10 SOILS: PROPERTIES AND MANAGEMENT 



There are certain of these minerals that merit especial 

 attention because of particular attributes which they may 

 impart to a soil. Quartz, for example, is very common 

 in all soils, making up usually from 85 to 99 per cent of 

 their composition. It is a makeweight material, how- 

 ever, as it is used to a very slight extent by most plants ; 

 but it adds a stability to the soil that perhaps the soil 

 would not otherwise have, and this function is of con- 

 siderable significance. Of greater importance from the 

 plant-food standpoint are the feldspars, of which orthoclase 

 is probably primary because it is the source and store- 

 house of the soil potash. Acted upon by physical and 

 chemical agencies, it slowly supplies the soil solution with 

 potassium, which in turn nourishes the plant. The micas 

 also may furnish considerable potash for crop growth. 

 The plagioclase, instead of being rich in potassium, as 

 the formula indicates, contain the more basic elements, 

 calcium and magnesium, as also do the pyroxenes and 

 amphiboles represented by augite and hornblende. Olivine 

 and serpentine, also silicates, are particularly rich in 

 magnesium. Practically all the phosphorus in the soil, 

 either organic or inorganic, has had its origin in the 

 mineral apatite ; yet this mineral is present in rocks and 

 soil usually in very small quantities, making up not more 

 than 0.6 per cent of the bulk of igneous rocks. More- 

 over it is a rather insoluble material. This fact, together 

 with the small quantities occurring in soil-forming rocks, 

 may account for the need of phosphorus in many otherwise 

 fertile soils. 



Calcium, so important as a basic material in soil, may 

 be supplied to a certain extent by other minerals besides 

 those already named — calcite, dolomite, and gypsum 

 being perhaps the most important, especially the calcium 



