THE FORMATION OF SOILS. 



323 



The oxide of iron appears also to be a requisite ingredient. The 

 proportion necessary for the formation of good soil, depends much 

 on the nature of the climate, but more on the quality of the sub-soil, 

 and its power of retaining or absorbing moisture. This alone may 

 make a soil barren, which upon a different sub-soil would be exceed- 

 ingly productive. When this is the case, drainage or irrigation offers 

 the only means of permanent improvement. 



Different vegetables also require different admixtures of earth. 

 They require it, first, because it is necessary to their growth that the 

 soil should be sufficiently stiff and deep to keep them firm in their 

 place ; and also that it should not be too stiff to permit the expan- 

 sion and growth of their roots : and, lastly, that it should supply 

 them with a constant quantity of water, neither too abundant nor 

 deficient. Hence we may learn why different degrees of tenacity, 

 depth, and power of retaining or absorbing moisture, are required in 

 soils for different kinds of plants. Thus, in uncultivated countries, 

 we find that certain vegetables affect particular situations in which 

 they flourish spontaneously and exclusively ; and it is only by imita- 

 ting nature, and profiting by the instruction she affords, that we can 

 hope to obtain advantageous results, or acquire certain fixed princi- 

 ples, to guide us in our attempts to bring barren lands into a state of 

 profitable cuUivation. When rocks contain, in their composition, a 

 due proportion of silex, clay, and lime, they furnish soils whose fer- 

 tility may be said to be permanent. The most fertile districts in 

 England were made so by nature ; their original fertility was inde- 

 pendent of human operation. 



Some small portion of the earths and alkalies is found by chem- 

 ical analysis in plants : but it would be contrary to fact and analogy, 

 to suppose that the earths, in a concrete state, form any part of the 

 food of plants ; the earths and alkalies which they contain, are in all 

 probability formed by the process of vegetation, from more simple 

 elements ; for it is now ascertained, that the earths and alkalies are 

 compound substances. 



The principal elements found in plants are hydrogen, carbon, and 

 oxygen ; and by experiments of Gay-Lussac and Thenard^* it ap- 

 pears, that the hydrogen and oxygen in starch, gum, vegetable oils, 

 and sugar, exist in precisely the same proportions that form water. 

 Carbon, the other principal elementary substance found in plants, 

 exists both in water and in the atmosphere. Water and the atmos- 

 phere contain in themselves, or in solution, all the elements necessary 

 for the support and growth of vegetables. But, most soils are either 

 too wet or too dry, too loose or too adhesive, to admit plants to ex- 

 tract these elements, in the proportions necessary for their growth. 

 Manures, by furnishing in great abundance the hydrogen, carbon, or 



* Recherches Physico-Chimiques. 



