28 THE BOOK OF CORN 



and in the case of legumes, nitrogen. From the soil 

 the plant takes the following essential elements: 

 phosphorus, nitrogen, potassium, calcium, magnesium, 

 iron, sulphur and chlorine. Of these eight elements, 

 calcium, magnesium, iron, sulphur and chlorine are 

 used by plants in very small amounts and all ordinary 

 soils contain them in such quantities that they are 

 practically never exhausted. The elements contained 

 in the organic portion of the plant are carbon, 

 hydrogen, oxygen and nitrogen. These elements occur 

 in approximate proportions as follows : carbon 45 per 

 cent; oxygen 49 per cent; hydrogen 6 per cent. In 

 addition nitrogen may be present in amounts varying 

 from 0.5 to i.o per cent. The inorganic elements are 

 phosphorus, potassium, calcium, magnesium, sulphur, 

 iron and chlorine. 



NATURE OF THE ELEMENTS USEFUL TO PLANTS 



Carbon — Carbon is found in the earth in the form 

 of a solid, as coal and diamonds. The compound of 

 carbon upon which vegetation depends is carbonic 

 acid. It is this acid in the air that feeds the plant. 

 This compound is 1.53 times as heavy as air, and 

 usually comprises about one-twenty-five-hundredth 

 part of the air. No green plant can grow without the 

 presence of carbonic acid. Although the carbonic acid 

 of the air comprises but a small part of the whole, 

 plants are able to secure enough to meet all of their, 

 needs. The supply of carbonic acid in the air is con- 

 stantly replenished by the oxidation of organic matter, 

 the combustion of wood or coal or peat, and by the 

 respiration of all kinds of animals. 



Comparatively little is known regarding the exact 

 way in which the decomposition of carbonic acid is 

 brought about in the plant. Experiments have shown 



