112 NATURE AND PROPERTIES OF SOILS 



terial was f omid not only near the surface of the soil but at 

 depths of fifteen or twenty feet. 



The exact origm of this material is problematical. Forest 

 and prairie fires, infiltration, mild oxidation, and lignifica- 

 tion might be mentioned. Of a certainty the agencies of dis- 

 tribution are the natural forces engaged in physical weather- 

 ing. Such material can be divided into two general groups, 

 organized and unorganized; in the former, the normal struc- 

 ture remains intact, while in the latter the original features 

 have been obliterated. Part of it belongs, therefore, in the 

 original plant tissue group ; a part of it with the partially de- 

 cayed material ; while some must be included with the simple 

 products of decomposition. This carbonized material is im- 

 portant, as it makes up no inconsiderable part of the soil 

 organic matter. It is very resistant, and consequently lends 

 stability to the organic constituents. 



60. The determination of soil organic matter.^ — ^A num- 

 ber of methods have been proposed for the direct or indirect 

 determination of the organic matter in soils, but none has 

 proved entirely satisfactory, since the composition of this ma- 

 terial is so indefinite and complicated and so likely to change 

 while under investigation. Other soil constituents also tend 

 to interfere with the determination. Three general methods 

 seem worthy of mention, as they have been used very widely 

 in soil analyses and at least give comparative, if not absolutely 

 accurate, results. They will be discussed in the inverse order 

 of their value. 



Loss of ignition.^ — This is a simple method which designs 

 to burn off the organic matter and determine its loss by dif- 

 ference. Five grams of dry soil are placed in a crucible and 

 ignited at a low red heat until the organic matter is all oxi- 



^Soil organic matter as here used refers only to the original and 

 partially decayed organic constituents. Carbon dioxide^ methane, nitrites, 

 nitrates and similar compounds are^ therefore, not included in this term. 



* Wiley, H. W., Official and Provisional Methods of Analysis; IT. S. 

 Dept. Agr., Bur. Chem., Bui. 107, p. 19, 1908, 



