206 ANNUAL, REPORT OF THE Off. I 



soil and thus its l>< nefit to the soil is no greater than a cereal or 

 other non-leguminous crop used as a green manure. 



No doubt you are all familiar with the second phase; the final es- 

 tablishment of such a crop as clover, alfalfa, or soy beans only after 

 repeated sewing on the same soil. This is due in part to the bacteria 

 carried into the soil with seed and chaff, and in part, no doubt, to the 

 gradual adaptation of other bacteria previously associated with some 

 native plant of the pea or clover family. If the figures could be 

 gathered representing the annual loss of seed and labor involved in 

 a i tempting to establish legumes on soils new to the plants, the ag- 

 gregate, I am sure, would convince the most skeptical of the 

 economy and, in many cases, the absolute necessity of introducing 

 the bacteria by some form of artificial inoculation. TJie method 

 which most readily suggests itself is the transfer of soil from an old 

 field. This method, while it has often given most satisfactory re- 

 sults, is still open to objection. The use of soil of unknown origin 

 for this purpose is liable to carry into new localities plant diseases 

 the damage from which would be greater than any possible benefits 

 obtained from the soil inoculation. Among the diseases which might 

 readily be carried in this way are the bacterial diseases of the 

 potato, tomato and egg plant, the wilt disease of cowpeas, melons 

 and cotton, and various root-rot and nematoid diseases widely dis- 

 tributed in the South. No careful fruit grower would dare use soil in 

 his orchards without first ascertaining that it had been taken from a 

 locality free from root-rot and crown gall diseases. 



The liability of spreading weed seeds and insect pests is of scarcely 

 less importance, so that in every case, soil used for this purpose 

 should come from a source which is definitely known and is safe to 

 use. This is a difficult matter to determine in many cases, so that, 

 under average conditions, the method of soil transfer is open to sus- 

 picion if not positive objection. 



To obviate such difficulties as these the attempt was made to cul- 

 tivate the bacteria in pure cultures and thus introduce them into the 

 soil. You are doubtless familiar with the failure of early attempts 

 along this line — the German product "Nitragin" proved unsuccessful 

 in practice, owing, as was afterward demonstrated, to faulty methods 

 of cultivation and distribution. The rich nitrogenous medium used 

 gave a luxuriant growth of the bacteria but the conditions of life 

 were so easy that the organisms deteriorated in vigor and failed in 

 their essential part as "nitrogen-fixers" when again released in the 

 soil. 



When the problem was taken up in the Laboratory of Plant Phy- 

 siology, at Washington, a different principle was adopted; the bac- 

 teria were given a food supply practically devoid of nitrogen and 

 only those organisms which were vigorous and able to utilize nitro- 



