THE ELEMENTS OF SOILS. 65 



3. Nitrogen. This is another gas that forms a part 

 of all plants and animals. It is colorless, tasteless, 

 and without smell. It is not a poison alone, and yet 

 animals will instantly die in it. No fire can burn in 

 it; neither will it burn, like hydrogen, when mixed 

 with oxygen. In the soil it appears as valuable com- 

 pounds of nitrogen and other elements that form the 

 principal foods of plants. It forms the larger part of 

 the atmosphere ; and if we wish our plants to be well 

 fed, and to grow large and luxuriant, we must have 

 nitrogen in the soil. To do this we must freely admit 

 the air to the soil, that, by enabling the nitrogen to 

 mingle with decaying vegetable-matter, it may make 

 new compounds on which plants may feed. 



4. Carbon. This element forms a part of every 

 plant. If we burn a plant, we shall find carbon in the 

 ashes left behind. It is very common. Coke, char- 

 coal, coal, black-lead, lampblack, and sugar are largely 

 composed of carbon. The diamond is pure crystallized 

 carbon. When carbon burns, it unites with oxygen to 

 form a heavy, suffocating gas, in which no animal can 

 live. At the same time, it is of the greatest value in all 

 soils, and forms a food for all plants. 



5. Silicon. This element, combined with others, 

 forms common sand, and makes a quarter-part of all 

 the solid crust of our world. As sand it is of the 

 utmost value in making the soil a fit home for plants. 



6. Sulphur. This element we often meet in sul- 

 phur matches. It combines with other elements to 

 make some of the most important plant- foods in our 



