SCIENCE AND PRACTICE. 123 



CHAPTER X. 



On Fallow. 



Agriculture is both an art and a science. Its scientific basil 

 embraces a knowledge of all the conditions of vegetable life, of 

 the origin of the elements of plants, and of the sources whence 

 they derive their nourishment. 



From this knowledge fixed rules are formed for the practice 

 of the art, that is, for the necessity or advantage of all the 

 mechanical operations of the farm, by which the land is prepared 

 for the growth of plants, and by which those causes are removed, 

 which might exercise an injurious influence upon them. 



Experience acquired in the practice of this art can never stand 

 in contradiction to its scientific principles : because the latter 

 have been deduced from all the observations of experience, and 

 are actually an intellectual expression of it. Neither can The- 

 ory ever stand in antagonism to Practice, for it is merely the 

 tracing back of a class of phenomena to their ultimate causes. 



A field upon which we cultivate the same plants successively 

 for a number of years, may become unfertile for these plants in 

 three years ; whilst another field may last seven, another twenty, 

 and another one hundred years, without losing its fertility. One 

 field bears wheat but not beans ; another bears turnips but not 

 tobacco ; and a third yields rich crops of turnips, but does not 

 bear clover. 



What is the reason that a field loses gradually its fertility for 

 the same plant ? What is the reason that a certain kind of plant 

 flourishes on it, and that another fails ? 



These questions are proposed by the Science of Agri- 

 culture. 



What means are necessary to enable a field to sustain its fer- 



