The Composition and Use of Fertilizers. 201 



Ammonia is a colorless gas, and it is this gas dissolved in water 

 which is familiar to us as ammonia water or " Spirits of Harts- 

 horn," and which causes the peculiar odor of " hartshorn." 



Ammonia unites with different acids and forms salts somewhat 

 as metals do ; these salts we call ammonium salts, compounds which 

 do not generally have any odor like ammonia. Thus, ammonia 

 combined with sulphuric acid forms ammonium sulphate, com- 

 monly called sulphate of ammonia; ammonia, combined with hy- 

 drochloric acid, forms ammonium chloride, sometimes called mu- 

 riate of ammonia, also known as sal ammoniac. 



(3) Nitrogen in Nitrates. — Nitrogen, combined with hy- 

 drogen and oxygen, forms nitric acid or aqua fortis. If in nitric 

 acid a metal as sodium, for example, takes the place of hydrogen, 

 we have formed a sodium salt of nitric acid or a nitrate, called 

 sodium nitrate, or nitrate of soda. 



When animal and vegetable substances decompose in rather 

 warm, moist places, the nitrogen is changed into nitrates. This 

 change of the nitrogen of organic matter into nitrates is caused 

 by germs called bacteria, which are very small living vegetable 

 organisms, and which exist everywhere in enormous numbers. The 

 process is known as " nitrification/' 



(4) Nitrogen in Animals and Plants, or Organic Nitro- 

 gen. — Nitrogen, combined with the elements, hydrogen, carbon 

 and oxygen occurs in plants and in animals. Such substances for 

 example are the casein or curd of milk, the gluten or gummy por- 

 tion of wheat, the fibrin of blood, the white of egg, etc. When 

 such compounds decompose, the nitrogen is first changed into 

 ammonia, and then, under proper conditions of warmth, moisture 

 and access of air, into nitric acid or nitrates. The nitrogen exist- 

 ing in animals and plants is generally called organic nitrogen. 



(b) In what Forms Nitrogen is Useful to Plants. — Plants can 

 use nitrogen in three different forms, viz. : 



(1) As nitrogen gas or uncombined nitrogen. 



(2) In the form of ammonia. 



(3) In the form of nitrates. 



All plants cannot use nitrogen in any of these three forms 

 equally well, but each form is found specially suited to certain 

 kinds of plants as will be noticed. 



(1) Nitrogen Gas Used by Plants. — Although we have ni- 

 trogen gas or uncombined nitrogen existing in the air in enormous 

 quantities, still the number and binds of plants which can use the 



