CHAPTER III 



PLANT FOOD AND PLANT GROWTH 



Oxygen. Oxygen is the most abundant element. It constitutes 

 about one half the sum of all known matter. It forms chemical 

 compounds with nearly all other elements, 1 and is, in consequence, 

 termed a chemically active element. In the free state (O 2 ), it is a 

 gas. In this form it constitutes about 23 per cent of the air. It 

 is thus present everywhere and ready to form compounds with other 

 elements, or to attack other compounds under favorable conditions. 

 The compounds formed with oxygen may at ordinary temperatures 

 be gases, such as carbon dioxid (CO 2 ), liquids, such as water (H 2 O), 

 solids, such as iron oxid (Fe 2 O 3 ). 



All ordinary combustion consists of chemical reaction with 

 oxygen, and the principal products formed are carbon dioxid and 

 water. 



Water is eight ninths oxygen, and carbon dioxid is eight 

 elevenths oxygen, as any one can determine for himself if he knows 

 the atomic weights given in Table 2. The grain of corn is nearly 

 one half oxygen (46 per cent). 



Carbon. This is a very common element, but not very abundant 

 as compared with nine other elements. Even titanium, a tetrava- 

 lent element belonging to the same periodic group as carbon and 

 silicon, is one half more abundant than carbon in the earth's crust. 

 But titanium has no agricultural value, while carbon is one of 

 the most important elements in the structure of plants and ani- 

 mals. About 45 per cent of the corn kernel is carbon. 



Carbon in the free state is the principal element in coal and char- 

 coal. Soft coal (bituminous) contains about 90 per cent of carbon, 

 and hard coal (anthracite) contains about 97 per cent of carbon. 



Graphite, the " lead " used in lead pencils, is not lead, but car- 



1 No oxygen compounds are known with fluorin, argon, or helium. 



26 



