276 IMPROVEMENT OF THE SOIL. 



substituted for another in some cases, we must seek still fur- 

 ther for facts and principles, fully to explain the reasons for 

 the rotation of crops, and their beneficial effect. 



III. The excretions toliich the roots of plants deposit in the 

 soil have been regarded by some as the most satisfactory mode 

 of explaining the effect of cultivating the same crop in suc- 

 cession on the same field, and of the benefits of rotation. Lie- 

 big considers the view now to be presented, as the only one 

 deserving " to be mentioned as resting on a firm basis." It 

 is the theory of M. De Condolle, " who supposes that the 

 roots of plants imbibe soluble matter of every kind from 

 the soil, and thus necessarily absorb a number of substances 

 which are not adapted to the purposes of nutrition, and must 

 subsequently be expelled by the roots, and returned to the 

 soil as excrements." Now as excrements cannot be assimi- 

 lated by the plant which ejected them, the more of these 

 matters the soil contains, the more unfertile must it be for 

 plants of the same species. These excrementitious matters 

 may, however, still be capable of assimilation by another kind 

 of plants, which would thus remove them from the soil, and 

 render it again fertile for the first. [Liehig.) In a word, 

 one species of plants excretes by its roots substances, which 

 are poisonous or innutritions to plants of the same family, but 

 which may be assimilated by plants of a different species. 



The experiments of iV/«cmVePrmfeps prove, that the roots 

 of plants do expel matters which cannot be converted into 

 any of their component parts. Some of these excrements are 

 of a gummy and resinous character, and are regarded as pois- 

 onous ; others, are compounds of carbon and are nutritious. 

 Liebig supposes that these excrements are not, according to 

 De Condolle, derived from the soil, but from the atmosphere ; 

 and that it is in this way that a soil receives as much carbon 

 from the plant as it yields to it. It now becomes an interest- 

 ing inquiry what state this excrementitious matter is in, 



