CARBON AND NITROGEN CYCLES IN THE SOIL 99 



bacterial reduction of nitrates, on the other hand, gives no single 

 product, but a number of products not in any simple ratio, whilst the 

 chemical reduction can readily be made to go quantitatively to 

 ammonia. 



Whether denitrification goes on to any extent in properly drained soils 

 is doubtful, because the three essential conditions, lack of air, presence 

 of much easily decomposable organic matter and of nitrate are rarely 

 attained. In 1895 Wagner and Maercker startled the agricultural 

 world by announcing that unrotted dung destroys the nitrates in the 

 soil and reduces the crop yield (291). Their experiments were criticised 

 by Warington (297) who pointed out that their dressings of dung 

 were enormous and their results would not apply to ordinary farm 

 practice. But it goes on to a marked extent in wet soils. Nagaoka 

 (211, see also 74) has shown that nitrate of soda frequently depresses, 

 instead of increasing, the yield of rice, sagittaria and juncus on the 

 swamp soils of Japan, an action which he attributes to the formation 

 of poisonous nitrites. Organic manures are always used on such soils. 



Assimilation of Ammonia and Nitrates by Bacteria and other 



M icro-organisms. 



Various bacteria and moulds capable under suitable conditions of 

 assimilating ammonia have been isolated from soils. They are not 

 active in ordinary arable soils rather poor in organic matter ; Schlos- 

 mgpere (245) recovered as nitrate 98 per cent, of the added ammonium 

 compounds, so also did Russell and Hutchinson. In peaty soils, how- 

 ever, the assimilation of added ammonia appears to be more pronounced, 

 amounting to nearly 30 per cent, in Lemmermann's experiments (170). 



Certain organisms are capable of taking up nitrates. Whether the 

 action goes on in the soil is not clear, but it is not excluded by the 

 conditions; it requires easily decomposed organic matter (38) and air, 

 in which respect it differs markedly from denitrification proper ; it 

 apparently goes on when sugar is, added to the soil (152). But such 

 assimilation does not necessarily involve any loss of nitrogen, for as the 

 bacteria die they are probably decomposed with formation of ammonia 

 and nitrates once again. 



