NEED OF INOCULATION 381 



bacteria from one of these plants may inoculate the other. At pres- 

 ent no other case is known of cross-inoculation. 



How to Inoculate. When the value of inoculation was first 

 discovered (1887), the method recommended was to take soil from 

 a field where the particular plant was known to grow well and spread 

 it over the new field. The soil from the old field should be taken 

 from the upper six inches and transferred, without permitting it to 

 dry out, to the new field. From 200 to 300 pounds per acre is suf- 

 ficient if carefully applied. As sunshine is a strong bactericide, it 

 is best to apply on a cloudy day or toward evening and immediately 

 harrow in. 



Artificial cultures are made by growing the bacteria in a culture 

 solution, and applying these pure cultures to the seed just before 

 sowing or to the soil. The first artificial cultures were made about 

 1896, but for many years were unsatisfactory, and the soil method of 

 inoculation was best up to recently. New methods of handling arti- 

 ficial cultures have been developed and they are now quite satisfac- 

 tory. Full directions for using the cultures are usually furnished 

 with each kind. 



Natural inoculation is sometimes secured through bacteria 

 which adhere to the seeds. This is more common with rough-coated 

 seeds like sweet clover in the hull or burr clover. It has frequently 

 been observed, in sowing alfalfa on a new field, that only an occasional 

 plant would survive the first season. Examination usually shows 

 those occasional plants to have nodules, probably from bacteria on 

 the seed. If such a field is replowed, harrowed to scatter the 

 bacteria and sown a second time, usually thorough inoculation is 

 secured. 



Need of Inoculation. Some legumes have greater need of in- 

 oculation than others. When alfalfa or red clover is sown on fields 

 where there are no bacteria, the plants usually live for three or four 

 months, then begin to die. Soy beans and cow peas, while greatly 

 benefited by bacteria, will do very well on good soils when there is 

 no evidence of nodules on the roots. Field peas, vetch, white clover, 

 and alsike clover are all grown with little difficulty on good soils, 

 without inoculation. However, the bacteria are beneficial in most 

 cases. If no bacteria are present the legume exhausts the nitrogen 

 supply of the soil the same as other plants. 



