SOILS AND SUB-SOILS. 27 



so abundant but that the texture* of the soil is mechanically suited 

 to some one valuable crop; as some plants require a degree of 

 closeness, and others of openness in the soil, which would cause 

 other plants to decline or perish. 



The depth of soil seldom extends more than a few inches below 

 the surface, as on the surface only are received those natural sup- 

 plies of vegetable and animal matters, which are necessary to con- 

 stitute soil. Valleys subject to inundation have washings of soils 

 brought from higher lands and deposited by the water, and there- 

 fore are of much greater depth. 



Below the soil is the suit-soil, of uncertain depth, and which need 

 not be considered as extending deeper than its texture or condition 

 may affect the production of the soil above, whether beneficially or 

 injuriously. It is, however, most common that the sub-soil is ap- 

 parently nearly of the same constitution with the subjacent mass for 

 several or many feet deeper. The sub-soil is usually a mixture of 

 two or more earths, and the same as may predominate in the soil 

 above. But the sub-soil is much more deficient in calcareous earth 

 (except under chalky soils), and lime in every state, and also in 

 humus } and, indeed, nearly all sub-soils in lower Virginia are 

 totally deficient in all those ingredients essential to vegetable pro- 

 duction. Even where such absolute deficiency may not exist, the 

 usual great excess of either sand or clay in sub-soils would alone 

 serve to render them nearly barren ; and, consequently, their mix- 

 ture with the better soil lying above would be injurious rather than 

 beneficial to its improvement. 



The qualities and value of soils depend on the proportions of 

 their ingredients. We can easily comprehend in what manner 

 silicious and aluminous earths, by their mixture, serve to cure the 

 defects of each other ; the open, loose, thirsty, and hot nature of 

 sand being corrected by, and correcting in turn, the close, adhesive, 

 and water-holding qualities of aluminous earth. This curative 

 operation is merely mechanical ; and in that manner it seems likely 

 that calcareous earth, when in large proportions, and serving as a 

 mechanical constituent, also acts, and aids the corrective powers of 

 both the other earths. This, however, is only supposition, as I 

 have met with scarcely any such natural soil. 



But besides the mechanical effects of calcareous earth (which 

 are weaker than those of the other two), that earth has chemical 

 powers far more effectual in altering the texture of soils, and for 

 which a comparatively small quantity is amply sufficient. The 

 chemical action of calcareous earth, as an ingredient of soils, will 

 be fully treated of hereafter ; it is only mentioned in this place to 



* The texture of a soil means the disposition of its parts, which produces 

 Buch sensible qualities as being close, adhesive, open, friable, &c. 



