I have been searching without success in the text & indexes both of your “Origin” & “Variation” for something about the doctrine of “rotation of crops” & the much disputed question which the elder De Candolle raised long ago & got Macaire & other chemists to experiment upon, as to the chemical effect of the growth of one crop upon the soil rendering it unfit for the same species to be planted in it the following year.¹

I understand their doctrine to have been that in the formation of seed & other nutritious parts of plants the sap is digested, that it takes up certain elements & deposits others which are the residue of the process & which exude by the roots— Further that as the excrement of certain animals is fitted for the support of other animals though quite useless to sustain life in the same species so a plant of a totally different genus or family may find good nutriment in the refuse left behind by the plant previously grown in the soil.

If you can refer me to nothing touching upon this point in your works, will you give me your opinion as to whether there is likely to be any truth in this statement.— In vol II of your Variation p 146–148 you come very near to the question but do not exactly touch it.²