112 BACTERIA AND SOIL FERTILITY 



Very old grassland 13.7 



Manitoba prairie soil 13.0 



Pasture recently laid down . .11.7 



Arable soil lo.i 



Clay subsoil , . . . . 6,0 



Other things being equal, a wide carbon-nitrogen ratio indi- 

 cates a more fertile soil than a narrow carbon-nitrogen ratio. 

 However, this must always be interpreted with regard to the cli- 

 matic conditions. In the arid regions the carbon-nitrogen ratio is 

 narrow when compared with soils of the humid regions, yet the 

 bacterial activity of the former is just as active as that of the 

 latter. 



The organic substances found within the soil are called humus 

 and result from the action of bacteria upon the plant residues. 

 The composition of this substance varies with the products from 

 which it has been formed, also the degree of humification which 

 has taken place and the specific microorganisms which have 

 brought about the change. Moreover, the quantity and speed with 

 which humus is formed depends upon the nature and condition of 

 the material used and the physical, chemical, and biological condi- 

 tions of the soil. Hilgard thinks that in the humid regions one 

 part of normal soil humus may be produced from five to six parts 

 of dry-plant debris, whereas in the arid regions from eighteen to 

 twenty parts of the same material would be required. Snyder 

 allowed various organic substances to humify for one year with 

 the following results: 



I part fresh cow manure yielded 23 parts humus containing 6.16% nitrogen 



I part green clover yielded 25 parts humus containing 8.24% nitrogen 



I part meat scraps yielded 11 parts humus containing 10.96% nitrogen 



I part sawdust yielded 10 parts humus containing 0.30% nitrogen 



1 part oat straw yielded 6 parts humus containing 2.5% nitrogen 



Humus is mainly valuable because of its physical effect upon 

 the soil and on account of its nitrogen, potassium, and phosporus 

 which are slowly liberated by bacteria. Its beneficial effect upon 

 the bacterial flora of the soil and soil fertility, however, is mainly 

 exerted before it reaches the stage of humus. 



