EARTHS AND SOILS. 51 



to serve as a basis for fixing the roots of plants, and main- 

 taining them in an upright position. 



The agents in vegetable nutrition, or growth, are air, 

 heat, and moisture. The seed cannot germinate and 

 grow, nor the food be prepared nor transmitted to the 

 plant, without the united co-operation of these agents. 

 Hence the utility of draining, ploughing, pulverizing, &c., 

 to render the soil permeable to solar and atmospheric in- 

 fluence. But of these matters we shall speak more fully 

 in another place. 



Subsoil. 



" The value of a soil depends much upon the nature 

 of the subsoil or under stratum. On various accounts its 

 properties merit peculiar attention. By examining the 

 subsoil, information may be obtained regarding the soil 

 itself ; for the materials of the latter, are often similar 

 to those which enter largely into the composition of the 

 former, though the substances in the soil are necessarily 

 altered, by various mixtures, in the course of cultivation. 

 The subsoil may be of use to the soil, by supplying its 

 deficiencies, and correcting its defects. The hazard and 

 expense of cultivating the surface, are often considerably 

 augmented by defects in the under stratum, but which, 

 in some cases, may be remedied. 



" Subsoils are, 1. Retentive ; or, 2, Porous. 



"1. Retentive subsoils consist of clay, or marl, or of 

 stone beds of various kinds. 



" A retentive, clayey, or tilly subsoil, is highly injuri- 

 ous. The land is soaked with water, is ploughed with 

 difficulty, and is not in a condition to exert its powers, 

 until the cold, sluggish moisture of the winter be exhaled. 

 By the water being retained in the upper soil, the putre- 

 factive process is of course interrupted, and manures are 

 prevented from operating. The plants likewise, from the 

 roots being chilled, can make but little progress. Hence, 

 when grain is cultivated, it is always of inferior quality, 

 and the herbage, when in grass, is coarse. 



" A clayey subsoil, however, may sometimes be of 

 material advantage to a sandy soil, by retaining moisture, 



