76 



MISCELLANEOUS PAPERS. 



to counteract the influence of substances in the soil unfavorable to 

 the growth of the bacteria.^ 



EFFECT OF SOIL CONDITIONS UPON BACTERIA. 



The constitution or character of the soil solution has great effect upon 

 the growth of nodule bacteria as well as upon the formation of nodules. 

 In the large number of soil samples tested simultaneously in the green- 

 house and in the laboratory it seems to hold true generally that the 

 possibilit}" or impossibility of securing effective inoculation on a par- 

 ticular crop in a certain soil can be predicted by the relative growth 

 of the specific culture in the sterile extract of the soil in question. 

 There also seems to be a somewhat close relation between the soil 

 solution necessary for the growth of host and bacteria, so that in many 

 cases, at least, it is possible to determine the suitability of a legume 

 crop to a certain soil b}^ the growth of the specific culture of Pseudo- 

 monas radicicola in the extract of the soil. The probability of an 

 extract not truly representing the soil solution and the disturbing 

 factor of sterilizing certain extracts will in some cases introduce error 

 in interpreting results. The use of sterilized soil extracts as culture 

 media for the bacteria has been found to give fairly ti'ue indications 

 of the growth of the respective hosts of these bacteria in a consider- 

 able series alread}^ tested, however, and contemplated improvements 

 in technique will doubtless increase the probability of securing trust- 

 worthy indications. The testing of the soil extract in its behavior 

 toward specific bacteria may even indicate that some treatment is 

 necessary, but can not prescribe the method of amelioration, and for 

 the present direct experimentation is necessary to determine the proper 

 treatment of an unsuitable soil. 



As an example of the relation between the growth of the specific 

 bacteria in the sterile soil extract and the growth of the legume host 

 in this soil may be cited the following result of a fairly representative 

 test, using different quantities of lime on an unfavorable soil which 

 responded favorably to treatment with lime: 



Quantity of lime. 



Green 



weight of 



plant. 



Unlimed 



Limed: 



2,000 pounds to the acre 

 4,000 pounds to the acre 



Number of 

 nodules. 



Number of 

 bacteria in a 

 cubic centi- 

 meter of soil 

 extract. 



71, 950 

 128,650 



«For the presence of substances in poor soils deleterious to higher plants, see 

 Bureau of Soils Bui. No. 28, 1905, ''Studies on the Properties of an L^nproductive 



Soil." 



