134 BACTERIA AND SOIL FERTILITY 



These conclusions were criticized by Warington who pointed 

 out that they were based on experiments in which the dressing of 

 dung was enormous and the same would not occur under ordin- 

 ary practice. This led to extensive experiments in many parts 

 of the world. The conclusions of this work can thus be sum- 

 marized: 



1. The losses of ammonia from manure are comparatively 

 slight, but the setting free of elementary nitrogen which is due 

 to microorganisms and not chemical means may be considerable. 



2. With a limited supply of air in manure, the loss of elemen- 

 tary nitrogen and of organic substance are not extensive, but the 

 greater the access of air the greater the loss of nitrogen, in some 

 cases becoming as great as 40 or 50 per cent. 



3. Ordinary conservation materials, when applied in the usual 

 quantities, do not stop entirely the loss of nitrogen, but burnt 

 lime is quite effective in stopping denitrification. Solid excreta 

 and straw lose their nitrogen very slowly and no conservation 

 material is needed. It is only the nitrogen of urine which re- 

 quires conservation. 



It is sometimes found that the addition of large quantities of 

 organic matter to a soil causes a decrease in crop yield. This 

 is especially true with regard to the carbohydrates and it has often 

 been interpreted as indicating rapid denitrification, but Pfeiffer 

 and Lemmermann have pointed out that there are at least three 

 factors which play a part, namely, ( i ) direct injury to the grow- 

 ing plants by large quantities of organic matter, (2) fixation of 

 soluble nitrogen by the increased activity of diflferent organisms, 

 and (3) denitrification proper. 



It ifquite probable that the last is of the least importance, for 

 Voorhees and Lipman after ten years' investigations under care- 

 fully controlled conditions conclude that "at least with cow ma- 

 nure, used at the rate of sixteen tons per annum for a period of 

 ten years, no destruction takes place. In view of the long dura- 

 tion of the experiment and of the comparatively large amounts 

 of manure used in the course of the ten seasons, we must assume 

 that denitrification is not a phenomenon of economic importance 



