548 The Storage of Farmyard Manure 



We are left therefore with the fact that loss of nitrogen only occurs 

 under the partial aerobic and anaerobic conditions that obtain when the 

 mixed trampled materials of farmyard manure are exposed to air; it 

 will not go on in simple aerobic or anaerobic conditions. This complex 

 air requirement indicates an alternate oxidation and reduction, as 

 postulated by the third hypothesis. 



The simplest possibility is that nitrates are formed in one part of 

 the heap, and decomposed in another. We have shown that nitrification 

 can set in in a manure heap when air is present and the moisture has 

 been reduced below a certain critical point. Further we have shown 

 that when this nitrate reaches the moister part of the heap it speedily 

 denitrifies. Whenever, therefore, nitrate can form, and can get into 

 the interior, there is the possibility of the loss of nitrogen. We have 

 never found nitrate in the wet part of the heap, or we might suppose 

 that it is formed when the current of air happens to travel that way, 

 and decomposed when the air passes elsewhere. Our failure to find 

 nitrate does not necessarily mean that none is formed there, but only 

 that nitiification, if it occurs, is slower than denitrification. 



It is not necessary, however, that the oxidation should proceed 

 as far as the nitrate stage : any intermediate stage would suit the hypo- 

 thesis equally well. 



Whatever the actual compound formed, this hypothesis requires 

 three stages for the loss of nitrogen : 



1. Ammonia is formed. This is necessary because so far as is 

 known no compound of nitrogen except ammonia is oxidised by bacteria 

 to give the compound postulated for the next steps. 



2. The ammonia is oxidised to form an oxygen compound of nitrogen. 



3. This compound is reduced, with liberation of gaseous nitrogen. 

 At first sight it may appear cumbersome that the ammonia should 



first be oxidised to an oxygen compound, and then reduced : why cannot 

 it be oxidised straight away to gaseous nitrogen? We do not consider 

 that this objection has any force. The oxidation of methane, even 

 during combustion, is known to proceed by the introduction of successive 

 oxygen atoms: 



H 



H H H O 



H— G— H -^ H— C>— O— H ^ H— C = ^ H— 0— C-0 ^ = C = 

 H O 



H 



