ORGANIC CONSTITUENTS OF SOILS 



147 



While this table does not represent exactly the changes that take 

 place in the soil under all conditions, yet Schriiier and Brown 

 have shown that many particles of coal and charcoal-like material 

 exist in the soil, indicating that this process probably occurs in soils. 

 Some of this may be the result of fires. 



Nitrogen Content of Humus. Moisture plays a very impor- 

 tant part in determining the composition of organic matter of soils, 

 indicating that the process of humification is quite varied under 

 different conditions. This difference is well shown in the nitrogen 

 content of humus from arid and humid regions. Hilgard has shown 

 that soils of California vary in humus content with the rainfall, but 

 their nitrogen appears to be independent of humidity. 



Humus and Nitrogen Content of Humid and Arid Soils of California 3 



Later investigations seem to indicate that the nitrogen content 

 of the humus in arid soils is not as high as the above figures show. 

 The organic matter of humid regions contains a somewhat vari- 

 able amount of nitrogen. One hundred and twenty-six surface sam- 

 ples of prairie soils contained 5.1 per cent of organic matter, the 

 nitrogen of which was approximately 4.88 per cent. The average 

 organic-matter content of M surface samples of timber soils wa^ l.!>3 

 per cent, which contained 5.09 per cent of nitrogen. By multiplying 

 the nitrogen content of a humid soil by 20 a fair approximation 

 of the amount of organic matter may be obtained. 



The character of soil humus will also depend upon the character 

 of the material from which it is derived. Snyder, of Minnesota, has 

 determined the amount of nitrogen in humus from different sources. 

 The Amount of Nitrogen in Humus from Different Materials 4 



Distribution of Organic Matter in the Soil Strata. The con- 

 tent of organic matter diminishes with depth in all normal soils so 

 that at from three to six feet the amount becomes very low. In arid 



