64 THE SOIL. 



of the weather, become fertile enough to produce those 

 plants which it formerly refused to bear. The dif- 

 ference between fertile arable land and barren untilled 

 soil is not the result of any dissimilarity ui the nutri- 

 tive substances which they contain ; because in cul- 

 tivation upon a large scale, to convert the untilled rough 

 soil into fertile arable land, the ground, so far from being 

 emiched, is rather impoverished by the cultivation of 

 other plants on it. 



The difference between the subsoil and the arable sur- 

 face soil, or the crude and the cultivated soil, supposing 

 that both contain the same amount of nutritive substances, 

 can only be founded upon this, that the cultivated ground 

 contains the nutritive substances of plants, not only in a 

 more uniform mixture, but also in another form. 



Now as from the influence of cultivation and weather 

 above-mentioned, the rough soil acquires the power of 

 furnishing the elements of food which it contains, in just 

 the same quantity and in the same time as cultivated soil, 

 a power which was formerly wanting in it with regard to 

 certain plants, it cannot be denied that an alteration must 

 have taken place in the original form and fashion of these 

 elements. 



Suppose we have a soil consisting of disintegrated 

 rocks : in the smallest particles of such a soil, the nutri- 

 tive substances of plants, as potash for instance in a 

 sihcate, are retained in combination by the chemical 

 attraction of sihcic acid, alumina, &c. This attraction 

 has to be overcome by one still more powerful, if the 

 potash is to be hberated and made available for passing 

 into plants. If we find that some plants are perfectly 

 developed in a soil of the kind, which remains unfruitful 

 for others, we are led to assume that the former are able 

 to overcome the chemical resistances opposed to their 



