11.] IVET AND DRV SEASONS 67 



permanent loss by liberating some of the nitrogen in 

 the form of gas, but the majority simply withdraw the 

 soluble nitrogen for a time from circulation, building it 

 up in their own tissues. The immediate result is, 

 however, a lessened availability of the manure, and this 

 loss will fall far more upon ammonium compounds than 

 upon nitrates, which are not so generally utilisable by 

 the organisms found in the soil. 



It is generally assumed that since nitrate of soda is 

 not retained by the soil, while ammonium salts are, 

 the former is a manure better suited to dry seasons and 

 climates, whereas under wetter conditions there is less 

 danger of the latter washing out of the soil. This view, 

 however, forgets that if the ammonium salts are to feed 

 the plant they must be nitrified, and that the calcium 

 nitrate produced is just as likely to be washed down in 

 a wet season. Indeed, the Rothamsted results do not 

 bear out the popular idea. In exceptionally dry seasons 

 there may be some advantage from the use of nitrate 

 of soda because of the deep-rooted habit it induces, 

 but the advantage is still more pronounced in seasons 

 of excessive wet. Taking an average of the wet seasons 

 against the dry, the ammonium salts do better in the 

 latter. Probably the nitrification of the ammonium salts 

 is checked in wet seasons when the temperature is low, 

 when also aeration is deficient through the repeated 

 saturation of the soil. 



In addition to the definite compounds which have 

 just been described, a very large number of waste 

 products from some industrial or manufacturing pro- 

 cess dealing with material containing nitrogen are 

 employed as nitrogenous manures. For example, 

 almost all animal products contain nitrogen, hence the 

 residues from slaughter-houses, fish-curing sheds, and 

 other processes concerned in the preparation of food. 



