MANURES 109 



are easily soluble in water (100 parts of water at the ordinary 

 temperature will dissolve about 73 parts of the salt). When 

 it is applied to soil it undergoes decomposition, the acid radicle 

 uniting with calcium from the calcium carbonate in the soil and 

 finding its way into the drainage, whilst the ammonium is held 

 back by the humus and other constituents of the soil ; 



(NH,),SO, + CaCOj = (NH,),C03 + CaSO,. 



Before it is available for plants ammonia has to be converted 

 into nitrates by the nitrification process, and this involves a 

 further loss of calcium carbonate : 



40, + (NH,),C03 + CaCOj = Ca(N03), + 2C0, + AJLfl. 

 Oxygen Ammonium 

 (from carbonate 



the air) 



It is evident from these reactions that sulphate of ammonia 

 is only suited to soils containing at least a moderate amount of 

 calcium carbonate, and that its use is then attended with con- 

 siderable loss of calcium to the soil. It is also clear that 

 sulphate of ammonia ought to be applied some time before the 

 crop requires its nitrogen, so that the necessary nitrification 

 can take place. In these respects it difiers essentially from 

 nitrate of soda, which is already available for plants, has little 

 or no influence upon the calcium carbonate of the soil, and, as 

 already stated, should not be applied before it is wanted. 



Ammonium sulphate has the advantage in a wet season, on 

 account of its retention in the soil. In a very dry season, when 

 nitrification may be checked and little loss of water by 

 drainage occurs, nitrate of soda often gives the better result. 



Potassium nitrate^ KNO3, ^^ ^^ double value as a manure, 

 but its price is so high that it is not very largely employed in 

 agriculture. 



Galcmm cyanamidey CaCN,, has lately been proposed as a 

 nitrogenous manure. The substance is obtained by heating 

 calcium carbide, CaC, (now so largely produced by means of 

 the electric furnace from carbon and lime, chiefly for the 



