322 SOILS AND MANUKES 



for a month after the animals were removed, and the loss 

 increased to 35 per cent., the weather at the time being 

 very warm. 



In ordinary farming practice the losses are generally 

 much greater than those recorded in the experiments re- 

 ferred to. Many of the animals are kept in stalls and 

 there is often a considerable waste of liquid excrement. 

 The manure is commonly carried out daily, and little 

 care is exercised to prevent loss by evaporation or by 

 drainage from the heap. When the amount retained by 

 the animals and all sources of loss are taken into 

 consideration, it is probable that, of the total nitrogen in 

 the food, only about from one-half to a quarter is re- 

 covered in the manure. The losses in all these ways fall 

 most heavily upon the soluble, i.e., the most active and 

 valuable part of the manure. 



AMMONIA FIXERS AND PRESERVATIVES. 



Gypmm, etc. A quantity of powdered gypsum is some- 

 times mixed with the manure in order to check the loss of 

 nitrogen by evaporation of ammonia. Its action is due to the 

 chemical change represented by the following equation : 



CaS0 4 + (NH 4 ) 2 C0 3 = CaC0 3 + (NH 4 ) 2 S0 4 . 



The efficiency depends to some extent upon the quantity 

 of the preservative used, and the saving effected by this 

 means is not commensurate with the cost of the material. 



Sulphate of magnesia and sulphate of iron (copperas) 

 have also been proposed as substitutes for gypsum. 

 They act in a similar manner but are more soluble 

 and probably more efficient than gypsum. 



Acid substances, e.g., sodium .bisulphate, superphos- 

 phate and even sulphuric acid itself have been tried and 

 have proved more effective than the neutral sulphates. 

 The best results were obtained by the use of superphos- 



