ROTATION OF CROPS. 275 



"then, after i,000 years, it must become sterile for this plant. 

 " If we were to remove the surface-soil and bring up the subsoil 

 *' to the surface, making what was formerly surface-soil now the 

 " subsoil, we would procure a surface much less exhausted than 

 *'the former, and this might suffice to supply a new series of 

 ** crops, but its state of fertility would also have a limit. 



" A soil will naturally reach its point of exhaustion sooner the 

 *' less rich it is in the mineral ingredients necessary as food for 

 " plants. But it is obvious that we can restore the soil to its 

 " original state of fertility by bringing it back to its former com- 

 " position ; that is, by returning to it the constituents removed by 

 " the various crops of plants. 



" Two plants may be cultivated side by side, or successively, 

 *' when they require unequal quantities of the same constituents, 

 " at different times ; they will grow luxuriantly without mutual 

 *■*■ injury, if they require for their development different ingredients 

 " of the soil." 



" Different genera of plants require for their growth and perfect 

 " maturity either the same inorganic means of nourishment, although 

 " in unequal quantities and at different times, or they require dif- 

 " ferent mineral ingredients. It is owing to the difference of the 

 " food necessary for the growth of plants, and which must be 

 " furnished by the soil, that different kinds of plants exert mutual 

 " injury when growing together, and that others, on the contrary, 

 "grow together with great luxuriance." 



******* 



" There are certain ashes of plants wholly soluble in water, 

 " others are only partially soluble, while certain kinds yield only 

 " traces of soluble ingredients. 



" When the parts of the ashes insoluble m water are treated 

 " with an acid, (muriatic acid,) this residue, in the case of many 

 " plants, is quite soluble in the acids, (as, for instance, the ashes 

 " of beet, turnips, and potatoes ;) with other plants, only half the 

 "residue dissolves, the other half resisting; the solvent action of 



