120 THE STORY OF GERM LIFE. 



pate some of the nitrogen into the air as free ni- 

 trogen. 



To meet this emergency and loss the farmer 

 has another method of enriching the soil, again 

 depending upon bacteria. This is the so-called 

 green manuring. Here certain plants which seize 

 nitrogen from the air are cultivated upon the field 

 to be fertilized, and, instead of harvesting a 

 crop, it is ploughed into the soil. Or perhaps 

 the tops may be harvested, the rest being 

 ploughed into the soil. The vegetable material 

 thus ploughed in lies over a season and enriches 

 the soil. Here the bacteria of the soil come into 

 play in several directions. First, if the crop 

 sowed be a legume, the soil bacteria assist it to 

 seize the nitrogen from the air. The only plants 

 which are of use in this green manuring are those 

 which can, through the agency of bacteria, obtain 

 nitrogen from the air and store it in their roots. 

 Second, after the crop is ploughed into the soil 

 various decomposing bacteria seize upon it, pulling 

 the compounds to pieces. The carbon is largely 

 dissipated into the air as carbonic dioxide, where 

 the next generation of plants can get hold of it. 

 The minerals and the nitrogen remain in the soil. 

 The nitrogenous portions go through the same 

 series of decomposition and synthetical changes 

 already described, and thus eventually the nitro- 

 gen seized from the air by the combined action 

 of the legumes and the bacteria is converted into 

 nitrates, and will serve for food for the next set 

 of plants grown on the same soil. Here is thus a 

 practical method of using the nitrogen assimila- 

 tion powers of bacteria, and reclaiming nitrogen 

 from the air to replace that which has been lost. 



Thus it is that the farmer's nitrogen problem 



