1911] The Ottawa Naturalist. 211 



wise almost unavailable nitrogen the air. Plants cannot use 

 the free nitrogen, but require it in the form of nitrates, and this 

 transformation is brought about by the root bacteria found in 

 association with these plants. This discovery afforded an 

 explanation of the long known fact that such leguminous crops 

 enriched the soil. Since that discovery, other bacteria living free 

 in the soil have been found that are capable of fixing the essential 

 nitrogen, and Hall has recently stated -that "We ma}^ with some 

 confidence attribute the vast stores of combined nitrogen con- 

 tained in the black virgin soils of places like Manitoba and the 

 Russian steppes to one of these organisms." Humus is rich in 

 nitrogen, and the bacterial organisms, together with the oxygen 

 of the air, convert this into available plant food. It is owing to 

 the nitrogen-enriching power of such leguminous crops as clover 

 and alfalfa that they are of so great value in enriching soil which 

 has been depleted of nitrogen by other crops. It is on this 

 single fact that the system of the rotation of crops 

 is based, that is, the alternate planting of leguminous crops 

 which increase the amount of available nitrogen in the 

 soil with crops, such as cereals, which use up the nitrogen. 

 This rotation, therefore, is one of the most important means of 

 maintaining the fertility of the soil. Furthermore, by the plant- 

 ing of these leguminous crops we are enabled to inoculate soils 

 previously deficient in nitrogen and accordingly increase their 

 productive power. 



Recently, at the Rothamsted laboratory in England, 

 Russell and Hutchinson have made some investigations 

 of more than ordinary interest upon this question of the relation 

 of soil bacteria to fertility. Subsequent to the discovery of the 

 nitrogen-fixing bacteria, the inexplicable fact was discovered 

 that when the soil was heated or treated with an antiseptic such 

 as chloroform to render it sterile, the fertility increased in an 

 astonishing manner. Heating the soil to a temperature of 70 to 

 100 degrees for two hours doubled the size of the crop. This fact 

 has been known since the time of the Romans and is practised 

 in intensive cultural systems, but the cause of the increased 

 productivity was unknown. These authors find that this in- 

 creased fertility appears to be due to the fact that when the 

 soil is heated or treated with an antiseptic, all the bacteria are 

 not destroyed, but larger protozoal organisms, akin to 

 Amoeba, which normally feed upon the bacteria, are killed. 

 In consequence, the bacteria released from their enemies increase 

 and multiply at a rapid rate, which results in an increase in the 

 amount of available nitrogen in the soil. 



These investigations will serve to indicate how these ques- 

 tions of the fertility of the soil and the maintenance of that 



