1166 
Journal of Agricultural Research 
Vol. XXVIII, No. 11 
SUMMARY 
This paper presents the results of laboratory studies on the effect of cyanamide 
and related compounds on bacterial numbers. In considering these results, it 
should be borne in mind that the bacterial counts were made on Lipman and 
Brown's synthetic agar, which, like all media, favors the growth of certain types 
of organisms; in this instance chiefly the ammonifiers. The use of other sub¬ 
strates would probably have given different results. 
Cyanamid produced unusually large increases in the number of bacteria in 
soils, the maximum increase usually occurring within two weeks after application, 
but this depended upon a number of factors, chiefly the rate of application and 
temperature of incubation. In one instance, the number reached was 70 times 
the control. 
In an attempt to determine the particular constituent in cyanamide primarily 
responsible for this rapid bacterial multiplication, it was found that both the 
cyanamide content and the lime were very important. Where used alone, either 
calcium hydroxid or calcium oxid in amounts equivalent to that found in 
cyanamid produced large increases in numbers. The larger the application, 
the larger was the stimulation within the limits of these experiments. In the 
case of pure cyanamide (H 2 CN 2 ) the two lower rates of application produced 
relatively small increases and the highest partially sterilized the soil. On the 
other hand, the intermediate rate produced a very large increase comparable to 
cyanamid. It is, therefore, impossible to state whether lime or cyanamide is 
the greater factor in explaining the effects of cyanamid. Undoubtedly, both 
are important in the moderate applications, but the cyanamide or its decom¬ 
position products are largely responsible for the partial sterilization with the 
higher applications. The enormous increases in numbers sometimes noted 
with certain concentrations of cyanamid can probably be attributed in large part 
to the cyanamide. Further work is needed to establish these points. 
Urea and ammonium sulphate produced only slight effects upon bacterial 
numbers, even though both materials are known to be excellent sources of plant 
food for higher plants. As previously pointed out, the absence of effect with 
urea was most likely due to the fact that the medium used did not contain urea 
and hence did not favor the urea bacteria. In the case of ammonium sulphate, 
no marked stimulation could be expected since the nitrogen is already in the 
ammonia stage and hence not attacked by the ammonifying organisms. 
The other materials, namely, dicyanodiamid, guanylurea sulphate, guanidin 
nitrate, and biguanid nitrate also failed to produce marked effects even though 
some of them are known to be injurious or at least unavailable for higher plants. 
In considering the results presented above, it should be borne in mind that the 
quantities of materials used were far in excess of the usual field applications. 
For instance, 50 mgm. of nitrogen per 50 gm. of soil would be the equivalent of 
slightly more than 10,000 pounds of ammonium sulphate per acre (figured as 
2,000,000 pounds of soil). Obviously, no such concentration would ever occur 
under field conditions except possibly in isolated cases where heavy fertilizations 
were made in the row. Applications corresponding to field practice, where the 
material is uniformly mixed with the soil would probably have little effect upon 
bacterial numbers. Further work is needed along all of the lines treated and 
broad generalizations are not justified from the data here presented. 
