80 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Bates (1910) gives us the following table of water-contents for soils 

 taken in the sandhills at Halsey, Nebraska: 



Table 17. — Amount of moisture in the soil, per cent of dry weight. 



From these data it may be seen that the soil was rather uniformly 

 moist to a depth of 6 feet. We must keep in mind, however, that these 

 data were taken under a rainfall of about 23 inches and during the wet 

 phase of the climatic cycle. 



A single set of duplicate determinations, taken in the area studied 

 on July 30, showed the water-content to be rather uniform to a depth 

 of 3 feet (about 4 per cent), 3 per cent of which was available for plant 

 growth. While further determinations were not made, in all of the 

 numerous trenches dug the sand was perceptibly drier at greater 

 depths. It seems certain that under the normal light rainfall the 

 surface soils from 4 to 36 inches would contain the most moisture, 

 while the deeper soils would become wet only during the years of 

 abnormal precipitation. The latter may account for the deep root 

 habit of Psoralen lanceolata and others. The deeper soils, once wetted, 

 would dry out very slowly, because of the relatively small number of 

 plants drawing their water-supply from them. Soil nutrients may be 

 a hmiting factor, but this seems improbable in view of the luxuriant 

 growth and complete occupancy of the soil by plants when sufficient 

 water is supplied. 



