mmmm^mmmmmi'f'^mmm^^Wfm^ ^mm 



30 ON SOILS. 



succession, because the soil requires time to 

 recover itself, or to reproduce a supply of the 

 various substances necessary for their perfec- 

 tion : but in addition to this cause there is ano- 

 ther, which, under the rotation of crops, is 

 indispensable in a judicious system of agricul- 

 ture. 



From experiments that have been made by 

 DecandoUe and others, and repeated by Leibig, 

 it is established beyond all question that the 

 roots of. plants throw off an excrement during 

 the progress of their growth, and that the ex- 

 crement of each plant is peculiar to itself. And 

 farther, that the presence of such matter in a 

 soil, impedes the growth of plants of the same 

 kind, whilst in some instances it becomes a 

 source of nourishment to others. 



This fact is another argument in favour of 

 the necessity of a rotation of crops. And 

 much will depend upon the nature and cha- 

 racter of the soil itself as to what time should 

 elapse before a certain crop may be success- 

 fully repeated. 



If Ihe land is light and porous this excre- 

 mentitious matter will in all probability be so 

 decomposed by the combined action of the air 

 and moisture, as to be entirely dissipated in 

 the course of one or two seasons; but if, on the 

 contrary, the soil is of a stiff retentive charac- 

 ter, four, five, and even six years may elapse 

 before this matter shall be so destroyed as to 

 allow a repetition of the first crop. 



It may be argued against this theory, that 

 many crops, such as Lucerne, for instance. 



