266 PLANT BIOCHEMISTRY 



several other compounds. Large quantities of ammonia are now being 

 used by direct addition to irrigation water. 



Organic nitrogen compounds. Urea is manufactured by combining 

 ammonia with carbon dioxide under pressure. Tlie reaction takes 

 place as follows: 



2NH3 + CO2 -^ NH2COONH4 -> CO(NH2)2 + H2O 



ammonium urea 



carbamate 



Urea is readily soluble in water and is probably absorbed by plants 

 in small amounts. But much of the urea is converted to ammonium 

 compounds and nitrates in the soil and is absorbed as ions of these 

 compounds. 



CO(NH2)2 + 2H2O -> (NH4)2C03 



urea ammonium 



carbonate 



(NH4)2C03 + 3O2 -^ 2HNO2 + 3H2O + CO2 



ammonium nitrous 



carbonate acid 



2HNO2 + 02-^ 2HNO3 



nitrous nitric 



acid acid 



Urea is considered to be one of the best nitrogenous fertilizers be- 

 cause it is cheap, is readily soluble, is rapidly converted to ammonium 

 and nitrate compounds, resists leaching, and increases soil acidity 

 only slightly. 



Calcium cyanamide, as produced for the fertilizer trade, is an im- 

 pure, grayish black powder, containing a considerable amount of lime 

 and free carbon. Pure calcium cyanamide (CaCNo) is white and con- 

 tains 35 per cent nitrogen. The commercial product contains about 

 21 per cent free CaO, 11 per cent free carbon, and smaller amounts of 

 several other compoimds. 



The raw materials for the manufacture of calcium cyanamide are 

 air, coal, and limestone. Nitrogen (prepared from air) and calcium 

 carbide (produced from coke and lime) are heated at 1000°C. to form 

 calcium cyanamide. 



CaC2 + No ^ CaCN2 + G 



calcium calcium 



carbide cyanamide 



Crude calciimi cyanamide is particularly suitable for application to 

 acid soil, owing to its high content of calcium oxide. When added to 

 a soil having a pH of 7.0 or less, cyanamide is hydrolyzed to urea 

 within a few days. It is this reaction which makes possible the use 



