ROTATION OP CROPS. 141 



which is exhausted by the continued growth of any race of plants 

 in the same spot, this being usually rather increased than 

 diminished; and therefore any plants, which require no other 

 nutriment than this, may be made to grow in a soil, which wheat, 

 or some other plant that takes up a large portion of some parti- 

 cular kind of mineral matter, had completely exhausted. Such 

 is the case, for example, with many of the Leguminous plants 

 (the tribe including the Pea, Bean, Clover, and other similar 

 vegetables), which absorb so little mineral matter, that they may 

 be grown between two crops of corn, with nearly the same 

 advantage for the latter, as if the land had lain fallow between. 

 Hence these are called fallow crops. On the other hand, the 

 injurious properties of many weeds that are apt to show them- 

 selves in corn-fields, result from their imbibing a large quantity 

 of the same ingredients, as those which the corn requires ; so that, 

 in proportion to the vigour of their growth, that of the corn must 

 decrease. Hence it not only conduces to the neatness in the 

 appearance of a corn-field, but also to its productiveness, to keep 

 it free from weeds. 



208. Now the principle that a succession of different crops 

 may be grown, where one could not be repeated without occa- 

 sional intervals, has gradually superseded the old system of 

 allowing the land to lie for a season, out of every three or four, 

 entirely unproductive; so that the quantity of vegetable sub- 

 stances, nutritious either to man or beast, which is now raised 

 from a given quantity of land, is much greater than formerly. 

 This principle has been fully established by experience ; but it 

 is still acted on to a very limited degree, because its conditions 

 are not yet fully understood. If there were nothing else to be 

 considered, than the kind of mineral substance which each plant 

 draws from the soil, it would not be difficult to say, what crops 

 might succeed each other most advantageously, since it would be 

 only necessary to find out the mineral ingredients which each 

 requires, and to make those succeed each other, which draw least 

 of the same. But there is another very important condition to 

 be attended to. 



209. Plants, as already stated (. 1.19), not only draw 



