58 DISEASE IN PLANTS. 



become fewer and fewer, and gradually disappear, 

 and (neglecting certain anaerobic bacteria of putre- 

 faction) they are rarely found in marked abundance 

 more than a few inches below the surface soil. 



These aerobic fungi and bacteria are the great 

 agents of continued fertility of a soil, and it is 

 they which, living and multiplying in the moist 

 and well-aerated warm interstices of a rich open 

 soil, carry out the useful destruction of organic 

 matter, breaking it up into mineral and gaseous 

 bodies, which are then dissolved in the water bath- 

 ing the root-hairs or escape into the atmosphere. 

 In this work of destruction they are aided by 

 the oxygen of the air and the solar heat: their 

 own fermentative action is also accompanied by 

 a marked rise of temperature, and the carbon- 

 dioxide and other products of their activity all go 

 to complicate the chemical changes going on in 

 the soil around the roots. 



Duclaux has calculated that Aspergillus 7iiger, 

 a common mould fungus, can break down organic 

 substances, such as carbohydrates, at such a rate 

 that a metre cube of the fungus would decompose 

 more than 3000 kilogr. of starch in a year, and 

 this may serve as an example giving some idea of 

 the possibilities in soil. 



Analyses of waters containing large quantities 

 of organic matter, as they enter such open soils 

 as those referred to, compared with the drainage 

 water after passing through the upper strata, show 

 that the carbonaceous and nitrogenous materials 

 are broken down to more or less completely 



