GENERAL FUNCTIONS OF MICROBES 19 



manured, /.*., from 5 to 9 milligrams of combined nitrogen 

 per kilo, of soil. Finally, the crops remove from the soil 

 much more nitrogen than the manuring supplies : the difference 

 varying with the rotation of the crops from 1*5 to 400 kilos, 

 per acre. Where does this nitrogen come from ? 



It can only come from the inexhaustible reservoir of the 

 atmosphere. 



Rain water carries into the earth the ammonia which has 

 evaporated from it and the oxidised compounds of nitrogen 

 which form during thunderstorms. But these gains about 

 1*5 kilos, per acre per annum are quite insufficient to com- 

 pensate for the losses occasioned by drainage and cultivation. 

 Plants must take up not only the ammonia and the nitrates 

 of the atmosphere supplied by rain, but also uncombined 

 nitrogen, the free nitrogen of the air. 



Cultivated soil, kept moist and exposed to the air, fixes 

 atmospheric nitrogen (Berthelot). This fixation does not occur 

 if the earth has been sterilised by heating to 120 C. Living 

 creatures must therefore be at work in this. 



These workers are the bacteria of the soil which are found 

 to a depth of one foot. They are also found in the sea, 

 especially in the neighbourhood of algae. They are more 

 abundant in soil, the better it is aerated and cultivated. They 

 include anaerobes (Clostridium pasteurianum) and aerobes 

 (genus Azotobacter : Pseudomonas leuconitrophilus). Their 

 organic food in soil as in laboratory cultures is carbohydrate, 

 glucose, saccharose, levulose, dextrine, mannite and other 

 sugars; butyrates, lactates and acetates; their mineral food 

 consists of salts of lime and phosphates. The Azoto- 

 bacter chroococcum will only develop in soil containing at least 

 o'i per cent, of lime. 



It is not exactly known how bacteria fix nitrogen. They 

 build it up into their substance and liberate it when they are 

 destroyed. Doubtless also they build up nitrogenous com- 

 pounds which are taken up by the nitrifying bacteria. 



Nitrogen can only be thus combined on condition that the 

 bacteria are supplied with energy in the form of carbohydrate 



C 2 



