PROPERTIES OP MIXED EARTHS. 41 



the north, but vegetation is more active in the first than in 

 the last, and the quality of their productions superior. 



The slope of lands likewise affects their fertility; a 

 piece of ground which lies upon a declivity, loses water 

 more readily than one which is horizontal, and vegetation 

 is less strong upon it, but the productions are of a better 

 quality. There is a vast difference between wines made 

 from the same kind of grape, raised in the same soil, if 

 one be the production of the harvest upon the declivity of 

 a hill, and the other of the plain at its foot. 



Inclined lands, where the slope is rapid, and the soil 

 light and porous, are liable to the evil of having the ma- 

 nures, applied to them, carried off by heavy rains; even the 

 soil sometimes experiences the same fate, and the surface 

 becomes furrowed^ by ravines laying bare the rocky foun- 

 dation. This frequently happens to lands cultivated upon 

 the sides of mountains, till they become at length com- 

 pletely barren; and hence we must conclude that it is 

 unwise to clear up the declivities of mountains, since a 

 temporary advantage reduces the land to a Jong period of 

 sterility. 



Soils composed of the same earthy principles, combined 

 in the same proportions, will still present very different 

 results, according to the nature and quantity of the salts 

 which they contain. I have made known those which are 

 usually found in plants, and which for this reason must be 

 regarded as suited to vegetation ; but their proportions are 

 limited, and if they are too abundant, they become hurt- 

 ful. The salts cannot be regarded as the actual food of 

 plants; they are only auxiliaries, though very powerful 

 ones, to their nutrition. The organs of vegetables require 

 exciting ; and heat and the salts act upon them as stimu- 

 lants. The salts are to plants what spices and marine 

 salts are to man. Independently of their stimulating 

 powers, the salts exert a chymical action upon the aliments 

 of plants, by combining with them, rendering some of them 

 soluble in water, and moderating the decomposition of 

 others; and thus contributing to regulate and facilitate 

 the process of nutrition. 



Even from the part which the salts perform, it is evident 

 that they ought to be supplied only in suitable proportions ; 

 if they are too abundant, or very soluble in water, they 

 will be absorbed by the organs of the plants in such a 

 quantity as to produce irritation and dryness. Thus th« 



