SOIL PHYSICS AND MANAGEMENT 



A verage Composition of the Known Earth l 



* The elements essential for crops are in bold type, 

 t The solid part of the earth's crust, 

 j The liquid part, oceans, seas, etc. 



The hardness of minerals is indicated by the following scale: 

 (1) talc finger nail scratches it easily; (2) gypsum thumb nail 

 scratches slightly; (3) calcite can be scratched by common soft 

 pin; (4) fluorite soft iron scratches it; (5) apatite scratched 

 by a good knife; (6) feldspar very hard knife scratches it; (7) 

 quartz scratches glass; (8) topaz scratches quartz; (9) corun- 

 dum sera tx-hes topaz ; (10) diamond. 



The number of minerals that form soils is not large, but they 

 may be found in many intermediate stages because the process of 

 decomposition is a gradual one. 



1. Quartz, silica (Si(X) is a very abundant mineral in rocks 

 and the most abundant in soils. When crystalline it possesses a 

 glassy appearance and is transparent in thin slices, but impurities 

 render it more or less opaque. The common crystalline varieties 

 are quartz crystal, rose, smoky and milky quartz. The non-crys- 

 talline varieties are usually opaque and include flints, cherts, 

 chalcedony and the different forms of agate. In rocks such as 



