ACTIVE AGENTS 191 



pionic, oxyphenylacetic, and oxyphenylpropionic acids; (4) phenol, 

 indol, skatol, pyrrol, and its derivatives, these bodies sometimes 

 being in very small quantities or even completely absent; (5) sulphur 

 derivatives like methyl-mercaptan; (6) various amino-acids, leucin, 

 tyrosin, tryptophan, and sometimes glycin, creatinin, etc.; (7) 

 various ptoma'ins, like putrescin and cadaverin, the guanidins, 

 cholin, and nurin, pyridin, hydrocolloidin, etc. 



In the process of decay the carbon and hydrogen is liberated as 

 carbon dioxid, methane, water, and other volatile products with the 

 result that the carbon in the soil tends to fall off relatively to the 

 nitrogen and the ratio --, which in the original plant material 

 is about 40, is reduced in the soil to 10. This carbon-nitrogen ratio 

 varies with climatic conditions, also with soil type and previous 

 treatment. Lawes and Gilbert, as quoted by Lipman, give the 

 following carbon-nitrogen ratio in the organic matter of different 

 soils: 



Cereal roots and stubble 43.0 



Leguminous stubble 23.0 



Dung 18.0 



Very old grassland 13.7 



Manitoba prairie soil 13.0 



Pasture recently laid down 11.7 



Arable soil 10.1 



Clay subsoil 6.0 



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

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

 this must always be interpreted with regard to the climatic condi- 

 tion. 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 the substance varies w T ith the products from which 

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

 place. 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 conditions of the soil. 

 Hilgard thinks that in the humid regions one part of normal soil 

 humus may be formed 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 : 



Per cent, 

 nitrogen 



1 part fresh cow manure yielded 33 parts humus containing 6.16 



1 green clover " 25 " 8.24 



1 meat scraps 11 " 10.96 



1 sawdust 10 " 0.30 



I " oat straw " 6 " " " 2.50 



