230 DEPTH OP SOIL. 



best conditions to successful agriculture. If we nave depth of soil, 

 and the routs of our plants do not penetrate sufficiently to derive 

 benefit froui the fertility that lies below, we can always, by working 

 a little deeper, bring up the inferior layers to the surface, and so 

 make them concur in fertilizing the soil. And, independently of 

 this great advantage, a deep soil suffers less either from excess or 

 deficiency of moisture ; the rain that falls has more to moisten, and 

 is therefore absorbed in greater quantity than by thin soils,, and, 

 once imbibed, it remains in store against drought. 



The layer upon which the vegetable earth rests, is the subsoil, 

 which it is of importance to examine, inasmuch as the qualities, and, 

 consequently, the value of an arable soil, have always a certain rela- 

 tion with the nature and properties of this subjacent stratum. Fre- 

 quently, and especially in hilly countries, the mineral constitution of 

 the subsoil is the same as that of the soil, and any difference that 

 the former may present is owing especially to the presence of humus, 

 and to the looser condition which results from the growth of vege- 

 tables, from ploughing, &c., and not from atmospheric influences. 

 By deep ploughing done cautiously, the thickness of the layer of 

 arable land may be increased at the expense of the subsoil, and, 

 when plenty of manure can be commanded, the operation will go on 

 with considerable rapidity. Still it is maintained, and indeed in 

 many cases it is unquestionable, that the soil loses temporarily some 

 portion of its fertility by the introduction of a certain quantity of the 

 subsoil, and that, under ordinary circumstances, several years elaps» 

 before any amelioration becomes perceptible. 



In plains, in high table-lands, the analogy, in point of constitution, 

 between the soil and subsoil is not so constant. In such situations 

 the arable land is frequently an alluvial deposite proceeding from the 

 d_es*ruction or disintegration of rocks situated at a great distance. 

 When the superior strata possess properties that are entirely differ- 

 ent from the subsoils, it may be understood how the vegetable earth 

 may be improved by the addition of a certain dose of the subsoil, and 

 this is the case in which the amelioration is the least expensive. 

 The impermeability of the subsoil is one grand cause of the too great 

 humidity of a cultivated soil. A strong soil, very tenacious through 

 the excess of clay which it contains, has its disadvantageous proper- 

 ties considerably lessened, if the subsoil upon which it rests is sandy, 

 1st, from the evident amelioration which must result from an ad- 

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

 advantage in having a soil which has a strong affinity for water 

 superposed upon a subsoil which is extremely permeable. The in- 

 verse situation is scarcely less desirable ; a light friable soil will have 

 greater value if it lies upon a bottom of a certain consistency, and 

 capable of retaining moisture ; wil i 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 may collect and stagnate ; an 

 impermeable subsoil, to act beneficially in such circumstances, must 

 have a suflScient inclination to admit of its draining itself. The most 

 essential distinction, then, in regard to the nature of subsoils is, int» 



