26 CHYMISTRY APPLIED TO AGRICULTURE. 



seventy-five miles from its source, and found it composed 

 of 



Siliceous gravel 32 



Calcareous gravel 11 



Silica 10 



Carbonate of lime . . • . . 19 



Alumina 21 



Vegetable remains 7 



The analysis of a soil in Touraine, which produced 

 excellent hemp, gave me of 



Coarse gravel 49 



Carbonate of lime .... 25 



Silica 16 



Alumina 10 



From the results of these analyses we find, that in the 

 best earths there is a large proportion of gravel, which 

 renders the soil light and easily worked, and facilitates 

 the passing off" of superabundant rains. In consulting the 

 analysis of less fertile soils, we find that their fertility 

 diminishes in proportion as one or the other of the three 

 principal earths predominates ; and becomes almost noth- 

 ing in those which possess the properties of but one. 

 The mixture of earths then is necessary to the formation 

 of a productive soil ; and their proportion can be varied 

 only according to the nature of the climate, and the 

 kind of plant to be cultivated. Siliceous and calcareous 

 earths may form a larger proportion of the soil in moist, 

 than in dry countries, and alumina may, in its turn, pre- 

 dominate in those lands, which, from their declivity, 

 suffer the water to flow off freely ; but a mixture of the 

 three earths can alone form a good soil, and too great a 

 disparity in their proportions materially affects the char- 

 acter of it. 



The constituent parts of a soil have a constant tenden- 

 cy to become pulverized, and at length, by frequent tilling, 

 by the action of salts, manures, and frosts, they are reduced 

 to so fine a powder, as to cease to be productive. Rain 

 falling upon ground in this state renders it perfect mud, 

 which when exposed to heat becomes so hard, that the air 

 cannot penetrate it, nor the tender fibres of plants force 

 their way through it. 



Davy has observed, that all soils composed of ^ of im- 

 palpable matter are completely barren. The use of barn- 



