ACTION OF MICRO-ORGANISMS 211 



meiice to grow and the processes of nitrification go on rapidly 

 before the soil is reinfected by protozoa. Sick soil can be 

 partially sterilised by steam or volatile substances such as 

 toluol ; care must be taken not to oversterilise as that kills 

 the bacteria also and prevents the advantages of a fresh 

 growth of bacteria. 



In cultivating the soil the main aim is to drain the soil so as 

 to prevent it from becoming waterlogged, to prevent acidity 

 by lime and to break up the soil so that there will be a good 

 supply of oxygen in the spaces of the soil. All these are steps 

 to promote the growth of nitrifying bacteria as well as aid the 

 " weathering " of mineral constituents so that potassium, 

 phosphates, etc., are rendered available for the roots of plants. 



The conditions in the soil present a superficial resemblance 

 to fermentation in the large intestine. When there is an 

 overgrowth of putrefactive organisms toxic substances are 

 produced which by absorption cause mental dullness and 

 headache and are said to be instrumental in hastening the 

 onset of old age. These organisms can be removed by infect- 

 ing the alimentary canal with lactic acid organisms. The 

 acid inhibits the growth of putrefactive organisms and allows 

 the normal bacteria to reassert themselves. 



We have seen that when the supply of oxygen is deficient 

 the process of nitrification is stopped and free nitrogen 

 is given off. This leads us to consider anaerobic metabolism, 

 sometimes miscalled anaerobic respiration. 



Certain bacteria grow best in the absence of oxygen 

 (anaerobes). Their energy changes must be carried out by 

 some molecular change which does not involve the addition of 

 oxygen. Such a change is exemplified by the explosion of 

 gunpowder or other explosive in which the various atoms are 

 rearranged with the liberation of considerable energy. 



A passing reference must also be made to the interesting 

 sulphur organisms which decompose hydrogen sulphide and 

 deposit sulphur in their protoplasm. When there is a de- 

 ficiency of energy-furnishing food the sulphur is oxidised and 

 the energy set free is utilised for the life processes of the 

 bacteria. These organisms can decompose carbon dioxide 

 and form carbohydrate from the energy of the abo.ve oxidation ; 

 in fact they grow better if there is no carbohydrate in their 

 surroundings.* This process of chemosynthesis can be 

 compared to the photosynthesis of green plants. In the one 

 case energy obtained from the sunlight and in the other from 



* S. Winogradsky, Bot. Zeitung, 1887, vol. 45, p. 490. For iron 

 bacteria see the same, iSSS, vol. 46. 



