124 CHEMISTRY OF FARM PRACTICE 



the end, and the reduction in price of the inferior sources 

 is usually not great. 



The crop grown and the character of the soil should be 

 the determining factors in the selection of the sources of 

 nitrogen. Light sandy soils are leachy, and do not easily 

 retain soluble plant food, therefore it is often advisable 

 to make use of organic sources with such soils. Some 

 crops have to be forced and require a large amount of 

 rapidly available fertilizer; but these crops are usually GO 

 valuable that the grower can afford a certain amount of 

 loss through leaching. 



92. Inorganic Sources of Nitrogen. The sources of 

 nitrogen may be divided into two classes organic and 

 inorganic. The inorganic sources of the nitrogen found 

 in commerce in large quantities are potassium nitrate, 

 (KNOa), sodium nitrate (NaNOs), calcium nitrate (Ca(NOs)2) 

 and sulphate of ammonia ((NH^SO-i). These are all 

 easily soluble in water. The nitrates remain soluble under 

 all conditions until they are either used as plant food, 

 leached out, or, as occurs under very unusual conditions, 

 lost through denitrification. The nitrates are so soluble 

 that they may be considered the most thoroughly predi- 

 gested nitrogenous plant food and, therefore, are most 

 efficacious when applied to a growing crop as a topdressing. 

 Sulphate of ammonia reacts with certain soil compounds 

 which render the ammonium radical (NH4) less soluble 

 in water and, consequently, more slowly available. This 

 reaction is supposed to take place between the ammonia 

 salts and compounds in the soils called zeolites. The zeolites 

 are double hydrated silicates of aluminium with some 

 other base which is interchangeable. Examples of zeolites 

 are Thomsonite, CaAl2Si2Og, and natronite, Na2Al2SisOio. 

 The bases that may be substituted in the zeolites are cal- 

 cium, sodium, potassium, and ammonium. Sodium will 

 displace calcium in the zeolitic compounds, while potas- 

 sium will displace sodium or calcium, and ammonium will 



