298 Dr Davy's Agricultural Discourse. 



this may be, — as when the crops, the produce of the soil, are 

 in no part returned to it ; but, it cannot be in ordinary course, 

 if the reverse practice be observed ; the tendency of vegeta- 

 tion being to enrich the soil : in brief, the great fertility of 

 what are called virgin soils, appears to be owing, not to a 

 state that the word implies, but to the plants which have 

 grown on such a soil, having undergone decay there, and by 

 their decay manured and enriched the soil. Take the ex- 

 ample of a native forest : — the trees composing it spread 

 their roots in all directions and to a considerable depth, from 

 whence they collect the inorganic materials requisite ; these 

 are conveyed into the leaves and into the wood of the trunk 

 and branches, in which also, carbon, derived from the atmo- 

 sphere, is stored ; the leaves fall and decay, and, in time, the 

 trees, and decompose, and in decomposing give carbonaceous 

 matter to the soil, and restore the inorganic materials ex- 

 tracted, and as it were, collected and concentrated. Such is 

 the natural tendency of vegetation ; and such also is the effect 

 of judicious management, when green crops are plough ed-in 

 as manure, or when the greater part of the ripe crop is re- 

 turned to the soil, w^hetlier directly, as in the instance of the 

 application mentioned of the leaves of the cane to the surface 

 of the soil ; or, indirectly in the state of stable-dung, or pen- 

 manure, the vegetable matter having been first used as 

 forage. This view is simple and clear, resting on the prin- 

 ciple that vegetable growth and the enrichment of the soil 

 are concomitant ; and, I believe, that generally it is a prin- 

 ciple of practical application. There are, however, facts de- 

 serving of attention, and of careful study, which prove that 

 the growing of plants of the same kind for a series of years, 

 the plants decaying where they grew, has an injurious effect 

 on the soil, as regards its power of supporting these plants, 

 indicated by their disappearing, followed by other species 

 which grow luxuriantly, proving unexhausted fertility, that 

 is, that the soil has been so modified by one species as to be 

 rendered unfavourable to it, and yet favourable to another 

 species. Now, what is witnessed in a striking manner as the 

 result of a long series of years in the soils under forest growth, 

 may take place, though not in a marked manner, from year 



