35 



of pure cultures from selected plants. The soil method, of course, 

 introduces not only the nodule-forming species, but numerous 

 other forms, snch as those previously discussed in this paper, and 

 these may often be as much needed as the tubercle forms. Where 

 they as well as the nodule forms are needed, soil transfer is usually 

 the most satisfactory method of inoculation. This is likely to be 

 the case in " sour" soils. It is, however, expensive and cumber- 

 some, especially where the soil has to be transported for some 

 distance. 



Another and much more serious drawback to this method is the 

 danger of introducing into a farm injurious and disease-producing 

 bacteria and fungi, as well as troublesome weeds. It is not safe 

 to use soils from areas in which such pests occur. Anyone who 

 has seen the great injury wrought by root nematodes, the wilt 

 diseases of melons, cowpeas, tobacco, cotton, tomatoes, potatoes, 

 flax, the black-rot of cabbage and cauliflower, the various stem and 

 root diseases of clover and alfalfa, and diseases of other crops 

 almost too numerous to mention, and widely distributed, will in- 

 vestigate carefully before importing soil that may contain the 

 spores of these organisms. 



PURE-CULTURE INOCULATION, 



For the reason above mentioned, pure culture inoculation must 

 eventually mainly replace the soil-transfer methods for all kinds 

 of soil inoculation, but this has not yet been put on a practical 

 basis for any except the nodule bacteria. The improved cultural 

 and selection methods introduced by this Department in the prac- 

 tical handling of these forms have brought within reach of every 

 farmer who may need them pure cultures of the most virile, vigo- 

 rous, and best types of root-nodule bacteria for each particular 

 leguminous crop. It requires care and skill, however, to make 

 and keep these cultures in good condition. For lack of this many 

 who have tried to make these cultures have failed. The pure cul- 

 ture method has come to stay, however. It will be improved by 

 experience aad continue to increase in usefulness in the hands of 

 careful and progressive workers. It is suggested, before using 

 cultures from any source, that farmers secure from this Depart- 

 ment and read carefully Farmers' Bulletin 240. The Department 

 of Agriculture is distributing the bacteria during the present sea- 

 son (1906) in nitrogen-free liquid cultures hermetically sealed. 

 Excellent results can also be obtained in distributing the cultures 

 in dry form. They must be dried quickly, however, from solutions 

 containing very small amounts of soluble salts, and kept dry un- 

 til ready for use ; otherwise they are likely to mould and spoil. 

 This Department has used this method very successfully tor seve- 

 ral years. It is also used successfully by Ferguson, of the Vir- 

 ginia Experiment Station.* 



THE FIXATION OF NITROGEN FROM THE ATMOSPHERE BY 

 ELECTRICAL METHODS. 

 While much can be accomplished through the agency of soil 



*See Bui. 159, Virginia Agr. Exp. Sta. 



