40 CHEMICAL COMPOSITION OF THE SOIL 



Iron is one of our very common metals. United with oxygen 

 and water it forms the yellow and red ochres used in painting. 

 It also enters largely into the coloring matter of red and yellow 

 soils. Iron is an important plant food and is very abundant in 

 the soils everywhere. It forms generally from 1 to 7 per cent of 

 the soil. 



Manganese is a grayish white metal and is very heavy. Like 

 iron, it is somewhat abundant and is a valuable constituent of 

 the soil. 



Non-metallic Elements. The elements found in this class are 

 of very great importance to plant growth and form a large part of 

 its food. They may be grouped as follows: 



Oxygen Chlorine 



Silicon Phosphorus 



Carbon Nitrogen 



Sulphur Fluorine 



Hydrogen Boron 



Oxygen is a colorless substance which comprises about 23 per 

 cent by weight of the atmosphere, eight ninths of the weight 

 of water, and 50 per cent of the earth's solid crust. In gaseous 

 form it is the life-giving principle of the atmosphere, without 

 which animal life is impossible. 



Silicon comprises about one fourth of the earth's soil and is the 

 second most abundant element of the soil ingredients. It is found 

 in the husks of grain and in the tissues of nearly all plants, but 

 its presence is not indispensable. 



Carbon occurs in the soil as a part of the humus or organic matter, 

 but the plant gets the carbon of its food from the carbon dioxide 

 of the air. Gaseous carbon dioxide is the product of a great num- 

 ber of reactions which take place on the earth's surface. It is 

 generated in the combustion of carbon and organic matters, in 

 the respiration of animals, and in the processes of decay and 

 fermentation. It also issues from the soil in many volcanic 

 regions. 



In plants carbon appears as one of the elements of starch and 

 it is especially abundant in the seeds of leguminous plants, in 

 cereals, and in potatoes. 



