Humus and Soil Fertility 



145 



4. It has an important cUcct upon the biological state of the soil, 

 making it a more favorable medium for tlie development of the root 

 svstoms of plants and for the growtli of microorganisms essential for 

 soil processes. 



Color of Soil 



\ 



4,0 



35 



3.0 



'^ 2.5 



2.0 



1.5 



C J2 



Is 



5 -Q 



■\ 



\ 



X) ^ 



^_ 



. '^, 



^^^ 



^ 



if 



St?; 



\ 



^ 



5 .- 



o o 



■^ c" 



^^X 



:v 



0.40 



0.35 



0.30 



0.25 .^ 



0.20 



0.15 



Surface Soil Subsoil 



Fig. 63. Carbon and nitrogen content of different soils and subsoils (from 



Brown and O'Neill). 



For countless generations, man depended upon the humus of the 

 soil to supply the necessary plant nutrients, through the activities of 

 the numerous microbes which inhabit the soil in thousands of mil- 

 lions per single gram. These nutrients were built up by the plants 

 into plant tissues, which were partly consumed by animals, including 

 man, and partly returned to the soil in the form of leaves, needles, 

 stems, and roots. The animals and their excreta found their way, 

 sooner or later, into the soil, to serve again as sources of humus, to 

 be later again decomposed, with the liberation of the constituent 

 nutrient elements for renewed plant growth. Humus can thus be 



