Methods of Anahsis of Humus 133 



gctlicr with the nc\\l>- s\ntheiiized microbial cell substance, com- 

 prises the humus. Humus is formed frorn the plant and animal resi- 

 dues which ha\e lost the readily decomposable constituents, have 

 gained the synthesized microbial substances, and have accumulated 

 the more resistant constituents. Humus is, therefore, chemically not 

 always the same. 



Humus accumulates under conditions not favorable to its further 

 decomposition. This is true particularly when humus is formed in 

 a water-saturated en\ ironment, as in peat bogs, or in an acid environ- 

 ment, as in raw humus layers in forest soils, or at very low tempera- 

 tures, as in high altitudes, when it is frequently designated as "alpine 

 humus." 



Methods of Analysis of Humus 



The total humus content can be determined by the loss on igni- 

 tion, especially in peats, composts, and other humus-rich materials. 

 In mineral soils, the best method of determinating humus is to calcu- 

 late it from the organic carbon content, by using the factor 1.724. 

 The fact that humus is not simple in chemical composition and that 

 it comprises a number of complex substances, both organic and 

 inorganic, can be demonstrated by the proximate method of analysis, 

 when it is possible to show that different types of humus have dif- 

 ferent chemical compositions (Tables 26 and 27). Not only does 



Table 26. Chemical Composition of the Organic Matter in Different 



Mineral Soils 



On basis of total dry soil, surface samples. 



forest humus vary from peat humus and from humus in composts, 

 but all these \ary considerably from the humus in mineral soils. The 

 humus in the different layers of forest soils varies greatly in chemical 

 composition. 



