392 THE NON- VASCULAR PLANTS 



Nitrogen, as you know, is necessary to plant life. Though 

 nearly four fifths of the air is nitrogen, green plants get 

 none of their nitrogen directly from it. They are unable 

 to use nitrogen in gaseous form. That which they use 

 comes from the soil . Compounds which contain it are present 

 in the soil, and these, when dissolved in soil water, are the 

 source of nitrogen for green plants. Certain compounds 

 called nitrates are of chief importance to plants as sources 

 of nitrogen. (See page 344.) 



The soil on which wild plants grow has its supply of 

 nitrates kept up. The wild plants return their nitrogen 

 to the soil when they die. But soil on which farm crops 

 grow is constantly losing nitrogen. The plants which it 

 produces do not return to it when they die. The farmer 

 is constantly taking away the crops and the crops are rich 

 in nitrogen. He does not allow them to rot upon the 

 ground. For this reason one of the great questions which 

 confronts farmers is how to keep the soil of their fields from 

 becoming too poor in nitrogen. Manure and certain other 

 fertilizers help keep up the nitrogen supply. 



Bacteria, as you have noted, are of much importance 

 in connection with this subject. It is the action of bacteria 

 upon manure and other organic materials which results in 

 the production of nitrates, the best available form of nitro- 

 gen for green plants. 



Probably you already know that it is not good farming 

 to grow the same crop upon the same soil year after year. 

 Farmers avoid doing this either by letting some fields rest 

 (lie fallow) in some seasons, or by rotation of crops. To 

 grow corn for two seasons and then clover for one upon the 

 same land is an example of the rotation of crops. Why 

 should such practice improve the fertiHty of the soil? 



