igig] 



ALWAY, MCDOLE, cp- TRUMBULL— SUBSOIL 



199 



ratios at all levels sampled had fallen to practically i.o and so 

 remained through the remainder of the season, there being no 

 evidence of further drying of the subsoil. 



Thus the data of both Shantz and Burr confirm our findings 

 regarding the stage of dryness to which the subsoil may be reduced 

 in a dry season by the short grass vegetation, while those of the 

 latter author agree also with our view that after the subsoil moisture 

 content has been reduced to approximately the hygroscopic coeffi- 

 cient it suffers but httle, if any, further lowering through a continua- 

 tion of the drought conditions, and not with that of Briggs and 

 Shantz that the subsoil continues to lose water through the plant 

 tissues until it approaches an air-dry condition (6, p. 8). 



TABLE X 



Ratio of moisture context to hygroscopic coefficient in a prairie field 

 AT North Platte in 191 2, computed from data of Burr 



Depth 

 foot 



Hygroscopic 

 coeflScient* 



6.8 

 6.8 

 6.8 

 6.8 

 7-S 

 7-5 



April 



18 



2.5 

 9 



2 



I 

 I 

 I 



29 



May 



June 



10 



1-4 

 1.3 

 1.6 

 1-4 

 1-3 

 1.2 



29 



July 



25 



26 



Aug. 



0.8 



o," 



o 



o 



o 



o 



Sept. 



* Computed from wilting coefficients of a representative set of samples (7). 



Extremes in eastern Nebraska 



The periods of extreme drought at Lincoln were not numerous, 

 and usually when these came an examination with the soil auger 

 showed that the moisture content of even the surface foot or two 

 was well above the hygroscopic coefiicient, and as it was only the 

 minimum moisture content that we were seeking in these prairie 

 fields we report data from only a few sets of samples. On only 3 

 occasions in the 6-year period (1906-1912) did we find in the surface 

 2-3 ft. the dry condition which indicates the approaching exhaustion 

 of available moisture (table XI). 



The first sampling, on August 23, 1909, had been preceded by a 

 comparatively dry period of 42 days, during which only 1.57 inches 

 of rain had fallen; none had fallen in the last 20 days, while the 



