X0.68, i. e., the amount of water a dry 

 soil will absorb when placed in a saturated atmosphere with a constant 

 temperature) was very striking, as though the plant roots, while not 

 recognizing the wilting coefficient, practically ceased to withdraw water 

 as soon as the hygroscopic coefficient had been reached. There was 

 little difference in moistness between the surface foot and the suc- 

 ceeding 2 or 3 feet, and even the deeper subsoil was but little if at all 

 moister. At no level and in none of the fields was there any growth 

 water, the moisture-content being below the computed wilting coeffi- 

 cient." They also computed the hygroscopic coefficients from the 

 data of Shantz (1911) in the short-grass plains at Akron, Colorado, and 

 of Burr (1914) at North Platte, Nebraska. At Akron the soil- water, 

 even during a season of excessive rainfall, was reduced below the wilt- 

 ing coefficient and approached or reached the hygroscopic coefficient at 

 depths of 0.5 to 1.0 and 1.0 to 1.5 feet and at the fifth and sixth foot 

 at certain periods during the growing season. At North Platte the 

 water-content at all levels to 6 feet fell by the end of June (1912) to a 

 point practically equal to the hygroscopic coefficient, there being no 

 evidence of further drying of the subsoil. These data throw much 

 light upon the degree of dryness to which the subsoil may be reduced 

 by natural vegetation, and lead to the conclusion that under field con- 

 ditions water is still available for growth after it has been reduced to 

 the wilting coefficient (c/. Alway 1913 and 1917 on movement of 

 capillary water in dry soils) . 



Eastward from the short-grass plains and through the mixed prairie 

 the gradual amelioration of the severe climatic conditions for growth 

 is indicated by a corresponding transition to a better developed type 

 of vegetation. At the Lincoln station not only the natural vegetation 

 and agricultural practice, but also the soil composition and the mois- 

 ture conditions of the subsoil, show that the region is much more humid 

 (Alway, 1916). Concerning investigations at Lincoln, Alway (1919) 

 states : 



"On only three occasions in the six-year period (1906-1912) did we find in 

 the surface two or three feet the dry condition which indicates the approach- 

 ing exhaustion of available moisture .... Thus as near the surface as the 

 sixth foot, when conditions were such as to develop the driest subsoil, the ratio 

 (of actual water-content to hygroscopic coefficient) was not much below that 

 found in the deep subsoil under normal conditions. This failure of the natural 

 vegetation of the prairies of eastern Nebraska to exhaust the free water of the 

 deeper sub-soil is in sharp contrast with the conditions found on the short- 

 grass prairies of the southwestern part of the State. " 



An examination of soil-moisture data from various prairie sites 

 taken during the growing-seasons of 1916 to 1918 reveals the fact that 



