THE PRAIRIES. 



19 



amount of 0.25 inch does not occur, are not infrequent. Indeed, 16 or 

 17 such periods have occurred at Lincoln during the past 22 years. 

 The average annual snowfall is about 24 inches. ^^As a rule snow 

 covers the ground but a few days at a time after each snow storm, and 

 the ground is covered with snow less than half of the time even during 

 the months of the heaviest snowfall" (Loveland, 1912). Much of 

 the snow is swept by high winds into depressions, and thus contributes 

 often but httle to the supply of soil moisture of the land upon which 

 it falls. Hence it may be seen that precipitation is only a general 

 indicator of conditions for plant growth. Obviously its influence upon 

 the distribution and seasonal activities of plants is exerted through its 

 power to replenish soil moisture. 



Table. 1. — Mean monthly and annual precipitation at Lincoln, Nebraska, in inches. 



Months. 



Precipi- 

 tation. 



Months. 



Precipi- 

 tation. 



Feb 



0.67 

 0.96 

 1.26 

 2.51 

 4.39 

 4.43 



July 



Aug 



Sept 



Oct 



4.01 

 3.72 

 2.91 

 1.94 

 0.94 

 0.85 



28.59 



May 



Dec 



Annual. . . 



The fertile, dark-colored prairie soil of the region is of the type com- 

 monly called loess, much of which, however, is confounded with glacial 

 drift. ^'The loess covers the hills and valleys alike to a depth of from 

 20 to 100 feet, being much thicker than this in places and much thinner 

 in others. Throughout the first 100 miles westward from the Missouri 

 it is underlain by Kansan till " (Alway, 1916) . ''The uniformity in the 

 physical properties, recognized as characterizing the material of the 

 loess, should tend to produce, under uniform climatic conditions, soils 

 uniform in chemical properties." The water-holding capacity of the 

 surface foot of soil is about 60 to 70 per cent of its dry weight, while the 

 moisture equivalent and wilting coefficient are 25 per cent and 13.5 

 per cent respectively.^ 



Studies of the water-content of upland prairie soils have been 

 carried on for more than two seasons. Table 2 gives the results of 

 water-content determinations during the growing season of 1916. 

 The minus sign indicates water non-available for plant growth. 



A glance at these results shows that at four different periods no 

 water was available for growth at a depth of 4 inches, while during 

 late August the same condition obtained for the 4 to 12 inch layer. 

 Unfortunately, deeper soil samples were not obtained, but during the 

 following seasons samples were taken at irregular intervals to a depth 

 of 5 feet. These data are shown in table 3. 



^ The writer is indebted to Dr. L. J. Briggs, of the Bureau of Plant Industry of the U. S. 

 Department of Agriculture, for determinations of moisture equivalents and wilting coefficients. 



