2 ORIGIN AND KINDS OF SOILS, CONSIDERED 



chanical 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 

 substratum 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 d6bris of the various rocks of that country, 

 and from every country containing nearly the same kinds of rocks, the 

 alluvial deposits on the banks of all the large rivers of the world con- 

 sist nearly of the same earths. But as the rocks or geological forma- 

 tions 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 uni- 

 formity 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 substrata or bases of soils, and each of which must 

 consequently be more or less different in its composition.* For all 

 practical purposes, however, soils may be characterized by their prevail- 

 ing primitive earths ; and, hence, they are reduced to sands and 

 gravels, clays, chalky and limestone soils, alluvial soils, and peatbogs. 



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 per- 

 fectly 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 prac- 

 tical test of this soil, when tolerably pure, is, that when moistened, it 

 cannot be formed into a plastic mass, or consolidated by pressure, 

 whether in a moist or dry state, so as to form a compact solid body. 

 Hence all sandy soils are loose, never present a firm surface, and are 

 seldom covered with a compact clothing of grass or other herbaceous 

 plants. Such soils, from being without cohesion, are incapable of re- 

 taining moisture, and, as they are readily permeable by both moisture 

 and air, they powerfully promote the putrefaction of organic matter, 

 whilst they as readily permit it to be washed away from them by 

 rains, or to escape in the form of gas. Hence, in manuring sandy soils, 

 no more should be applied at once than can be consumed by the crop 



* See Morton 'On Soils,' 4th edit. 8vo, 1843. 



