no ELEMENTARY AGRICULTURAL CHEMISTRY 



manufacture of acetylene), in a current of air which has been 

 deprived of oxygen. The crude product, which contains about 

 20 per cent, of nitrogen instead of the theoretical 35 per cent., 

 is a black powder, resembling basic slag in appearance. When 

 used as a manure it has in many cases yielded as good results 

 as the same amount of nitrogen applied as nitrate of soda or 

 sulphate of ammonia ; but on peaty soil it has been found to 

 be harmful to plants, owing, it is said, to the formation of 

 dicyandiamide by the action of the acids present in the soil. 

 This substance is a powerful plant poison. 



Under ordinary conditions — i.e., in the absence of acids — 

 the substance probably decomposes thus : 



CaCN^ + dR,0 = CaCOj + 2NH3, 



the whole of the nitrogen being obtained as ammonia, and 

 thus capable of nitrification. 



(B) Phosphatic Manures. — Bones, especially steamed and 

 burnt bones, and guano are chiefly valued for the phosphates 

 they contain. These manures have already been described. 

 Other more important (because more abundant) sources of 

 phosphoric acid are known. Before dealing with them it may 

 be advisable to describe briefly the various forms in which 

 phosphoric acid occurs in fertilisers. 



1. As Free phosphoric acid, H3PO4. — This, when pure, is a 

 thick, semi-solid mass, obtained by acting upon a phosphate 

 with sulphuric acid ; 



CagPPs + SH^SO, = 3CaS0, + 2H3PO,. 



It is soluble to any extent in water. It is found in small 

 quantities in some superphosphates. 



2. As Monocalcium tetrahydrogen phosphate, CaH^P^Og. — 

 This substance is obtained by the action of a smaller proportion 

 of sulphuric acid upon*calcium phosphate : 



CajP.Oj + 2H,S0, = CaH.P.Og + 2CaS0,. 



