8 Southern Oardener^s Practical Manual 



About four -fifths of the ocean of air which bathes 

 the surface of our globe is uncombined nitrogeu. The 

 only known means of collecting this nitrogen from the 

 air is the power possessed by leguminous plants, such as 

 peas, beans, clover, alfalfa, vetches and others, which 

 collect it through the agency of microscopic organisms 

 known as bacteria, that have their homes in excrescent 

 nodules, formed upon the roots of such plants. While 

 these bacteria possess the power of fixing the nitrogen 

 from the air, it is supposed to be changed to the form 

 of nitrates before being appropriated by plants. We 

 understand by nitrate, nitric acid combined with some 

 metal, as sodium, a familiar form of which is known to 

 chemists as sodium nitrate or, in commerce, as nitrate of 

 soda. If it combines with potassium, we have potassium 

 nitrate, nitrate of potash or saltpeter. 



Of the fourteen elements found by chemical analysis, 

 to be present in plants, only three, — nitrogen, phosphorus 

 and potassium — are usually considered in the application 

 of fertilizers to the soil. On some soils lime is a needed 

 constituent. 



The principal commercial sources of supply of these 

 are: 



Nitrogen — (a) Mineral sources: Sulphate of ammonia, 

 nitrate of soda and (in Europe) nitrate of potash; (h) 

 Animal sources: Dried blood and tankage, two by-prod- 

 ucts from slaughter-houses; fish-scrap, etc.; (c) Vege- 

 table sources: Cottonseed-meal, castor-bean, pomace and 

 tobacco stems. Natural guanos and animal manures are 

 also important sources. 



Thie most cheaply available and inexhaustible source 



