170 | | Sepncnubal ‘tnd Ph psiological Botany. 
f 
~ 
‘quantitative ‘selective power’ has been ascribed to plants. 
aside nitrogenous food-materials—different species take up + 
distinguished as calcareous, alkaline, siliceous, &c. Legu- 
ing, and irrigating of the ground. 
-number of crops of an alkaline plant causes the soil to become for a 
-time—and at length when all the alkalies have been consumed, per- — 
ve z ‘ hn { hers + Bs Fete 
- ‘ fea , sa SA ar es 2 a, Ck ha ee * \ 
Watt ie ied vy = xr ‘ wal \ 
stances are not taken up in quantities dependent. on “afk ey 
concentration of the fluid, nor on the proportion of the 
various substances contained in it ; but that different species _ 
of plants, on the contrary, supply their needs from the same’ 
fluid in different ways. In consequence of this pheno- 
menon, which depends on causes at present unknown, a 
Since plants possess no volition, the term is an unfor- 
tunate one ; but it seems necessary to retain it, notwithe § | 
standing its ambiguity, since no better one has been sug- ee 
gested. With reference to the peculiarity that—settmg  - ~ 
in preference different nutrient substances, plants may be 
minosee, for example, are specially dependent on hme, pota- 
toes and turnips on potash, all plants in which the seed 
is remarkably developed on phosphoric acid, cereals and 
grasses generally on silica, &c. 
The cultivation of crops withdraws from the soil immense quantities 
ee 
of substances ; so that sooner or later the moment must arrive when the 
_produce will diminish, and at length altogether cease, unless those 
constituents of the soil which the crops have withdrawn from it be 
replaced. The object of the agriculturist must therefore be so to 
work his ground that the soil never becomes exhausted, but that he 
shall constantly obtain abundant crops and an adequate interest on his : 
capital, the soil. There are two modes of securing this : by a carefully ee 
selected succession of different crops, and by manures. He must of he 
course in addition depend on a suitable mechanical working, loosen- 
In consequence of the selective power of plants, a succession of a Peer 
manently—unproductive for plants of this description. But while the ~ ‘ 
alkaline plant has the power of withdrawing an unusual proportion of a 
alkalies from the soil, it leaves all the other nutrient substances com- . 
paratively untouched. These substances therefore accumulate in the ee 
soil, the small consumption of. them being outweighed by the decom- 
posing influence of the atmosphere, which is constantly reducing to a 
soluble condition small quantities of soil, and thus rendering their con- 
