92 , Journal of Agricultural Research voi. xix, No. a 



The optimum reaction for the germination of the seeds of the five 

 plants studied is probably below 7.71 Pg and above 2.96 Pg. 



GENERAL APPLICATION OF RESULTS TO FIELD PRACTICE 



Most experiment stations recommend the use of such amounts of lime 

 as will neutralize the total acidity present and maintain a soil at a neutral 

 or slightly alkaline reaction. The results herein reported would indicate 

 that if the direct physiological efifect of excessive acidity upon plant 

 growth were the only factor concerned, it would be more desirable to rec- 

 ommend such amounts of lime as would maintain the soil at a slightly 

 acid reaction such as would be represented by a Pq value of 5 or 6. On 

 the other hand, attention has been called to the necessity of further inves- 

 tigation of the other factors associated with acidity before this con- 

 clusion is warranted. Thus, a high optimum reaction (in Ph) for the 

 development of the nitrogen-transforming organisms of a soil might 

 counterbalance completely the advantages of a slightly acid reaction for the 

 growth of the plant itself. The authors have investigations in progress, 

 including solution, pot, and field plot studies, which it is hoped will give 

 further evidence upon the part played by the other factors concerned. 



SUMMARY 



(i) A study has been made of the efifects of reaction, as measured by 

 hydrogen-ion concentration, upon the growth of the seedling of wheat, 

 soybeans, com, and alfalfa in solution culture and upon the germination 

 of the seeds of wheat, soybeans, corn, alfalfa, and red clover, under con- 

 ditions permitting the elimination or control of factors other than re- 

 action. 



(2) Citric acid was found unsuitable for adjusting the reaction of cul- 

 ture solutions for such work on account of bacterial infection which 

 produced rapid changes in reaction and nitrate content. The nitrate- 

 assimilating bacteria in this case were found more sensitive to acidity 

 than were wheat seedlings. 



(3) A satisfactory method of adjusting the reaction of the culture so- 

 lutions was found to be the addition of a uniform amount of H3PO4 to all 

 cultures and increasing amounts of NaOH to successive cultures. 



(4) Maximum growth of seedlings of wheat, soybeans, and alfalfa 

 occurred in cultures having a reaction of 5.94 Pa, while com produced 

 greatest growth in the cultures having a reaction of 5. 16 Pg. 



(5) A reaction of 5.16 Pg was approximately equal to 5.94 Pg for the 

 growth of soybeans and wheat but decidedly less favorable for the growth 

 of alfalfa. 



(6) A reaction of 4.1 1 Pg was somewhat less favorable to soybeans and 

 distinctly less favorable to com, wheat, and alfalfa than a reaction of 

 5-i6Ph. 



