Relations of Higher Plants to the Formation of Nitrates 79 



28 give these results. In both tests the nitrate-forming bacteria in the 

 alfalfa soil are more active than are those in the timothy soil. 



From these data the conclusion may be drawn that the growth of alfalfa 

 on this soil has given rise to greater activity on the part of the nitrate- 

 producing bacteria, or that at least it has not had so pronounced an inhib- 

 iting action as the timothy has had. 



It is difficult to determine whether the effect of the alfalfa on the soil has 

 been to increase the activity of the nitrifying bacteria as compared with the 

 unplanted soil, or only as compared with the timothy soil. In other words, 

 it may be the case merely that alfalfa depresses nitrification less than does 

 timothy. One reason why it is difficult to ascertain the relative activity 

 of the nitrifying bacteria in the planted and the unplanted soil is because 

 nitrates are higher in the unplanted soil as it comes from the field and the 

 increase during incubation reaches a point at which the products formed 

 may interfere with further action. 



The columns in Table 28 in which is stated the excess of nitrates when 

 the soil was incubated with dried blood over those with direct incubation, 

 possibly give some light on this question. It will be noticed that the 

 planted soil produced about the same excess, in the main, as did the 

 unplanted soil. Unfortunately, incubation with dried blood was not per- 

 formed in the test made later in the season. It seems probable that the 

 character of the organic matter left in the soil by the plants determines to 

 some extent the rate of nitrification in this soil. This is indicated by the 



TABLE 29. 



Nitrates Produced by Incubation both with and without Dried Blood 

 IN Unlimed Soil (Parts per Million) 







Nitrates producec 



in fourteen daj'S 



Excess of nitrates 













with dried blood 











over those with 







Ma: 



' 10 



May 29 



direct incubation 



Plat 



Crop 















Incubation 



Incubation 



Incubation 



Incubation 











with 

 natural 



with 

 dried 



with 



natural 



with 

 dried 



May 10 



May 29 







soil 



blood 



soil 



blood 







4001 C 



Alfalfa 



66 



213 



82 



227 



147 



145 



4001 C 



None 



05 



206 



42 



193 



141 



151 



4002 C 



Timothy 



56 



169 



47 



136 



113 



89 



4002 C 



None 



49 



170 



29 



112 



121 



83 



