1162 
Journal of Agricultural Research 
Vol. XXVIII, No. 11 
The pouring of the plates was done in the same manner as previously, using 
Lipman and Brown’s synthetic agar. The average number of organisms, in terms 
of millions per gram of soil, is given in Table II for the different treatments. All 
plates which were badly overrun by fungi or were for other reasons decidedly 
abnormal were eliminated in determining the averages. 
From the data it will be observed that ammonium sulphate produced only a 
slight increase in the numbers of bacteria and then only with the two largest 
rates of application. Likewise, dicyanodiamid, guanylurea sulphate, and urea 
produced no appreciable effects, the slight changes in numbers usually being within 
the experimental error. On the other hand, cyanamid, whether oiled and hydrated 
or untreated, produced an increase of approximately 700 per cent at the largest 
rate of application and smaller increases with lesser amounts. The maximum 
effect on the numbers of organisms was observed at the first analytical period, 
namely, 3 days. After 7 days the numbers had decreased to about half and con¬ 
tinued to decrease until at the end of 3 weeks there were actually fewer in the 
treated than in the untreated soils. At the end of the experiment, 38 days, all soils 
regardless of the kind of fertilizer applied and even the controls showed more 
microorganisms than at the previous incubation period of 31 days. It is of con¬ 
siderable practical importance to know that the coating of oil which treated 
cyanamid receives does not appreciably affect the numbers of bacteria in the soil. 
It should be remembered, however, that most of the organisms which grow on the 
synthetic agar used are ammonifiers and hence the results are in no way an 
indication of the effect of the different materials on either the numbers of nitrifiers 
or the rate of the oxidation produced by them. The conditions required for the 
optimum growth of the two classes of organisms, especially on laboratory media, 
are quite different, the nitrifiers being especially susceptible to slight variations 
in reaction, concentration, source of food supply, and presence of undesirable 
substances. The ammonifiers are quite hardy and grow under widely different 
environments. 
In comparing the results given in Tables I and II it is necessary to bear in mind 
the radical differences between the two sets of experiments. The first experi¬ 
ment was with a very fertile soil, the second with a very poor soil deficient in 
organic matter, nitrogen, and phosphorus. Again, the first work was conducted 
during the winter months with a soil taken from the field from underneath a 
crust of frozen soil; the second experiment was made during the hot summer 
months with soil secured from the field at the time. Furthermore, there were 
considerable differences, as already noted, in the methods of conducting the 
experiments, time of incubation, rates of application, etc. All work was done at 
room temperature which varied during the winter from 15° to 24° C., and during 
the summer from 21° to 37° C. We could, therefore, scarcely expect closely 
agreeing results between the two sets of experiments. 
The rates at which the changes in the numbers occurred are very different in 
the two sets of experiments. For instance, in the winter work the maximum 
numbers were observed in the soils receiving cyanamid at the 31-day incubatioiji 
period while in the summer studies the maximum numbers were found after 
3 days and rapidly dwindled thereafter. There was a marked increase in num¬ 
bers in both instances, except where too much of the material was used. Of 
the other materials, ammonium sulphate, urea, and dicyanodiamid produced 
slightly greater stimulations in numbers in the first set of experiments. A dis¬ 
crepancy occurred in the case of guanylurea sulphate, this material producing 
a maximum stimulation of 126 per cent with the richer soil and only about 32 
per cent with the poorer soil. In fact, in the latter case, the material usually 
acted as entirely inert, slight depressions in numbers being about as frequent as 
stimulations. 
