THEORY OF THE INTERCHANGE OF CROPS. 277 



whether it is already fitted to nourish other species of plants, 

 or must first pass through some chemical change 1 



It appears that the excrementitious matter of De CondoUe 

 is matter derived from the soil, and is not fitted to nourish 

 that species, but may be indispensable to some other plant. 

 It is undigested matter, and resembles the undigested excre- 

 ments of animals, which, though unfitted to be assimilated by 

 one animal, may prove nutritious to another. 



The excrements of Macaire Princeps may be derived from 

 the soil, but they are matters formed in the vegetable organs. 

 They are compounds produced in consequence of the trans- 

 formations of the food, and of the new forms which it assumes 

 by entering into the composition of the vegetable organs. 

 They are not, therefore, supposed capable of nourishing 

 other species of vegetables, until a change is wrought upon 

 them. This change is effected by the agency of the atmos- 

 phere, water, etc., and they are converted into humus. 



These views do not contradict each other ; both may be, and 

 doubtless are true ; both explain why it is that after wheat, 

 wheat will not flourish so well on the same soil, and why 

 one crop must succeed another to keep up the quantity of 

 produce. 



The latter theory, however, explains the fact that the ex- 

 crements of some plants, affect the same species longer than 

 others ; for it is evident that the time required for the decay 

 of the excrements may depend upon their nature, quantity, 

 and the composition and character of the soil. In a calcare- 

 ous soil it would be rapidly effected, and hence it is found 

 that such soils admit of the same crop after the second year ; 

 or its decay may be effected by alkalies, and this is doubt- 

 less one of the good effects of adding these substances to the 

 soil. But when the soil is siliceous or argillaceous, the same 

 crop cannot be cultivated with advantage until the fourth or 

 ninth year. Thus for example, " clover will not flourish in 

 some soils oftener than once in six years, on other soils, once 



