ON THE FOOD OF PLANTS. 141 



grand essentials for making them prolific ; and 

 when vegetable aud animal substances are laid 

 on the surface of the soil they are intended to 

 enrich, they are in a situation best calculated 

 for decomposition, by oxydizement ; for then, 

 as they are reduced to a soluble state, they are 

 dissolved by water, and carried to the roots ; 

 and when there is not moisture enough to carry 

 down the soluble part, it lies dry on the surface ; 

 and whilst dry, as Sir H. Davy observes, 

 no fermentation can take place ; and thus 

 circumstanced, under the alternate state of wet 

 and dry, the decomposition is gradual, complete, 

 and rapid. This may be observed by a stake of 

 wood, that has been some time fixed into the 

 earth ; for that part which is under the earth, and 

 that which is in the air, will remain sound, long 

 after that part which is between both, or on a 

 level with the surface of the earth, is completely 

 decomposed. 



Besides, there is a natural operation constantly 

 working, to assist in the decomposition of vege- 

 table substances which are laid on the surface, 

 and to facilitate their conveyance to the roots of 

 plants ; which, although little thought of) is of 

 great importance, and this is conducted by the 

 earth worms. Whoever will notice the surface of 

 the earth, will find those creatures not only con- 



