ROOT-NODULES OF LEGUMINOSAE. 



123 



The following table gives the harvested crops in kilograms per plot, omitting fractions less than 

 0.5 and adding those more than 0.5 : 



In summing up the 46 experiments of this year (1901) at Bremen it may be said that 54 per cent 

 were favorable, some extraordinarily so, in spite of the many unfavorable conditions. This result 

 certainly refutes the claim that inoculations in the open give recognizable results only in quite isolated 

 cases. It is also plain that results are the more certain the less often the land used has borne the 

 species of legume in question. The method of inoculation is of the utmost importance and must be 

 selected with reference to the soil and the species of legume. 



In the experiments at Dahlem, i. e., under Hiltner's direct supervision, Soja hispida was used 

 exclusively, since it is not there subject to spontaneous infection, but forms, when inoculated, large 

 easily counted nodules in which, at a temperature suited to the Soja, active nitrogen assimilation 

 takes place. In 1901 experiments were made to determine how long nodule bacteria remain active 

 in the soil. Plots were used which had borne soy-beans the previous year, some uninoculated, others 

 inoculated at that time. 



Both yellow and brown seeded varieties formed an average of 75 nodules on all plants grown in 

 previously inoculated soil. Out of 356 plants examined none had less than 50 nodules. From this 

 it is evident that the nodule bacteria which had wintered in the soil must have retained a high degree 

 of virulence. Hence, when nodule bacteria prove virulent one year, they are also virulent for the 

 same crop the next year, and even increasingly effective. This conclusion agrees with the results 

 of growers who have found that one inoculation suffices for a series of years where the same crop is 

 cultivated, and that further inoculation in such cases is useless. 



On soil which had been inoculated with Soja earth and cultivated in oats in 1900, the Soja-beans 

 of 1901 were sparingly infected. These nodules were due, however, to an infection from neighboring 

 plots rather than to bacteria which had persisted in the soil, as was shown by comparison of the two 

 varieties in the four oat plots of 1900. The bacteria, therefore, can live from one year to the next 

 in the Dahlem soil only when the legume to which they belong is at hand so that nodules are formed. 

 They are able to draw nourishment from the decaying roots, and, when these are gone, to hold their 

 own with the other soil organisms for a limited time only. 



Another experiment in 1901 was made with soy-beans on land that had in 1900 borne various 

 crops. In the uninoculated plots the average number of nodules varied from o to 1.2. Most of the 

 plants were free. On the contrary in the plots where Phaseolus had grown in 1900, 41 per cent of the 

 plants bore nodules. The results when inoculation took place 6 weeks before sowing were very 

 moderate (3 to 14 nodules), and no difference could be observed between the plots which in 1900 

 had borne legumes and those which had not. A 30 times larger amount of inoculating material 

 caused a larger number of nodules, but even then the number did not equal those formed when inocu- 

 lation took place at the time of sowing. What caused this failure, when 6 weeks clasped between 

 the inoculation of the soil and the sowing of the seed, is not certain. At any rate it was not due to 

 lack of moisture, as moisture conditions were excellent. 



Experiments with soy-bean to determine the best method of inoculation again showed the action 

 of an injurious substance in the seed-coat. Inoculation made by strewing Dahlem sand which had 

 been moistened with a pure culture produced plants which were absolutely nodule free. The most 

 favorable results were obtained by germinating the seed and then inoculating just before sowing. 

 These facts indicate that seeds which have been soaked exert no longer an injurious influence : This 

 decreases as the germination progresses. The inoculation with pure cultures in all the experiments 

 showed itself equal to natural earth inoculations and in most cases was superior to the latter. 



The method of inoculation by strewing earth previously infected with pure cultures proved 

 beneficial only on cultivated moor soil, or in moist weather, or with seeds which germinate very rapidly. 



The method by seed-inoculation may entirely miscarry owing to destruction of the bacteria by 

 poisonous substances extruded from the seed-coat during germination. This danger is much greater 

 with large seeds, such as lupins, peas, and soy-beans, than with small seeds, but may occur with the 

 latter, e. g., clover seed, if the soil is very dry and germination slow. 



