440 IOWA DEPARTMENT OF AGRICULTURE 



proper kind. Under such conditions growing the same crop for severaJ 

 years would probably be necessary before thoro inoculation would be 

 accomplished. 



Inoculation is also desirable when a legume is to be grown again 

 on a certain field after several years of cropping to other plants. How 

 long bacteria retain their efficiency in the soil is not definitely known, 

 but experience indicates that reinoculation is desirable if the legume 

 in question has not been grown for from five to seven years. In the 

 absence of definite proof of the presence of the bacteria in a vigorous 

 condition in the soil it is a wise precaution to introduce them and in- 

 sure the proper growth of the legume. 



WHEN INOCULATION IS UNNECESSARY 



If satisfactory yields of a legume have been obtained recently on a 

 soil and the plants carry a large number of root nodules, this indicates 

 that inoculation has been thoro and the introduction of bacteria will 

 be unnecessary. Within comparatively short periods of time bacteria 

 apparently retain their efficiency and inoculation is not profitable, be- 

 cause it will not increase the yield or value of the crop. 



METHODS OF SOlLi INOCULATION 



Soil may be inoculated by two methods: By the transfer of soil from 

 a field where the sa^me legume has previously been successfully grown 

 and inoculated or by the use of commercial preparations which are on 

 the market. 



The soil transfer method was the first to be employed and its value 

 has been demonstrated in many experiments and in extensive practice. 

 The method followed is to scatter 300 to 500 pounds of the inoculated 

 soil over each acre of the field to be seeded and disk it in thoroly 

 before seeding. Inoculated soil should not be allowed to remain exposed 

 to heat and light but should be mixed with the surface soil immediately 

 for the desirable bacteria are rather readily killed by exposure to the 

 sun. 



Certain other precuations should be observed. The same legume must 

 have been grown on the soil which is to be used, for cross inoculation 

 between legumes occur only in the case of alfalfa and sweet clover. 

 Further, the legumes grown in the soil used must have been well 

 inoculated as shown by the presence of many root nodules. Also the 

 soil should be free from plant diseases and this may be determined 

 by a careful observation of the crop grown on it. Finally, no objectionable 

 weed seeds should be present in the soil. If these precautions are 

 followed and the soil and climatic conditions are favorable, successful 

 inoculation can be practically guaranteed. 



OBJECTIONS TO THE SOIL METHOD 



There are, however, certain objections to the soil transfer method 

 of inoculation. If the soil must be shipped any great distance the 

 cost of transportation and spreading will be rather large. If soil that 

 is safe cannot be secured in the immediate vicinity, other methods of 

 inoculation may be more desirable. 



