CONDITIONS AFFECTING LEGUME INOCULATION. 79 



extract of untreated soil gave a growth of the alfalfa bacteria which 

 might be designated ''fair." while with lime, with lime and humus, 

 and with humus alone the extracts gave uniforml}^ a growth which 

 might be designated ''excellent." In the pots, how^ever, no nodules 

 developed in the humus series. The sterilization b}' heat of the 

 extract from a soil containing such a relatively large quantity of 

 humus, one-fourth bv volume, probably changes the material so that 

 the solution becomes favorable to the growth of the bacteria, but this 

 material in the soil pots, not heated, remains unchanged and maj^ in 

 this form be unfavorable to the activity of the bacteria introduced 

 into the soil. 



Aside from the effect of the soil on nodule formation there seems 

 to exist a marked difference among different varieties of legumes in 

 their susceptibility to infection — that is, their readiness to form 

 nodules. With a similar soil and one which is favorable to the 

 growth of the respective species, nodules will occur in abundance upon 

 one species or variety and another species or variety will exhibit none 

 or only a few. This is particularly noticed with varieties of soy beans 

 and with certain species of Phaseolus, the Lima bean {Phaseohis 

 lunatus) and some varieties of S03" beans being ver}^ difficult to inocu- 

 late. It would seem that such species were actually resistant to 

 infection, especially in soils rich in nitrogenous matter. 



EFFECT OF HEAVY INOCUL.ATION. 



The decided advantage of very heavy applications of pure cultures 

 reported from some field and pot experiments and reports of other 

 experiments showing that cultures diluted to almost infinitesimal limits 

 gave as good results as undiluted cultures seem at first diametrically 

 opposed. Greenhouse tests seem to confirm the beneficial effect to be 

 expected from heav}" inoculation provided excessive quantities of cul- 

 ture are applied. The following experiment is fairljr representative: 



Effect of light and heavy inoculation of garden peas on soil from Blue Hill, Me. 



Number 

 Treatment. 1 of plants 



I grown. 



Number of I Average 

 plants with number of 



nodules 



Check, without inoculation. 



Light inoculation 



Heavy inoculation 



' n 



nodules. 



It is here shown that heavy inoculation not only doubled the average 

 number of nodules upon inoculated plants, but insured all plants 

 becoming* infected; while with light inoculation in this unfavorable 

 soil one-third of the plants failed to produce nodules, and with no 

 inoculation nodules were almost entirely lacking. 



100— VIII 



