6 NATURE AND PROPERTIES OF SOILS 



4. Importance of soil minerals. — Quartz is found in al- 

 most all soils, making up often from 80 to 90 per cent, of 

 the composition, although a range from 40 to 70 per cent, 

 is more common. Its universal presence is due to its hard- 

 ness and insolubility. Quartz is a make-weight material, 

 however, as it probably contributes but little to plant nutri- 

 tion. In the form of sand, quartz has a great influence on 

 the friability of soil, improving and maintaining the phys- 

 ical condition to a marked degree. 



Orthoclase, microcline, muscovite and, to a lesser degree, 

 biotite are important because of their potash content. 1 They 

 decompose, often rather readily, into kaolinite and similar 

 products, thus liberating potassium in soluble form. The 

 plagioclase feldspars also give rise to kaolinite. They carry, 

 however, sodium and calcium. The latter element 2 plays an 

 important role in soil both as a nutrient and as an amend- 

 ment. When not sufficiently active it must be applied in 

 some form. Calcite and dolomite also carry calcium. Horn- 

 blende and augite bear calcium as well as magnesium and 

 iron. Olivine is a magnesium and iron silicate. The oxida- 

 tion of the iron of the above minerals gives rise to hematite, 

 so common as a red coloring matter of soil. 



Practically all of the phosphorus of the soil, either organic 

 or inorganic, has its origin in apatite, yet this mineral occurs 

 but sparingly either in rock or soil. It makes up but 6 per 

 cent, of igneous rocks. This accounts for the small percent- 

 age of phosphoric acid in most soils and explains why it is 

 often added in fertilizers. 3 



1 Plummer, J. K., Availability of Potash in Some Common Soil- 

 forming Minerals, Jour. Agr. Bes., Vol. XIV, No. 8, pp. 297-315. 

 Aug., 1918. de Turk, E., Potassium-bearing Minerals as a Source of 

 Potassium for Plant Growth; Soil Sci., Vol. 8, No. 4, pp. 269-301. 1919. 



a Shorey, E. C. et al., Calcium Compounds in Soils; Jour. Agr. Ees., 

 Vol. VII, No. 3, pp. 57-77. Jan., 1917. 



8 Fry, W. H., Condition of Phosphoric Acid Insoluble in Hydro- 

 chloric Acid; Jour. Ind. and Eng. Chem., Vol. V, No. 8, pp. 664- 

 665. 1913. 



