156 AGRICULTURAL BACTERIOLOGY. 



which can be relied upon by the farmer as always producing 

 the advantage of an increased yield. Lastly it is quite expen- 

 sive and difficult to obtain in good condition. 



3. Utilization of the Nitrogen. The next problem is how 

 such a store of nitrogen, fixed in the soil, may be best utilized for 

 the benefit of the next crop. There are two methods by which 

 this nitrogen may be made available for crops subsequently 

 growing in the same soil. The first, which is commonly called 

 green manuring, consists in allowing the crop to grow vigorously 

 for a time, and then plowing the whole crop immediately into the 

 soil, with the expectation that the nitrogen stored up in the 

 plants will be available in the soil for the next crop. The 

 method by which the nitrogen becomes available is here 

 very simple and based upon the facts already noticed. When 

 these crops are thus plowed into the soil they are brought 

 at once within reach of the numerous soil bacteria, whose 

 action upon organic products has been described. The putre- 

 factive bacteria seize hold of the proteid products in the plants, 

 as well as the cellulose, fats, and other organic substances, 

 and cause their rapid decomposition. An ammoniacal fer- 

 mentation begins ; doubtless the denitrifying bacteria seize hold 

 of the ammonia and other ingredients formed, extracting their 

 oxygen to a certain extent and setting free the nitrogen. 

 After this process is finished the nitrifying bacteria in the soil 

 seize hold of the ammonia and the nitrates that are left after 

 the decomposition ceases. These are oxidized by the process 

 of nitrification, and the result is a quantity of nitrates. Thus, 

 after a few weeks, a considerable portion of the nitrogen ma- 

 terial which is fixed in the legume has been converted into 

 some form of nitrogen salt, cither a salt of ammonium or a 

 nitrate, either of which is available for plant life. These salts 

 remain in the soil and may be used by the next crop of plants 

 sown on the same field, thus increasing its yield by means 

 of the nitrogen which has been fixed by the legume and the 



