16 



6. The use of horse or cow dung likewise seriously diminishes the 

 return obtained from applications of sulphate of ammonium, urine or 

 green manures. 



4. Straw is also practically without value as a manure, and like 

 dung, energetically destroys the nitrates in the soil. 



5. Ordinary farmyard manure when applied to the soil acts in the 

 same general manner as the dung and straw which form its principal 

 constituents. 



6. The denitrifying power of dung, straw and farmyard manure, is 

 due to the presence in these substances of a special organism having 

 the power of reducing nitrates. 



7. The denitrifying power of dung, or of farmyard manure, is con- 

 siderably diminished when the manure has become old and humified. 

 The destruction of nitrate of sodium in the soil is also much less when 

 the manure has been applied some time before the nitrate. 



8. When the farmyard manure has been preserved by mixing with 

 it superphosphate or kainit, its denitrifying power is little altered by 

 age. 



The whole of the German experiments, showing the actual effects of 

 various manures on crops, appear to have been carried out in large zinc 

 pots. Such pots are usually without drainage. They are provided with an 

 outside pipe communicating with the bottom of the pot ; through this 

 pipe water is supplied. An experimental series of pots usually occupies 

 a truck, running on rails, and during inclement weather the whole of 

 the pots can be brought under cover. This mode of experimenting has 

 been carried to great perfection in Germany. 



It is evident, from what has been said above, that the destruction of 

 nitrates in the soil is regarded by the German investigators as the chief 

 cause of the prejudicial action- observed by them when farmyard manure 

 or its solid constituents, were applied to the soil; and this destruction 

 of nitrates they believe to brought about by a special organism which 

 the manure supplies. To gain a clearer light on the subject we will 

 therefore, in the first place, briefly consider what takes place during the 

 process of denitrification, and what conditions are favourable to the 

 occurrence of this action. 



The Process of Denitrification. 



The subject of denitrification, has been carefully investigated by 

 many scientists during the past thirty years, with the usual result that 

 a great deal of the work done has been already forgotten, and is now 

 buried in the ignorance of a fresh generation of workers. This scarcely 

 avoidable occurrence happens most frequently when as in the 

 present case, the modern worker belongs to a different nationality 

 from that which produced the earlier investigators. An historical treat- 

 ment of the subject would perhaps be unsuitable for these pages. Pre- 

 facing, therefore, our remarks by saying that Dr. Angus Smith of Man- 

 chester, in 1867, was apparently the first to observe the destruction of 



