DECOMPOSITION OF HUMUS. 129 



how it arises, I conjecture, may be explained as 

 follows : 



" In the more tenacious soils the carbonic acid 

 evolved in the decomposition of humus forms a 

 little cavity of humous substances; consequently 

 the whole mass becomes traversed by fine pores, 

 just as the carbonic acid in the fermentation of 

 bread renders the tough mass of paste porous. 

 The legion of animals, from the earth-worm to the 

 infusorium, act in the same way, namely, to loosen ; 

 since they penetrate and perforate the soil in all 

 directions, as do, moreover, the roots of plants in 

 proportion as the tenacity of the soil allows them 

 to spread. The effect will be greatest in all these 

 directions when the soil is kept for a long period 

 undisturbed and in a uniform state of moisture, as 

 is the case when it exists under an overshadowing 

 cover of vegetation. Hence the soft and open con- 

 sistence which the soil exhibits where it has borne 

 clover, lucerne, rape, or lupines, or has lain fallow 

 for some time. If such soil is worked while wet, 

 the cavities become effaced, and the mass close and 

 clammy, just as wet fermented dough would be if 

 violently beaten. In freer, looser soils, or those too 

 loose by nature, the earth also attains a mellow con- 

 dition through the cultivation of the above-named 

 crops ; in such, however, this condition is charac- 

 terized by the soil becoming less loose, pulverulent, 

 and consequently more cohesive. This beneficial 



