A SOIL WHEN SAID TO BE FERTILE. 75 



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 

 silicate, are retained in combination by the chemical 

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

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

 potash is to be liberated and made available for passing 

 into plants. If we find that some plants are perfectly 

 developed in a soil of the kind, which remains unfruit- 

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

 able to overcome the chemical resistances opposed to 

 their growth, and that the latter are not. Further, if 

 we find the same soil gradually acquiring the power of 

 producing these latter plants also, we can assign no 

 other reason than this, that by the combined action of 

 air, water, and carbonic acid, aided by mechanical 

 operations, the chemical resistances have been over- 

 come, and the alimentary substances have been reduced 

 to a form in which they are available for absorption 

 even by plants endowed with the feeblest powers of 

 vegetation. 



A soil can only then be said to be perfectly fertile 

 for a given species of plant, e.g. wheat, when every part 

 of its horizontal section which is in contact with the 

 roots contains the amount of food required by the plant, 

 in a form allowing the roots to absorb such food at the 

 proper time, and in the proper quantity, during every 

 stage of its developement. 



In a former section mention has been made of a 

 property possessed by arable soil, viz. that when 



