Decomposition Processes 



105 



humus is loft. Materials low iu nitrogcu decompose slowly, liberat- 

 ing at first no available nitrogen and leaving a large amount of 

 humus. This is illustrated in Fig. 49. There are many e.xceptions 

 to this rule, depending on the nature of the residues and their treat- 

 ment and on the nature of the soil. 



25 



20 - 



15 - 



10 - 



5 - 



60 



-40 



- 20 



2 4 6 8 10 12 14 16 18 



Time, days 



Fig. 49. Decomposition of oat straw (from Norman). 



Straw, stubble, and forest litter, unless properly supplemented 

 with essential nutrients, are useful primarily as sources of humus 

 and less as fertilizing materials for plant growth; they may leave, 

 after 3-10 months of decomposition, as much as 50-60 per cent 

 hiunus. 



In the process of humus formation from plant residues, three dis- 

 tinct phases are recognized: (a) rapid decomposition of some of the 

 chemical constituents by microorganisms, (/?) synthesis of new sub- 



