SOIL-FORMING MINERALS 5 



composition, usually a distinct crystalline form and 

 definite physical properties. A very large number of 

 species of minerals which differ greatly from each other 

 in composition and physical properties have been recog- 

 nized. It is these differences which renders necessary 

 a study of those important species which are found in 

 the soil, in order to gain a thorough knowledge of the 

 relations which they bear to plant nutrition and the phys- 

 ical and chemical characteristics of the soil mass. By their 

 chemical and physical weakness or resistance, they 

 modify the supply of food elements and determine the 

 physical make-up of the soil, with all the attendant 

 physical conditions of heat, moisture, air, etc., which 

 this limits. 



While the number of minerals known is very great, 

 only a comparatively small number occur in the soil 

 in important amounts; but these are thoroughly repre- 

 sentative. All minerals may be divided into two groups: 

 (1) The original or primary constituents which were 

 formed at the first consolidation. (2) The secondary 

 constituents which result from changes in the minerals 

 subsequent to their first consolidation, and which are due 

 in large part to the chemical action of percolating water. 



3. Soil-forming minerals; their composition and 

 properties. The soil is composed of a great variety 

 of minerals and probably almost every recognized 

 species could be found in some soil. But the number 

 of minerals which make up the bulk of soil is rela- 

 tively small. The following table includes the most 

 important soil-forming minerals and their leading 

 properties: 



