lo Oct., 1911-] The Nitrogen Cycle as it A-ffects Agriculture. 68r 



as gaseous nitrogen, in which form it is not readily avaihihle to ordinary- 

 plants, and animal life. 



Xow we have the two important facts, that nitrogen is an extremely 

 valuable plant food ; and, at the same time, it is easily rendered un- 

 available so far as its usefulness as food to ordinary plants or animals 

 is concerned. Recognizing both these facts, it becomes a question of no 

 little moment to the farmer as to how he may best prevent the loss of 

 an element in the absence of which his crops will not come to maturity ; 

 but, before considering how losses mav best be prevented, we must fir.'-t 

 consider the changes which nitrogen compounds undergo in the nitrogen 

 cvcle. and the chief conditions which lead to the loss of this element. 



Xatural Changes. 



On reference to the diagram, we may take, as a starting point, the 

 nitrogenous vegetable compounds, that is, nitrogen contained as com- 

 pounds in i)lants. When these are devoured by animals as food, thev 

 are converted, by digestion and change in the animal br,dy. into nitro- 

 genous animal com])ound . \\ithin the animal organism, continuous 

 building up and breaking down of the tissues of the f)ody is taking place, 

 and in the latter process nitrogen becomes a waste product and is thrown 

 out of the body in the liquirl excrement in the form of urea. 



This urea is an interesting compound from many points of view, Imt 

 its chief interest to the farmer centres in the fact that it is very easily 

 converted by certain bacteria and ferments into ammonia, and it is here 

 where a great deal of the loss of nitrogen from farmyard manure occurs. 

 The marked smell of ammonia noticed in an ill-ventilated stable is due 

 to the decomposition of urea re.su king in the formation of free ammonia, 

 and this ammonia being volatile is practically lost to the farmer as a 

 manure, as it becomes (hssipated into the air and so is lost. Certain 

 species of nitrifying bacteria, however, are ca])able under certain condi- 

 tions, such as free aeration, absence of light and presence of a slightly 

 alkaline medium, of converting ammonia into nitrous acid which forms 

 nitrites with such substances as lime present in the medium in which the 

 bacteria are working. 



Hence, if the manure containing the urea is properly looked after and 

 subxefjuently applied to the soil, the nitrogen in the ammonia formed from 

 the decomposition of the urea is fixed in the form of nitrites. These 

 nitrites are not available to plants as food, but are converted Ijy still 

 other bacteria, working also under a certain .set of conditions, intO' 

 nitrates, and it is in this form that nitrogen is assimilated by plants. 

 It must be rememlH^red. however, that these changes from ammonia to 

 nitrites, and nitrites to nitrates, take place to the fullest extent onlv under 

 favourable c<;nf]itions. and if the soil Ix- water-logged, sour, and uncul- 

 tivated, the rever.se actions may be brought about bv denitrifving bacteria, 

 and th<> nitrogen will be lost. 



The nitrogen cycle <loes not of necessitv inclurle the 'tage through the 

 .'•nimal kingdom, for nitrogenous vegetable matter decaying in the .soil 

 is acted on by the nitrifying l)acteria. It is converted int » nitrites, and 

 later into nitrates, whence it is again available to plants, and the cyck^ 

 completed. Another cycle is completed when the nitrogen, set free into 

 the air when vegetable matter is burned, is returned to the vegetable 

 kingdom again l)y that class of |)lants known as the legumes. whicJi. 

 acting in symbiosis with certain forms of bacteria, have the |X)wer of 

 (ixing atmospheric nitrogen for their own u.se as food. 



