SOILS AND FERTILIZERS. 395 



Potassium and sodium enter into the composition of many rocks, 

 and as these become eroded by weather they are scattered through 

 the soil, whence their salts are extracted by rootlets and enter into 

 the formation of vegetable tissue. 



Behind these stands iron. The green coloring matter of plants 

 is a very complex substance known as chlorophyll, the duty of which 

 is to take carbonic oxide from the air, utilize the carbon, and restore 

 the oxygen. Iron enters into the composition of chlorophyll, and 

 to it is due the brown color of dead leaves. This metal is well-nigh 

 universal, all the reds and browns in soils and rocks being made 

 by it, and so it is rarely lacking anywhere. 



So much for the metals in soils; but, important as they are, plants 

 can not live on them alone. Among the nonmetallic bodies phos- 

 phorus stands high among essentials, and for it we are indebted to 

 the sea and the interior of the earth. Many living creatures extract 

 phosphorus from the sea water — combine it chiefly with lime, and 

 use the phosphate for making skeletons or shells, as the case may be. 

 After the death of the possessors the bones or shells sink to the bot- 

 tom, as do the Globigerina, and in time are either lifted up, as were 

 the limestones, and form " phosphate beds " like those of Georgia and 

 Florida, or are dredged up and ground into powder with bones of 

 land animals. 



Much of the matter forced up from the interior of the earth con- 

 tains phosphorus; indeed, it is the bane of Southern iron ores; but 

 though iron masters dread it, farmers welcome it, as the rains and 

 frosts crumble the phosphatic rocks and add them to the mass of 

 debris that forms our soil. 



Now let us take a test tube and put into it lime, potash, soda, 

 iron, silicon, or sand, and phosphorus, add to it a grain of corn, and 

 watch results. Under suitable conditions of warmth and moisture 

 the grain will sprout, but when the store of food laid up in it is 

 exhausted our little plant will die. It is obvious that something 

 else is needed for a soil, and analysis shows that it is nitrogen, the gas 

 that forms nearly four fifths of our atmosphere — a gas useless, as 

 such, to animals, but essential to plants. Nitrogen is abundant in 

 Nature. Besides being nearly four fifths of the air,. it forms twenty- 

 two per cent of nitric acid, forty-five per cent of saltpeter or niter, 

 eighty-two per cent of ammonia, and about twenty-five per cent of 

 sal ammoniac. Plants can not use nitrogen in its pure form, but 

 one or another of these forms will be found in the soil, whence 

 it may be extracted. 



Now we have the chief articles of plant food, and it is necessary 

 to know how they are to be used. A plant usually consists of two 

 parts, one that appears above ground, bearing branches, twigs, and 



