THE ORGANIC MATTER OF THE SOIL 141 



pounds, a considerable mass of material remains, which 

 is insoluble in water, alkali, and other ordinary solvents. 

 By the extraction of a large amount of soil, Schreiner 1 

 was able to study this material. He found it susceptible 

 to division into six groups,' as follows: (1) plant tissue, 

 (2) insect and other organized material, (3) charcoal 

 particles, (4) lignite, (5) coal particles, and (6) materials 

 resembling natural hydrocarbons, as bitumen, asphalt, and" 

 the like. Such material was found not only near the sur- 

 face of the soil, but at depths of fifteen or twenty feet be- 

 low. All the groups above listec? were found by Schreiner 

 to be represented in the thirty-four soils collected from 

 all parts of the United States and subjected to rigid test. 



The exact origin of such material is problematical.. 

 Forest and prairie fires, infiltration, mild oxidation, and 

 lignification might be mentioned. Of a certainty, the 

 agencies of distribution are the natural forces engaged 

 in physical weathering. This carbonized material is 

 important, as it makes up no inconsiderable part of the 

 soil humus. It is very resistant, and consequently lends 

 stability to the soil organic matter. It can be divided 

 into two general groups, organized and unorganized; 

 in the former the original structure remains intact, while 

 in the latter the original features have been obliterated. 

 The study of such material and the changes that it under- 

 goes not only increases the list of known organic com- 

 pounds existing in the soil, but throws considerable light 

 on the nature of the soil organic matter as a whole. 



96. The estimation of the soil organic matter. — 

 Many methods have been proposed for the determination 



1 Schreiner, O., and Brown, B. E. Occurrence and Nature 

 of Carbonized Material in Soils. U. S. D. A., Bur. Soils, 

 Bui. 90. 1912. 



