Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 211 



instances is held firmly among the subterranean organs of various 

 marsh and wet meadow plants. 



The contour and exposure of the land has considerable to do 

 with the distribution of soil-moisture. The distribution of species 

 and of plant associations is therefore naturally related to these fac- 

 tors. Add to this temperature and the mechanical effect of the wind 

 and we have the most important forces that are brought to bear upon 

 the vegetation of the sandhills. The influence of exposure and 

 depth of soil upon soil-moisture is shown especiallv well in the fol- 

 lowing table from Bates' investigations (3). 



AMOUNT OF MOISTURE IN SOIL 

 Per Cent of Dry Weight 



Station 



South slope.. 



Bottom , 



North slope.. 

 Ridge 



At 12 in. 



At 24 in. 



At 36 in. 



At 72 in. 



May I July [Sept. 



4.06 

 5.06 

 5.11 

 3.3.i 



1.94 

 2.94 

 3.02 

 4.01 



3.11 



4.72 

 4.70 

 3.58 



May ! July jSept. 



4.77 

 4.82 

 5.26 

 4.38 



1.62 

 2.11 

 3.33 

 3.79 



3.12 

 4.18 

 5.07 

 3.85 



May I July |Sept. 



4.29 



5.43 

 5.87 

 5.22 



1.78 

 3.30 

 5.14 

 3.67 



3.18 

 4.21 

 6.65 



3.70 



May I July |Sept. 



3.87 2.75 

 6.54 f 7.44 



5.88 7.31 

 5.36 I 4.18 



3.30 

 8.56 



7.41 

 4.94 



It will be observed from the above table that the fluctuation in 

 soil-moisture as the various sites are compared reaches a minimum 

 in July. 



The relative retentiveness of different sandhill soils for mois- 

 ture is determined by comparing the moisture equivalents of the 

 various soils. These values have been determined in the laboratory 

 of the office of Alkali and Drought Resistant Plant Breeding Inves- 

 tigations, United States Department of Agriculture, and are pub- 

 lished for the first time in the following table. 



MOISTURE EQUIVALENT OF SANDHILL SOILS 



The value given is the mean of four or more determinations in 

 each case. The number following the plus and minus sign is the 

 probable error in the mean value. 



