8 



THE SOIL AND THE PLANT 



• Prairie 

 oTimber 



finally reach the ocean, which becomes a vast reservoir of these 

 removed materials. In general the basic minerals enter solution 

 more rapidly than the acidic rock components. This weathered 

 soil material consists largely of silicates, aluminum and iron — a 

 mere skeleton of the parent material, but enriched by a clothing 

 of the organic residues of the decomposing vegetative cover. 



The climate exerts even a more important influence upon the 



organic than upon the 

 inorganic soil fraction. 

 The soils in cool and 

 moist regions are, as a 

 rule, richer in organic 

 matter than those in 

 warm and dry regions. 

 As shown in Fig. 2 the 

 nitrogen contents of soils 

 of cool regions are higher 

 than the nitrogen con- 

 tents of soils of warmer 

 regions. Since there is 

 generally a close rela- 

 tionship between the 

 nitrogen content of a soil 

 and the total amount of 

 organic matter, these 

 results may be inter- 

 preted as indicating the 

 presence of greater 

 amounts of organic mat- 

 ter in the cooler soils. 

 Not only is the total organic matter in the soil influenced by 

 climate, but also its chemical nature, such as the relation between 

 the elements carbon and nitrogen. 



We may consider then that the soil is continually exposed to a 

 variety of influences which modify its physical structure, chemical 

 composition, and even its location. Strictly non-biological 

 factors may exert pronounced effects, but the development of 

 higher plants and microorganisms is to a large degree responsible 

 for the creation of fertile agricultural soils from the inorganic 

 substances. 



N% 



0,3 



0.2 



0.1 



Wisconsin Illinois Ky. Tenn. Mississippi 



40° 



50° 



60° Annual Temp. F. 



Fig. 2. — Influence of temperature upon the ni- 

 trogen content of prairie and timber soils (after 

 Jenny). 



