110 AGRONOMY 



and when this is missing it cannot thrive. In some cases, 

 however, the form of bacteria associated with one species may 

 be gradually induced to form partnerships with another. 

 When the necessary bacteria are lacking, the soil may be inocu- 

 lated by a few bushels of soil brought from another field in 

 which the desired crop grows well. This is scattered over 

 the field at the time of planting exactly as one would scatter 

 seeds. In a few instances the bacteria of two species seem to 

 be interchangeable. Fields in which red clover or alfalfa will 

 not grow because their bacterium is absent, may be made to 

 produce these crops by inoculating with soil brought from 

 the nearest patch of wild sweet clover. In the same way the 

 bacterial symbiont of cowpeas may be supplied from soils in 

 which the wild partridge pea occurs. Several attempts, more 

 or less successful, have been made by the national govern- 

 ment and by private parties to send out dormant cultures of 

 bacteria for use with certain crops. The seeds of the crop 

 desired are inoculated with the bacteria before sowing. In 

 the case of many cultivated species of legumes, and possibly 

 all wild ones, the bacteria with which they form associations 

 are transported into new soils by clinging to the seeds. 



Denitrifying bacteria. Along with the bacteria in the soil 

 which turn nitrogenous substances to nitrates are found other 

 bacteria which reverse the process, and, by extracting the 

 oxygen from nitrates, set free the nitrogen. This process 

 goes on most rapidly in soils which are not properly aerated. 

 Stable manure, left in piles, loses much nitrogen in this way. 

 When plenty of oxygen is present the bacteria do not attack 

 the nitrates. 



Harmful organisms in the soil. As we have seen, the living 

 elements of the soil are quite as important as its mineral 

 constituents. In addition to the nitrifying and denitrifying 

 bacteria and the nitrogen-fixing bacteria, there are many 

 yeasts, algse, fungi, germs of plant diseases, and hosts of 



