Chapter VII. 



THE MINERAL CONSTITUENTS OF THE SOU SOLUTION. 1 



The mineral constituents of the soil are products of the dis- 

 integration, degradation and decomposition of rocks. The de- 

 composition products are mainly silica in the form of quartz, 

 ferruginous material consisting of more or less hydrated ferric 

 oxide and alumina, and hydrated aluminum silicate. The 

 ferruginous material, being deposited or formed in the soil in a 

 very finely divided condition, frequently coats the soil frag- 

 ments to such an extent as completely to mask their true char- 

 acter. But if a soil be thoroughly shaken with water, and 

 especially in the presence of some deflocculating agent such as a 

 slight excess of ammonia, as in the ordinary preparation of a 

 soil sample for mechanical analysis 2 the coating material is 

 generally removed quite readily, and the mineral particles appear 

 as fragments and splinters of the ordinary rock- forming 

 minerals. Sometimes these fragments are more or less worn 

 and rounded at the edges, showing mechanical abrasion or 

 solvent action; sometimes they show evidences of partial alter- 

 ation and decomposition; but surfaces of the unaltered mineral 

 individuals always are found. These unaltered minerals occur 

 as fragments of all sizes, and are to be found in the sands, silts, 

 and presumably in the clays. As might be anticipated, the 

 minerals other than quartz generally show a tendency to segre- 

 gate in the finer mechanical separations of the soil. The presence 

 of these unaltered mineral fragments in the clays has so far 

 defied direct experimental proof because of the limitations of 

 the microscope, but from chemical reasoning and a priori con- 



1 For a more detailed discussion and citations of the literature, see 

 The mineral constituents of the soil solution, by Frank K. Cameron 

 and James M. Bell, Bull. No. 30, Bureau of Soils, U. S. Dept. Agricul- 

 ture, 1905. 



1 Centrifugal methods of mechanical soil analysis, by L,. J. Briggs, 

 F. O. Martin and J. R. Pearce, Bull. No. 24, Bureau of Soils, U. S. Dept. 

 Agriculture, 1904. 



