194 



tually produced. It is well known that processes are constantly at work in the soil 

 unfavorable to the accumulation of nitrates. Entirely apart from possible losses 

 by leaching, there is the more or less remote but still real danger of denitrification. 

 In addition to this there is the constant draft on the store of soil nitrates by bac- 

 teria, molds, yeasts, and algae, not to mention higher plants when these are included 

 in the experiment. 



In view of these facts the more recent investigations on the decay of organic matter 

 in the soil frequently attempt at least a partial differentiation of the single stages 

 of the processr I am not aware of systematic attempts to determine albumoses and 

 peptones among the fragments of protein decomposition in the soil. There are, 

 however, systematic studies of ammonia formation as something independent of 

 nitrite or nitrate formation. Indeed, we have come to accept the term ammoni- 

 fication (or ammonization) as expressing a definite change or series of changes. 

 However, before taking up the discussion of methods relating to the study of am- 

 monification, nitrification, and denitrification in the soil itself, it would be proper 

 to consider here certain methods a which deal with the same reactions from a some- 

 what different standpoint. 



The methods in question are based on the changes which occur in solutions of 

 known composition when inoculated with a given weight of soil. For instance, 

 a sterile solution of peptone or of gelatin, when inoculated with soil, will undergo 

 decay, and a portion of the organic nitrogen will be split off as ammonia, which can 

 be readily distilled off and estimated. Now, it happens that in equal quantities 

 of the same solution inoculated with equal weights of different soils the amounts 

 of ammonia produced may differ widely. Otherwise stated, soils vary in their 

 ammonifying power. But since ammonification is a biological process, we are 

 forced to the conclusion that the differences noted are due either to the unequal 

 numbers of bacteria introduced into the sterile solutions by the different soils, or 

 to differences in the species or vigor of the organisms, or perhaps to both. Be it 

 as it may, the ammonification coefficients show fairly constant characteristics bear- 

 ing a more or less definite relation to the productive capacity of the corresponding 

 soils. Similarly, solutions have been prepared to favor the growth of nitrifying, 

 denitrifying, or nitrogen-fixing bacteria looking toward the determination of the 

 nitrifying, denitrifying, or nitrogen-fixing coefficients of soils. The latter methods 

 have not, on the whole, proved as consistent in their results as have the ammonifica- 

 tion methods. However, it would be out of place here to discuss them in detail, par- 

 ticularly since they have been considered elsewhere. 6 



On the other hand, it would be well worth while to consider here the fundamental 

 differences between the methods just outlined and those based on the study of bac- 

 teriological processes in the soil itself. We can well appreciate, of course, how 10 

 grams of one soil might cause the production of more ammonia in peptone solutions 

 than 10 grams of another soil under identical experimental conditions. One soil 

 might have two or three times as many ammonifying bacteria as another; or it might 

 have not only larger numbers, but also species and individuals with a particularly 

 well-developed power of ammonia formation. Moreover, it appears quite logical to 

 assume that large numbers and vigorous species may produce large quantities of 

 ammonia in the soil itself as well as in the culture solutions. Hence the analogy 

 between the changes in suitable culture solutions and the returns from pot or field 

 experiments. 



Theoretically, however, this analogy could not always be expected to exist. It 

 must be remembered that we are dealing here with micro-organisms entirely detached 



U. S. Dept. Agr., Office of Experiment Stations, Bui. 194, p. 10. 

 *>New Jersey Agr. Exp. Sta., Bui. 210; Annual Report, 1905, p. 225; 1906, p. 119; 

 1907, p. 186. 



