62 DISEASE IN PLANTS. 



The researches of recent years, however, and 

 especially those of Frank, Winogradsky, Hellriegel, 

 and Stahl, liave brought to light a series of re- 

 lationships between certain of these soil-organisms 

 and the higher plants which place the matter of 

 soil-biology in quite new lights. 



On the one hand it has been discovered that 

 groups of bacteria are the active agents in bring- 

 ing about the destruction of organic nitrogenous 

 matter with the formation of ammonia, in oxidising 

 this ammonia to nitrous and to nitric acids, which 

 combine with bases in the soil to form the corre- 

 sponding salts ; while, on the other hand, other 

 forms can decompose the nitrates and reduce them 

 to nitrites, or set free ammonia or even nitrogen 

 from them. Moreover, there are certain species 

 which can fix the free nitrogen of the atmo- 

 sphere, and start the cycle of up-building of this 

 inert element into the complex higher compounds 

 we term organic. It is impossible to over-estimate 

 the importance of these processes of nitrification 

 and denitrification going on in the soil about the 

 root-hairs of the higher plants. 



But, in addition to this circulation of nitrogen 

 in the soil, it turns out that the life-actions of 

 bacteria, and not mere chemical decompositions, 

 are largely responsible for the circulation of carbon, 

 of iron, of sulphur and other elements formed from 

 the decomposition also by bacterial and fungal 

 agency of animal and vegetable remains in the 

 soil. 



Even more startling are the biological relations 



