THE ACTIVITIES OP NITROGEN-BACTERIA IN SOIL 175 



With regard to the Algae, as far back as twenty years ago it was 

 stated that they possessed this power of fixing nitrogen. The facts are 

 as follows : If a plot of ground be cultivated with grass, potatoes or 

 turnips, it does not gain nitrogen from the air if it be covered with sand 

 to prevent the growth of Algae on it. On the other hand a gain does 

 take place if Algae and Mosses are allowed to grow in the soil. Again, 

 if a piece of ground be covered with an alga- vegetation, and another 

 with a moss-vegetation, the former shows that nitrogen has been gained 

 from the atmosphere, the latter that it has not. The conclusion that 

 Algae can " fix " nitrogen must, however, be received with suspicion, as 

 in the experiments care was not taken to remove the bacteria from the 

 soil. It has been since shown that no Algae in a pure cultivaiion can 

 assimilate nitrogen, but that many Algae in combination with earth 

 bacteria can effect this fixation. It has therefore been suggested that 

 they supply the carbohydrate material, got by them by assimilation, to 

 the bacteria which effect the nitrogen fixation. In this way they may 

 be regarded as helping to bring about the sum total of conditions which 

 enable the bacteria to fix nitrogen, rather than as organisms that effect 

 the fixation themselves. Finally, with regard to the Higher Given 

 Plants, many researches have been made to determine their power of 

 fixing nitrogen, but in all those in which positive results have been 

 obtained, it has been found that the earth had not been sterilised. In 

 all researches in which the earth had been sterilised negative results 

 have invariably been obtained. 



§3. CONDITIONS OF NITROGEN-FIXATION. 



It is extremely important to the agriculturist to know the conditions 

 which govern and regulate this nitrogen-fixation. Numerous researches 

 have brought out the following results : 



1. Of inorganic materials calcium and phosphorus are absolutely 

 "essential to the furtherance of fixation. Potassium and sodium seem 



to be unessential except in the case of Azotobacter, in which case the 

 presence of one or both of these seems to help the fixation. 



2. It is important that Algae should be present in the soil, for, as 

 shown, the gain of nitrogen is considerably greater when they are 

 present, because they probably supply the necessary carbon to the 

 bacteria, which are thus able to fix nitrogen with greater energy than 

 would otherwise be the case. It is not likely that any alga will serve 

 for this purpose, and an alga that is favourable to, say, Azotobacter 



