58 
Journal of Agricultural Research 
Vol. XXVIII, No. 1 
The disappearance of dicyanodiamid from the soil, as shown in Table X, takes 
place fairly rapidly. At the two concentrations used only 61 to 65 per cent of the 
dicyanodiamid was present after four weeks. With further incubation of the 
mixtures the dicyanodiamid decreased slowly, but IB to 27 per cent remained 
even after 40 weeks. It is quite probable, however, that all of the analytic a 
figures for dicyanodiamid are too high since silver nitrate precipitates several 
other decomposition products of cyanamid. These other products, if present 
at all, were doubtless in small amounts. The figures show that dicyanodiamid 
slowly decomposes in the soil to ammonia which is in turn more slowly converted 
into nitrates. With 12.72 mgm. of dicyanodiamid nitrogen per 250 gm. of soil 
actual nitrification of the added nitrogen apparently did not commence until 
after 36 weeks. With twice this quantity of dicyanodiamid the nitrate present 
after 40 weeks was still less than in the untreated soil. 
Dicyanodiamid inhibits the action of the nitrifying organisms of the soil 
causing an accumulation of ammonia in the soil during the early stages of its 
decomposition. Normal nitrification is not resumed until the concentration of 
undecomposed dicyanodiamid is reduced to such a point that it is no longer 
markedly toxic to the soil organisms. The ammonia accumulation in soils 
containing dicyanodiamid is due both to the decomposition of the organic matter 
present and also to the breaking up of the dicyanodiamid itself. 
The recovery of nitrogen from the soils receiving the largest application of 
dicyanodiamid was never higher than 85 per cent during the first 36 weeks. This 
indicates that either the dicyanodiamid was absorbed by the soil to a certain 
extent and was not extracted with water or else some of it was converted into 
compounds that were not determined. In this experiment at the end of 4C weeks, 
apparently all of the added nitrogen was present as nitrate, ammonia and dicyan¬ 
odiamid. With the smaller application the recovery of nitrogen was more than 
100 per cent in some instances. This may have been due to a slight stimulation 
of the ammonification of the soil organic matter by the dicyanodiamid but more 
likely was experimental error. Extreme accuracy can not be expected with the 
methods used, especially when dealing with such small amounts of nitrogen in 
soils containing various forms of nitrogen. 
THE EFFECT OF DICYANODIAMID UPON THE RATE OF NITRIFICATION OF AMMONIUM 
SULPHATE 
Since previous experiments had shown that soils receiving dicyanodiamid con¬ 
tain only a small fraction of the quantity of nitrates present in untreated soil for 
several days or weeks following the application, it was of interest to know to what 
extent the dicyanodiamid prevents the nitrification of ammonia nitrogen. The 
results of the analyses by the reduction method are given in Table XI and shown 
diagrammatically in figure 9. 
The procedure adopted was, in general, the same as used previously except 
that ammonia determinations were not made. Two samples of soil taken from 
the same held but at different times wer® used. The results reported in the upper 
portion of the table, using the two smaller rates of application of dicyanodiamid, 
were with one soil sample while those with the two higher rates were with the 
other sample. This slight variation had little effect upon the results. 
The marked toxicity of very small percentages of dicyanodiamid is very strik¬ 
ingly brought out in Table XI. Even as little as 0.1 mgm. per 100 gm. of soil 
delayed the nitrification of ammonium sulphate. As the concentration of 
dicyanodiamid was increased this effect became more pronounced until at a 
concentration of 10.5 mgm. of dicyanodiamid per 100 gm. of soil absolutely no 
nitrification of ammonia nitrogen occurred in 210 days and at the end of 280 days 
a conversion of only 26.12 per cent of the added ammonia nitrogen had taken place 
