46 ORIGIN AND KINDS OP SOILS. 



SECT. I. Origin and Kinds of Soils. 



152. The earthy part of all soils must necessarily have been derived from 

 the debris of rocks, and the organic part from the intermixture of decayed 

 vegetable or animal matter. The earthy mass so produced varies in colour, 

 but, from containing humus and mould, (161) it is always darker in a greater 

 or less degree than subsoils, which hi general are without organic matter. 

 Soils also contain mineral salts and metallic oxides, some of which are bene- 

 ficial, others harmless, and some few injurious, to plants. The chemical 

 constitution of a soil can only be known by analysis, which cannot, in gene- 

 ral, be depended on, unless performed by professional or experienced che- 

 mists * ; the mechanical state or texture of a soil is ascertained by digging 

 up a portion of it ; and its actual fitness for plants, by examining the species 

 growing on its surface. The rock or geological formation, the earth of which 

 forms the basis of any soil, will frequently be found to constitute the substra- 

 tum on which that soil rests ; but this is frequently not the case, because the 

 earths of many soils have been held in suspension by water in a state of motion, 

 and by that means have been transported to a great distance from the rocks of 

 which they are the debris. From this suspension of the earths of soils in 

 water, and their transportation to a distance, we are able to account for the 

 circumstance of several different kinds of earths being almost always found 

 in the same soil. Thus in alluvial deposits, on the banks of rivers, we find 

 the earth of various rocks of the country through which the river has taken 

 its course ; and as such soils are always the most fertile, we may conclude 

 that a mixture of various earths in a soil is to be preferred to any one kind 

 of earth alone. From the earth of the alluvial deposits of every country 

 being formed of the debris of the various rocks of that country, and from 

 every country containing nearly the same kinds of rocks, hence the alluvial 

 deposits on the banks of all the larger rivers of the world consist nearly of 

 the same earths. But as the rocks or geological formations from which 

 the earths of soils are washed away still remain in their places, and are of 

 many different kinds, it follows that there must be as great a variety in the 

 upland soils of a country as there is uniformity in those of the lowlands, 

 and of the banks of rivers. Thus there are between twenty and thirty 

 geological formations in England, which form the substratum or bases of 

 soils, and each of which must consequently be more or less different in its 

 composition. t For all practical purposes, however, soils may be charac- 

 terised by their prevailing primitive earths; and hence, they are reduced 

 to sands and gravels, clays, chalky and limestone soils, alluvial soils, and 

 peat-bogs. 



153. Sandy Soil. Silica, which is the basis of sandy soils, is, perhaps, the 

 most universal of all earths ; and there is scarcely a species or variety of 

 rock in which it does not abound more or less. Silica is found perfectly 

 pure in rock crystal, and tolerably so in what is called silver sand, and 

 also in the sand of some rivers and of the sea. The practical test of the 

 earth, when tolerably pure, is, that when moistened it cannot be formed into 



* At the Museum of Economic Geology, attached to the board of Woods and Forests, 

 Craig's-court, Charing Cross, London, an analysis of a pound of soil, sent from any part of 

 the country, -will he made by Mr. Richard Phillips, one of the best analytical chemists in 

 Europe, for a fee of about 20s. 



t See Morton on Soils. 2d edit. 12mo. 1840. 



