324 THE PRINCIPLES OF SOIL MANAGEMENT 



nutrients to the soil, usually in a form more readily 

 soluble than those already present in large quantity. 

 While other beneficial effects may be produced by 

 certain fertilizers, they are usually of secondary import- 

 ance, as compared with the addition of the plant nutri- 

 ents. 



191. Fertilizer constituents. Prepared fertilizers, as 

 found on the market, are usually composed of a number 

 of ingredients. As these are the carriers of the fertilizing 

 material, and as it is upon their composition and solu- 

 bility that the value of the fertilizer depends, a knowl- 

 edge of the properties of these constituents is of interest 

 to every user of fertilizers, and is a valuable aid in their 

 purchase. 



192. Fertilizers used for their nitrogen. Nitrogen 

 is the most expensive constituent of manures, and is of 

 great importance, as it is very likely to be deficient in 

 soils. A commercial fertilizer may have its nitrogen 

 in the form of soluble inorganic salt, or combined as 

 organic material. Upon the form of combination de- 

 pends to a certain extent the value of the nitrogen, 

 as the soluble inorganic salts are very readily available 

 to the plant, while the organic forms must pass through 

 the various processes leading to nitrification before the 

 plant can use the nitrogen so contained. The inorganic 

 nitrogen fertilizers are sodium nitrate, ammonium sul- 

 fate, calcium nitrate and calcium cyanamid. 



193. Sodium nitrate. This fertilizer now constitutes 

 the principal source of inorganic nitrogen in commercial 

 fertilizers. The salt occurs in the crude condition in 

 Northern Chili, and is believed to be due to the action 



