82 FOOD OF THE VEGETATING PLANT. CHAP. II. 



measure upon its capacity for retaining water : and 

 if so, soils containing the same ingredients must be 

 also equally fertile, all other circumstances being 

 the same ; though it is plain that their actual fertility 

 will depend ultimately upon the quantity of rain 

 that falls, because the quantity suited to a wet soil 

 cannot be the same that is suited to a dry soil. And 

 hence it often happens that the ingredients of the 

 soil do not correspond to the character of the cli- 

 mate. Silica exists in the soil under the modification 

 of sand, and alumine under the modification of clay. 

 But the one or the other is often to be met with in 

 excess or defect. Soils in which the sand prepon- 

 derates retain the least moisture ; arid soils in which 

 the clay preponderates retain the most : the former 

 are dry soils ; the latter are wet soils. But it may 

 happen that neither of them is sufficiently favour- 

 able to culture ; in which case their peculiar defect 

 or excess must be supplied or retrenched before they 

 can be brought to a state of fertility. 



Poororex- But soils in a state of culture, though consisting 

 Boils'ame- originally of the due proportion of ingredients, may 

 horated, yet b ecome exhausted of the principle of fertility by 

 means of 'too frequent cropping, whether by repeti- 

 tion or rotation of the same, or of different crops. 

 And in this case, it should be the object of the 

 phytologist, as well as of the practical cultivator, to 

 ascertain by what means fertility is to be restored to 

 an exhausted soil ; or communicated to a new one. 

 By drain- In the breaking up of new soils, if the ground has 



ing, par- 



