118 Decomposition of Cyanamide and Dleyanodiamide 



amide. This indicates that the decomposition begins normally; ammonia 

 is formed but the sensitive nitrifying; organisms are inhibited "by the 

 dicyanodiamide. 



To ascertain whether this hypothesis be correct, the effect of di- 

 cyanodiamide on the nitrification of ammonium sulphate was next 

 determined. The results (plotted in Fig. '?>) show a marked retardation 

 of the nitrification of ammonium sulphate in the presence of dicyano- 

 diamide. 



N present as Nitrate N present as NH3 



per million dry soil per million dry soil 



At After After After After At After After After After 



start 7 days 21 days 42 days 94 daj'S start 7 days 21 days 42 days 94 days 



Control 8-8 12-4 18-8 175 20-6 2-6 3-8 1-3 3-9 5-2 



Ammonium sul- "| 



phate alone J 



Ammonium sul 



19-5 33-7 41-4 550 — 28-5 16-8 16-5 70 



phate + dicyano- \ — 10-9 13-5 132 14-9 — 35 32-4 34- 1 30G 



diamide 



J 



Added 50 mgs. ammonium sulphate N per kilo soil = 00 parts of N per million dry soil. 

 15 mgs. dicyanodiamide N per kilo .soil. 



For the first 42 days the mixture of ammonium sulphate and dicyano- 

 diamide shows but a small production of nitrate and practically no 

 reduction in ammonia content. Even after 94 days the action has only 

 proceeded to an insignificant extent. 



We are now able to explain the discrepancy between the pot and 

 the laboratory nitrification results in the case of the mixture of cyan- 

 amide and dicyanodiamide (Figs. 1 and 2). In the laboratory tests the 

 cyanamide breaks down to ammonia, but the action goes no further 

 because the nitrifying organisms are adversely affected by dicyano- 

 diamide; therefore nitrates do not accumulate. In the pots the plants 

 are able to make use of the ammonia formed, either because the retard- 

 ation of nitrification does not operate to the same extent as in the 

 laboratory tests or the plants have taken up some of their nitrogen in 

 the form of ammonia. This inhibition of nitrification by dicyanodiamide 

 no doubt explains the slower action of cyanamide on plant growth in 

 the presence of even a small amount of this compound. 



Injliience of Dicynnodiantide on the Soil Bacteria. 



Experiments were next made to ascertain whether dicyanodiamide 

 affects other organisms of the soil besides the nitrifying group. 



Conjointly with the nitrification tests counts were made of the 



