﻿19031 UPLAND PLANT SOCIETIES 47 



analyses of the soil water, for it is this which affects the plant 

 directly. Such analyses were not made in connection with the pres- 

 ent work, but it is hoped that they will be made at some future 

 time, either for this region or some similar one. It is very 

 probable, however, that soil waters from the surface soil in 

 different parts of this area will be found to be very nearly 

 uniform in the salts contained. This conclusion results from 

 two considerations : first, the chemical nature of the soluble part 

 of the surface soil itself is very nearly uniform throughout the 

 county ; second, the washed soils are usually comparatively 

 shallow, and upward diffusion of dissolved substances probably 

 takes place with comparative rapidity, especially when aided by 

 the soil currents produced by changes of temperature, evapora- 

 tion, etc. The only localities where it is at all probable that a 

 paucity in soluble salts will be found to occur in the soil water 

 are the deep sand plains. There is some rather questionable 

 evidence from the vegetation that such is the case in these 



localities, but as yet no definite decision can be made in this 

 regard. 



In classifying soils according to their physical nature, the 

 only question which has any direct bearing upon plant growth 

 is that of the ability of the soil to retain water by capillarity, 

 so-called. Primarily, this ability depends upon the size of the 

 soil particles. Thus sand will retain less water than will loam, and 

 loam will retain less than clay. Sandy soil may be made to retain 

 more water in two different ways, either by the addition of clay or 

 by the addition of humus. The physical effect of humus is very 

 well marked. Of course the humus also adds some nitric acid 

 and certain organic materials which are of benefit to the plant, 

 and it also increases the amount of soluble salts at or near the 

 surface; for humus is formed mainly from leaves, and in these 

 organs the mineral part of the plant body is concentrated. This 

 IS perhaps an important fact in the growth of hard w^ood upon 

 deep sand which is well covered with humus. Where drainage 

 complete and rapid, as in sand, and oxidation is also rapid, 

 humus does not readily accumulate ; but where it does accumu- 

 late as a surface layer, the ability of the soil to retain water 

 approaches that of clay. 



is 



