VIII. CHEMICAL COMPOSITION OF THE SOIL 



THE substances that enter into the composition of the soil may 

 be roughly divided into two general classes: organic and inorganic. 

 The organic elements are derived chiefly from the decay of plants 

 and animals and comprise less than 10 per cent of the soil. The 

 inorganic elements are derived from the decay of rocks which form 

 the surface. 



The Metallic Elements. More than 90 per cent of the soil is 

 mineral matter composed of metals and non-metals. The metals 

 which are of the most importance to agriculture are: 



Aluminum Manganese 



Calcium Potassium 



Iron Sodium 

 Magnesium 



Aluminum is a beautiful white metal which is very abundant in 

 the earth's surface. It is one of the chief constituents of clay, which 

 plays an important part in the water-holding power of soils. 



Calcium is a yellowish colored metal that enters into the compo- 

 sition of limestone and gypsum. In limestone it occurs in the form 

 of calcium carbonate and in gypsum it occurs as calcium sulphate. 



Magnesium is a hard white metal and is an indispensable plant 

 food. Both magnesium and calcium collect chiefly in the seeds of 

 plants. 



Potassium is a soft white metal which is widely distributed in 

 the earth's crust. It is one of the essential elements of plant food. 

 Its compounds are very soluble and hence are rapidly leached 

 out and carried away in drainage waters. 



Sodium is a soft white metal so light that it readily floats on 

 water. It is widely distributed, and in the form of sodium nitrate 

 it is largely used as a fertilizer. Vast beds of this nitrate are found 

 in Chile. Sodium is also the basis of common salt, which is known 

 to chemists as sodium chloride. 



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