AZOTOBACTER 33 



compounds which can be assimilated by the plant. Legu- 

 minous plants grown from sterile seeds in a sterile soil 

 dwindle and die, but if inoculated with the organisms 

 derived from another plant of the same species growth 

 becomes vigorous ; if inoculated with those derived from 

 another species growth still takes place, but not nearly to 

 the same extent. The Leguminosse thus store up one of 

 the most important elements of plant food, and hence 

 their value in the rotation of crops. There is apparently 

 no increase of nitrogen compounds in the soil, the excess 

 found being due to the root residues remaining. A sub- 

 stance termed " nitragin," consisting of a culture of these 

 root organisms, has been prepared as a fertiliser. Nobbe's 

 " nitragin " did not prove a success, apparently because 

 the organisms soon lose their vitality. A better prepara- 

 tion, " nitro-bacterine," was devised by Moore of the 

 United States Department of Agriculture. Besides the 

 leguminous organisms, other bacteria are present in the 

 surface layers of the soil which fix atmospheric nitrogen. 

 The principal of these are ovoid organisms known as 

 Azotobacter. This group can be cultivated in a mannite 

 medium, e.g. di-potassium phosphate 0-2 grm., mannite 

 20 grm., water 1 litre. This may be used for isolation 

 by converting into an agar medium by the addition of 

 2 per cent. agar. Prof. Bottomley has succeeded in 

 obtaining a powder preparation of Azotobacter, which 

 retains its vitality for months, and the preparation properly 

 applied to poor soils produces astonishing results. 



It has been found that partial sterilisation of the soil, 

 e.g. by heat, increases its fertility, whereas it might have 

 been supposed that such a procedure would decrease the 

 fertility by destruction of nitrogen-fixers. Russell and 

 Hutchinson suggest that in ordinary soil amoebae and other 

 protozoa devour and keep down the bacteria ; by the 

 sterilisation the protozoa are destroyed and the more 



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