SOME GENERAL CONSIDERATIONS 11 



carbonate in either the crystalline or the amor- 

 phous forms. Plenty of calcium in a soil tends not only 

 to better physical conditions, but also to improve chemical 

 reactions and biological activity. The iron of the soil 

 minerals is of importance in its color relationships, for 

 when oxidized to the hematite form a bright red may be 

 imparted, while a yellow may result if limonite is pro- 

 duced. Color has great significance in a general estimate 

 of soil productivity and is always an important factor in 

 soil identification and survey. The tendency of most 

 iron compounds in the soil is toward the hematite or the 

 limonite form when subjected to oxidation and hydration. 



Kaolinite is a product of rock decomposition and is 

 considered to be of considerable importance in most clays 

 or clay loams. It is almost always impure and in this 

 form is designated as kaolin. Kaolin and the soil zeoHtes, 

 which are hydrated aluminium silicates carrying chiefly 

 calcium, sodium, and potassium, are really the end prod- 

 ucts of rock decay and therefore are secondary minerals. 

 Consequently they must always be considered in any 

 study of soil formation or of soil utilization, particularly 

 as they may serve to enrich the soil solution in plant-food 

 held by them in physical and chemical combinations. 



7. Relative abundance of minerals. ■— D'Orbigny ^ pre- 

 sents the following table as a result of his calculation on 

 the distribution of certain minerals in the earth's crust : — 



Percentage Percentage 



Feldspars 48 Carbonates .... 1 



Quartz 35 Hornblende, augite, etc. 1 



Mica 8 All other minerals . . 2 



Talc 5 



1 HaU, A. D. The Soil, p. 16. New York City. 1907. 



