Jan. 28, i9i8 Nitrogen- Assimilating Bacteria 185 



The direct influence of nitrates on bacteria has been studied to a limited 

 extent. The influence of various nitrates on soil bacteria has been 

 studied by Greaves (19). He added sodium, potassium, calcium, mag- 

 nesium, manganous and ferric nitrates to soil in varying quantities. 

 The amount added to the soil was such that in each case equivalent 

 quantities of the anion (NO3) in the various forms were added. The 

 effect of these salts on the bacteria was determined by using ammonifi- 

 cation as an index of the bacterial activity. He found that sodium- 

 potassium, manganous and ferric nitrates in small amounts, approxi- 

 mately 0.97 to 5.5 mgm. of nitrate in 100 gm. of soil, slightly stimulated 

 ammonification. Greater concentrations of these salts proved toxic as 

 evidenced by a decrease in the amount of ammonia formed. Sodium 

 nitrate was much more beneficial to ammonification than potassium^ 

 nitrate. From his results as a whole Greaves concludes that it is the 

 electronegative ion which stimulates bacterial activity. Calcium and 

 magnesium nitrates proved toxic in all concentrations studied. 



However, a majority of the investigations have been directed toward 

 a determination of the effect of the bacteria on the nitrates. But little 

 work appears to have been done on the direct action of nitrates on 

 bacteria. Pfeffer (j8, p. 351) cites some experiments showing the 

 repellant action of potassium nitrate toward certain bacteria. Spir- 

 illum undula was repelled by a solution of potassium nitrate having an 

 osmotic concentration equivalent to 0.5 to i.o per cent. With Spirillum 

 voluians a much higher concentration was necessary to bring about the 

 same reaction. It was found that different organisms required different 

 quantities of the same nitrate to repel them. 



It can be readily seen that by far the greatest amount of work on the 

 relation of nitrates to plant growth has been done in the realm of the 

 higher plants. Obviously further investigations should be made in 

 respect to the effect of nitrates on the lower forms of plant life, especially 

 the bacteria. In this paper an attempt is made to set forth the results 

 secured in a study of the influence which nitrates exert on certain groups 

 of soil bacteria, including not only their reproduction but also some of 

 their physiological properties. 



EXPERIMENTAL WORK 



OUTLINE OF PROBLEM 



The results of much careful experimentation show that nitrate nitro- 

 gen is most readily assimilated by higher plants. As a rule it seems to 

 stimulate the plant to increased activity. In some cases this is un- 

 doubtedly due to increased nutrition, while in others it is a result of 

 nuclear stimulation with a consequent cell multiplication. No sharp 

 line can be drawn between these two effects. Probably one overlaps 

 the other, and the increased growth of the organism can be attributed 

 to a combination of the two actions. 



