58 ELEMENTS OF AGRICULTURE 



they are said to be unavailable. For instance, nitrogen, 

 which makes up four-fifths of the air, is a very im- 

 portant plant food, but plants cannot take in free nitro- 

 gen gas; it must first be combined with some other ele- 

 ments, usually hydrogen and oxygen, before it can be- 

 come plant food. Phosphorus is another valuable plant 

 food, and when combined with certain quantities of 

 calcium and oxygen, is readily taken up by plants, but 

 alone or combined with larger quantities of calcium 

 and oxygen it is not fit for plant food. 



Nitrogen, phosphorus, and potash are three im- 

 portant foods that the plant takes up from the soil. A 

 proper supply of these foods is often lacking in soils, so 

 they are added in the form of fertilizers. Iron is an- 

 other important plant food. It is necessary for the for- 

 mation of chlorophyll, the green coloring matter of 

 plants, which utilizes the carbon dioxide of the air in 

 the formation of starch. As all soils contain a great 

 abundance of iron it is not necessary to add it as a fer- 

 tilizer. Calcium is still another important plant food. 

 Soils usually contain an abundant supply of this ele- 

 ment, but it is sometimes found necessary to add it to 

 the soil in the form of lime. These foods — nitrogen, 

 phosphoric acid, potash, lime, and iron — are necessary 

 to the growth of plants, and no crop can be grown with- 

 out a proper supply of them. Nitrogen, phosphoric 

 acid, potash, and lime are the four most often lacking 

 in soils, and to make up any deficiency they are added 

 in the form of fertilizers. 



