Dr Davy's Agricultural Discourse. 301 



as involving common principles, universally applicable. Not 

 a less important part of the subject, is that of special manures, 

 or of the choice of manures for particular crops, and this is 

 by far the most difficult part, and the one hitherto least 

 studied. In animals, their coarser organization is distinct ; 

 if we examine the teeth of any particular animal, its stomach, 

 its intestines, the main organs concerned in its nourishment, 

 we have no difficulty in deciding from their structure, whe- 

 ther the individual is carnivorous or herbivorous, or fitted for, 

 and requiring a mixed diet of animal and vegetable food ; and 

 this, be it one of the mammalia, birds, or insects. But not 

 so as regards plants ; the organs concerned in their nourish- 

 ment are so minute as to escape detection by the eye unaided 

 by the microscope ; and even when examined by this help, no 

 differences characteristic have hitherto been detected, ad- 

 mitting of being associated with the quality of the nourish- 

 ment best fitted for the individual plants. This, then, taking 

 it for granted, and it seems to be proved by experience, that 

 different kinds of plants do not for their coming to perfection, 

 require one kind, but different kinds of manure, must be de- 

 termined by other means. How is this important object to 

 be accomplished ? I do not know how it can be well accom- 

 plished, except by enlightened experience and by chemical 

 research. In the instances of the corn-bearing grasses, such 

 experience, such inquiry have been highly useful. These are 

 crops which are exhausting to the soil, — the grain being con- 

 sumed at a distance, and the more exhausting when the straw 

 also is removed from the farm. To correct the exhausting 

 effect, one of two measures is adopted ; either to allow the 

 land to remain fallow for a certain period, during which, 

 owing to the decomposition and disintegration of mineral 

 particles in the soil, and the addition made to it from the at- 

 mosphere by the agency of the elements, and by rains and 

 winds, and from the subsoil, by the penetrating roots of na- 

 tive grasses and other indigenous plants, the loss is made 

 good of those inorganic materials carried away : — or, in a 

 more summary manner, by restoring in manure (aided by in- 

 tervening green crops), the ingi^edients abstracted and lost 



