PROPERTIES OF MIXED EARTHS. 39 



their mixture with alumina preserves plants from suffering 

 so often from drought ; without the presence of alumina, 

 they would be alternately inundated, and dried up. Clay 

 alone does not permit the roots of plants to extend them- 

 selves, nor allow the air to penetrate to them ; but mixed 

 with silica, carbonate of lime, and sand, it forms a porous 

 soil, which possesses these properties. Chalk preserves 

 animal and vegetable substances from a too rapid decom- 

 position. Alumina and the oils combined together pro- 

 duce a saponaceous mixture, which can be imbibed by 

 plants, and thus furnishes them with two principles, which 

 separately are insoluble in water. 



The composition of soils varies according to the differ- 

 ence in climate, otherwise their fertility would be les- 

 sened. The quantity of rain that falls is so various, that 

 even within the extent of France, it ranges, according to 

 situation, from twenty to thirty, and according to Giobert, 

 at Turin, to thirty-four inches. There are some countries 

 where the atmosphere is almost always cloudy, and the air 

 laden with moisture ; whilst there are others in which the 

 sun is not obscured for six months together. It is evident 

 that in those countries where the air is uniformly damp, 

 and in those where rain is abundant, the soil may be, with- 

 out inconvenience, more calcareous than argillaceous ; and 

 that the best soils in the two divisions would differ very 

 widely as to the proportions in which their several earths 

 would be combined. 



Soils should vary according to the nature of the plants 

 to be cultivated in them ; some prefer a porous, dry, and 

 arid soil, others flourish only in land constantly moist ; 

 there are some that require a great degree of heat, others 

 vegetate in the midst of snows. These peculiar tastes of 

 plants ought to be known to the agriculturist, that he may 

 select for each one the soil best adapted to it ; or change 

 the characters of those he possesses, so as to afford to each 

 plant the soil most congenial to it. 



In order that a plant should flourish in a soil, it is not 

 always sufficient that the earths composing it are of the 

 right kind, or suitably proportioned; it is necessary to 

 unite other circumstances which are not always to be met 

 with ; for example, the arable soils which are based upon 

 rocks, vary considerably in depth ; and the thickness of 

 the bed not only exerts an influence upon the powers of 

 ye^getation, but determines the kind of plant which can be 



