50 DISEASES OF CULTIVATED PLANTS 



broadly, lime and sodium phosphate or calcium phosphate are 

 highly favourable to bacteria of the Azotobacter group, and 

 furnish respectively the calcium and phosphorus necessary for 

 their existence. 



It is important to bear in mind, that although exact 

 research has established the fact that these organisms possess 

 the power of fixing free nitrogen obtained from the atmo- 

 sphere, when placed under favourable conditions, we are as 

 yet very much in the dark as to what goes on in a state of 

 nature; that is, we cannot as yet utilise these bacteria as 

 a source of nitrogen on a scale necessary to meet the 

 demands of those employed in the cultivation of plants. 



The discovery by Oberlin that the application of carbon 

 bisulphide to the soil greatly increased its fertility has 

 opened up a new vista as to the great importance of bacteria 

 in enriching the soil. Oberlin demonstrated that grape-sick 

 soils could be rendered continuously fertile by the applica- 

 tion of carbon bisulphide, thus abolishing the necessity for 

 fallowing and rotation previously necessary. Much discus- 

 sion followed this discovery as to the manner of its action. 

 The explanation accepted at present is briefly as follows : 

 The introduction of carbon dioxide into the soil at first results 

 in the decimation of both the useful and injurious species of 

 bacteria present. This results in the destruction or tem- 

 porary retardation of the plants growing on the land, accord- 

 ing to the amount of carbon bisulphide used. At a later stage 

 a new bacterial flora appears in the soil, but there is a 

 relative suppression of denitrifying forms, and a relatively 

 rapid development of nitrifying bacteria, which results in 

 the accumulation of nitrogen in a form readily available for 

 plants. 



It has been observed that when a crop of mustard is 

 ploughed into the land green, its action is somewhat similar 

 to that of carbon bisulphide on the bacteria present in the 

 soil. It retards the development of acid-forming species, 

 and favours the development of nitrifying species. 



Among the difficulties to be overcome before we are in 

 a position to utilise the abstract knowledge we possess, 

 regarding those bacteria able to fix free nitrogen in the form 

 of soluble nitrates, directly useful to plants, is the fact that 

 there also exist in the soil bacteria capable of effecting an 

 exactly opposite process. These are usually termed denitri- 

 fying bacteria, and their business in the scheme of nature 



