20 MICROBES AND TOXINS 



food. No hydrocarbons, no bacterial activity, no fixation of 

 nitrogen. To fix a gram of nitrogen, experiments show, loo 

 to 200 grams of glucose are required. 



Just as we do not know all about a disease when we know 

 its microbe, so it is not sufficient to have simply the bacteria 

 which fix free nitrogen in order to enrich at will a soil with 

 nitrogen. Nature is not the laboratory. The inoculation of 

 poor soil with these beneficent bacteria produces scarcely any 

 increase in the crops. To excite bacterial multiplication it 

 would be necessary to distribute sugars over the soil — 14 

 kilos., it is calculated, to fix the nitrogen to which corresponds 

 I kilo, of nitrate. Nitrogen at such a price is far from cheap. 

 Besides, the hydrocarbons favour the denitrifying action of the 

 microbes which results in a loss of nitrogen. The useful 

 germs are already present in good soil ; it is literally the soil 

 itself which must be altered, by adding marl, by tilling, drainage, 

 and all the operations which change its physical properties. 



There exist certain moulds which fix the nitrogen of the air, 

 for example the Penicillia and the Sterigmatocystes. The algse 

 of the nostoc group, Chlorella, Stichococcus and Cystococcus, fix 

 nitrogen only when in symbiosis with the fixing bacteria : it is 

 then the bacteria which fix nitrogen ; the algse, as green plants, 

 nourish their associated bacteria by means of the hydrocarbons 

 which they synthesize in virtue of their chlorophyll activity. 

 According to Beijerinck's experiments the Cyanophyceae are 

 capable of taking nitrogen into their tissues independently just 

 as they do carbon. But if all the algse possessed this double 

 function they would be so powerfully adapted for life that they 

 must have long ago invaded the whole universe. 



There is a third group of microbes which fix nitrogen ; but 

 in this case they do not live free in the soil, but are 

 confined to the roots of Leguminosse. 



The soils which are found to be most rich in nitrogen are 

 those in which there have been longest grown crops of this 

 family. Georges Ville in 1852 thought that the leguminosse 

 took up oxygen from the air. He sowed leguminosse in sand 

 which had been washed and calcined, thus being sterile and 



