38 A MANUAL OF BACTERIOLOGY 



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 substance 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 preparation, " 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 

 aie 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 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 amcebse and 

 other protozoa devour and keep down the bacteria ; 

 by the sterilisation the protozoa are destroyed and the 

 more resistant bacteria are then free to develop. Greig- 

 Smith, 1 however, denies that phagocytic protozoa possess 

 any power of limiting the number of bacteria in the soil, 

 and ascribes the effect of soil sterilisation to an action on 

 the bacterio-toxins and nutrients of the soil. 



1 Proc. Linn. Soc. N.S.W., xxxvii, 1913, p. 655. 



