iQig] ALWAY, MCDOLE, b- TRUMBULL—SUBSOIL 187 



Location of prairies sampled 



The fields from which we secured the following data form two 

 groups, one in the southwestern corner of Nebraska adjacent to the 

 towns of McCook, Wauneta, Imperial, and Madrid; and the other 

 close to the Nebraska Experiment Station at Lincoln. As all the 

 former were at a distance of 200 miles or more from the experiment 

 station, the sampling of them was feasible only at long intervals, 

 and many of the data from these were secured incidental to the 

 collection of samples for chemical studies. The western group of 

 fields is beyond question well within the semi-arid region, while 

 those at Lincoln He ahnost as far to the west as the strictly humid 

 climate extends on the American prairies. Some may consider 

 that even Lincoln falls within the eastern limit of the great semi- 

 arid region, but the composition of the soil (4, p. 414), the growth 

 of vegetation and its agricultural history, as well as the moisture 

 conditions of the subsoil distinguish it from the drier country west 

 of Holdrege. All the factors which determine the difference in 

 climate alter so gradually from east to west that it is impossible to 

 place any definite line of demarcation between the humid and the 

 semi-arid regions, the most that we are justified in assuming being 

 that for every advance of a few miles to the westward of Hastings 

 there is a nearer approach to strictly semi-arid conditions. At 

 Hastings we appear to be still within the humid region, while at 

 Holdrege, 50 miles farther west, most of the characteristics of semi- 

 arid regions are discernible. Also the distribution of carbonates 

 in the subsoil indicates that the district between Hastings and 

 Holdrege is the region of most rapid transition (4, p. 414). 



Favorable weather conditions 

 The weather of the period covering our work proved extremely 

 favorable for the development of dry subsoils in both localities. 

 At Lincoln it included the driest two-year period (1911-1912) of 

 the past 20 years, 1897 to 1916, although in two years, 1895 ^i^d 

 1 901, there had been a lower annual precipitation than in either of 

 these (table I) . Accordingly the soil moisture conditions we found 

 there may be considered to include those representing the eft'ects of 

 extreme drought. 



