FERTILIZERS. 81 



be uiipi'oductive, the luek of one or all of the three 

 elements mentioned is almost nniversal. 



Plants reqnire larger (luantities of these elements than 

 of any other, and hence they fail the soonest. 



Nitrogen, — Nitrogen is an element both essential to 

 plants and dittieult to obtain. While the air contains 

 an abundance of it, but few plants can make use of it 

 from that source. It is not generally availahle to the 

 plant in its free state, but in combination with other 

 elements. 



There are two compounds of nitrogen in which it is 

 believed to be mostly serviceable to plants : they are 

 nitric acid and ammonia. 



The plant seems to obtain most of its nitrogen by tak- 

 ing njj through its roots either of these substances, or 

 compounds formed by the chemical action of these upon 

 other substances. 



1. Nitric Acid (HNO3). — Nitric acid is formed in 

 the soil by the decay of organic matter. A small 

 quantity of it also exists in the atmosi)here, some of 

 Avhich arises from the decay of organic substances, and 

 some is believed to be produced out of the free nitro- 

 gen and oxygen of the air by electric currents passing 

 through it. 



This acid, either formed in the soil or washed into it 

 from the atmosphere, enters the roots of the plant, either 

 directly, or in the form of salts, called nitrates, produced 

 by its union with alkaline substances. 



2. Ammonia (NH^).- — Ammonia is also produced by 

 the decomposition of animal and vegetable matter, from 

 which, unless absorbed and retained by other sulistances, 

 it easily escapes into the atmosphere. It is very soluble 

 in water, and is absorbed by various substances, as peat 



WI^'S Agu. — Q 



