Apr. IS, 1920 Effect of Solution Reaction on Germination and Growth 85 



brown in color and supported vigorous growths of mold. It seems 

 probable, therefore, that all plants would have died in these cultures 

 and that 2.96 Pg is really below the critical reaction for all crops studied. 

 A reaction of 2.17 Pg killed all seedlings a few days after transplant- 

 ing, and while in Table IV weights are included for the seedlings from 

 cultures having this reaction, these represent the weights of the dead 

 seedlings at time of harvesting. Abundant growth of, molds occurred 

 upon the roots of all plants in cultures of this reaction. A reaction 

 of approximate neutrality, 6.97 Pg, was found less favorable to the 

 growth of seedlings of all four crops than a slightly acid reaction, while 

 a reaction of 7.71 Pg still further depressed the growth of all crops ex- 

 cepting com, where a slight increase was observed, the latter probably 

 falling within experimental error. A study of the growth curves (fig. 4), 

 shows that the optimum reaction for alfalfa was apparently higher than 

 for the other crops studied. While maximum growth occurred at 5.94 Pg, 

 a reaction of 6.97 Pg had a less injurious effect and a reaction of 5.16 Pq 

 a more injurious effect than was found with wheat, soybeans, and com. 

 This agrees with the relative adaptation to soil reaction of the several 

 crops commonly observed in field practice. In this coimection it is 

 interesting to note that Fred and Davenport (5) have recently shown 

 that the critical reaction for the bacterium Rhizohium leguminosarum, 

 symbiotically associated with alfalfa, is 4.9 Pg, while that for the corre- 

 sponding organism associated with the soybean is 3.3 Pg. 



change; 01'' REACTION INCIDENT TO GROWTH 



As previously noted, determinations of reaction by means of the 

 hydrogen electrode were made upon the cultures of the wheat series at 

 the begiiming and end of each 4-day period — that is, before and after 

 renewing the solution on each culture. The average reaction at the 

 beginning and at the end of the 4-day periods for wheat in series B and 

 the changes observed in reaction are given in Table V. The relation 

 between the change of reaction and the position of a given culture with 

 respect to the electrometric titration curve is brought out by a compari- 

 son of the curves shown in figure 2, A, and figure 2, B. 



Table V. — Change in reaction during 4-day periods 



