OF POTTING. 271 



ly relieved them ; this matter being assumed to be 

 unsuitable to themselves, although harmless to differ- 

 ent species. The subject has been hitherto so little 

 investigated that it is not safe, perhaps, to take it as 

 the basis of a theory ; but it certainly appears to offer 

 a more probable explanation of the deterioration of 

 soil than any other yet proposed. There are those, 

 indeed, who seem willing to deny altogether that soil 

 is deteriorated ; and cases are adduced of Peach trees 

 not repotted for twenty years, which did not die ; of 

 Strawberry beds not renewed for a long series of 

 years, which still bore fruit : but I do not know that 

 any one ever asserted that trees would perish if re- 

 planted in their own deteriorated soil; it has only 

 been said that they would become unhealthy and un- 

 productive, and I think few gardeners will deny that. 

 Neither has it been pretended that the root-secretions 

 of every plant are deleterious at all. It is quite con- 

 ceivable that one plant may secrete a deleterious mat- 

 ter that is very slowly decomposable, but which may, 

 nevertheless, be soluble enough to enter into the food 

 of other roots; and in such a case an injurious effect 

 may be produced : while, in another case, the secreted 

 matter may be rapidly decomposable, when it will 

 enter into new combinations, and lose whatever dele- 

 terious property it originally possessed, if any. At 

 all events, be the theory what it may, it is an un- 

 doubted fact that soil is deteriorated by a plant 

 which has grown in it for a long time ; and that, to 

 be maintained in a healthy condition, that soil must 

 be changed. This explains why potted plants, care- 



