76 PERMEABLE SUBSOILS. 



entirely different from the subsoils, it is easily un- 

 derstood how the vegetable earth may be improved 

 by the addition of a certain quantity of the subsoil. 

 The impermeability of the subsoil is one grand 

 cause of the too great humidity of much-cultivated 

 land. A strong soil, very tenacious through the 

 excess of clay which it contains, has its disadvanta- 

 geous properties considerably lessened if the subsoil 

 upon which it rests is sandy; first, from the evident 

 amelioration which must result from an admixture 

 of the two layers ; and next, because it is always a 

 positive advantage to have a soil which has a strong 

 affinity for water superposed upon a subsoil which 

 is extremely permeable. The inverse situation is 

 scarcely less desirable. A light, friable soil will 

 have a greater value if it lies upon a bottom of a 

 certain consistency which is capable of retaining 

 moisture; with this condition, however, — that the 

 clayey layer shall not be too uneven in its surface, 

 that it shall not present great hollows in which 

 water shall collect and stagnate. An impermeable 

 subsoil, to act beneficially in such circumstances, 

 must have a sufficient inclination to admit of its 

 draining itself The most essential question, then, 

 to be considered with regard to the nature of sub- 

 soils, is, whether they are or are not permeable. 

 Acquainted with the nature of vegetable earth, it 

 is easy to judge of the advantages or disadvantages 

 which will be presented by subsoil having the 



