S2 ON SOILS. 



from two causes : the first, that each plant or 

 crop takes away from the soil the matter suited 

 to itself and thereby exhausts it; the other, 

 that it leaves an excrement behind which acts 

 as a poison to plants of the same kind ; but 

 which excrement in some instances serves as 

 the nutriment of other plants, and in all cases 

 may be destroyed by exposing it to the action 

 of the air, or by mixing with it such manures 

 as will readily effect its decomposition. 



After what has been already said on the ex- 

 haustion of soils by a particular crop, it will be 

 hardly necessary for us to point out the advan- 

 tages that may be derived from a judicious ad- 

 mixture of various soils, in which the defi- 

 ciencies of the one may be supplied by the 

 excess of the others. Indeed it is on this 

 principle, that of supplying a soil with the 

 ingredients necessary for the support of plants, 

 and neutralizing other substances, which may 

 be prejudicial to them, that the whole merit 

 of agricultural chemistry belongs, and much 

 may be done in this way, in every situation 

 however destitute ; and at the present time 

 when the facilities for transporting matter of 

 all kinds, by means of railroads, are taken into 

 consideration, it may not be an unreasonable ex- 

 pectation to hope to see eventually the barren 

 hills of one district, manured and covered with 

 the alluvial deposites of others, in a high state 

 of cultivation; and then the benefits which 

 science has conferred on the community at 

 large, and which in this instance serves in 

 some degree to annihilate space, and to equalize 



