152 LECTURES ON 



the silicates of alumina and potash, lime, &c, yield carbonates of 

 potash, lime, &c, which dissolve and wash away, while a silicate of 

 alumina and water, mingled with free silica, and mechanically retain- 

 ing more or less of the other substances remains, and this is clay. 



"When formed from feldspar alone it is often pure white in color, 

 and bears the name kaolin. This, the purest form of clay, is the 

 material which constitutes the basis of porcelain. In mines, excavated 

 through feldspathic rocks, nothing is more common than to find masses 

 of the whitest kaolin in the fissures or cavities, which give a down- 

 ward passage to the percolating water. The clay of ordinary soils, 

 is, however, a material greatly admixed with other substances, and 

 therefore exceedingly different and variable in its composition, and 

 all the better adapted by this for its agricultural applications. 



Many soils contain much carbonate of lime in an impalpable form, 

 having been derived chiefly from the mechanical wearing down of 

 lime rocks, as marble and chalk — from the shells of mollusks or coral 

 branches, or, finally, being clays that have originated by the chemi- 

 cal decomposition of feldspathic rocks containing much lime. 



Organic matter, especially the debris of former vegetation, is 

 almost never absent from the impalpable portion of the soil, existing 

 there in some of the many forms assumed by the Protean humus. 



From consideration of the relative proportions of the principal me- 

 chanical ingredients has chiefly arisen the customary classification and 

 nomenclature of soils. Silicious sand (grains of quartz, feldspar, &c.) 

 and clay make up the chief proportion of many soils. The mixture 

 of the two forms a loam which may be sandy (light) or clayey (heavy.) 

 A further division is into loamy sand and loamy clay. When, in ad- 

 dition to these, lime is present the soil is said to be a marl, either 

 sandy, clayey, loamy, &c, according to the relative quantities of the 

 ingredients. 



Soils containing organic matter to the amount of 5 to 10 per cent, 

 are termed vegetable moulds; if this ingredient exceeds 10 percent., 

 which rarely occurs, except in wet situations, we have & peaty soil. 



If coarse rounded fragments of rock be present in large quantity 

 the soil is gravelly. Where much oxyd of iron exists, as evinced by 

 a red or yellow color, the soil is ochery. The epithets peaty, gravelly, 

 ochery, come then, in man}- cases, to further modify the designations 

 of sands, clays, marls, and moulds. 



Other divisions are current among practical men, as, for example, 

 surface and sub-soil, active and inert soil, tilth, and hard pan. These 

 terms mostly explain themselves. When, at the depth of four inches 

 to one foot or more, the soil assumes a different color and texture, 

 these distinctions have meaning. The surface soil, active soil, or 

 tilth, is the portion that is wrought by the instruments of tillage — 

 that which is moistened by the rains, warmed by the sun, permeated 

 by the atmosphere, in which the plant extends its roots, gathers its 

 soil food, and which, by the decay of the subterranean organs of 

 vegetation, acquires a content of humus. Where the soil originally 

 had the same characters to a great depth, it often becomes modified 

 down to a certain point, by the agencies just enumerated, in such a 



