268 



MANUAL OF AGRICULTURE. 



plants are included under the three heads of salts of potash and 

 soda, of lime and magnesia, and of silica, bears out the fact : — 



Until quite recently, the general opinion was entertained that 

 by the continuous growing of one particular species of cro]^ on 

 the same piece of ground, and that even when the appropriate 

 manures were supplied to it, both quantity and quality of the 

 returns gradually diminished and deteriorated, until at length its 

 cultivation proved fruitless. As accounting for this, the theory 

 was advanced of plants excreting matter from their roots, and 

 that any accumulation of this was hurtful to plants of the same 

 species as the excreting one. But this opinion must now be 

 surrendered; and where failures occurred in cultivation after 

 such a sort, they were due, doubtless, to mistaken ideas as to the 

 requisite manures, and the neglect of adopting means for the 

 check of fungoid and insect ravages. Ample proof is now aftbrded 

 by results in the case of grain crops, that the same species of 

 crop may be continuously raised on the same plot of ground 

 with profit, provided suitable manures are applied, freedom from 

 weeds secured, and the ravages of insects and fungi averted. 

 But hitherto this admission has had but little effect in altering 

 the customary and fixed rotations of cropping peculiar to differ- 

 ent districts and estates. 'No doubt, so long as the great majority 

 of soils are subjected to the present mixed method of husbandry, 

 which must of necessity accompany the rearing and feeding of 

 live-stock, rotation of cropping must continue as an institution, 

 jjioviding as it does for the requisite proportions of litter and dry 

 and green food for the various species of animals. Its different 

 systenis are determined by the nature of climate and soil, the 

 situation and demand of the more convenient markets, and the 



