CONSTITUTION^ OF THE SOIL. 3O7 



fcefore. We gather, then, that there are three items to be 

 regarded in the simplest view of the chemical compo- 

 sition of the soil, viz., the inert mechanical basis, the 

 presently available nutritive ingredients, and the reserve 

 matters from which the available ingredients are supplied 

 as needed. 



In a previons chapter we have traced the formation of 

 the soil from rocks by the conjoint agencies of mechanical 

 and chemical disintegration. It is the i>erpetual operation 

 of these agencies, especially those of the chemical kind, 

 which serves to maintain fertility. The /ragments of rock, 

 and the insoluble matters generally that exist in the soil, 

 are constantly suffering decomposition, whereby the ele- 

 ments that feed vegetation become available. What, 

 therefore, we have designated as the inert basis of the soil, 

 is inert for the moment only. From it, by perpetual 

 change, is preparing the available food of crops. Various 

 attempts have been made to distinguish in fact between 

 these three classes or conditions of soil-ingredients ; but 

 the distinction is to us one of idea only. We cannot realize 

 their separation, nor can we even define their peculiar con- 

 ditions. We are ignorant in great degree of the power 

 of the roots of plants to imbibe their food; we are equally 

 ignorant of the mode in which the elements of the soil are 

 associated and combined ; we have, too, a very imperfect 

 knowledge of the chemical transformations and decomposi- 

 tions th?.t occur within it. We cannot, therefore, dissect 

 the soil and decide what and how much is immediately 

 available, and what is not. Furthermore, tlie soil is chem- 

 ically so complex, and its relations to the plant are so com- 

 plicated by physical and physiological conditions, that we 

 may, perhaps, never arrive at a clear and unconfused idea 

 of the mode by which it nourishes a crop. ISTevertheless, 

 what we have attained of knowledge and insight in this 

 direction is full of value and encoui-agemcnt. 



Deportment of the Soil towards Solvents,— When wo 



