One of De Candolle's contemporaries, Macaire-Prinsep, carried out some experiments, which for a time gave promise of firmly establishing the existence of plant (or root) excretions. Macaire-Prinsep took up a number of adolescent plants and after carefully washing their roots, placed them in vessels containing rain water. He reported that after eight days the water in which healthy specimens of Chondrilla had grown had acquired a yellow tint and a strong odor. The water had a bitter taste and gave a precipitate when added to solutions of lead acetate. The same investigator also reported that water containing the excretions from the roots of peas was fatal to other plants of the same species, but not to wheat plants.
Using these ideas and experiments as a basis, De Candolle formulated a theory to account for the well-known benefits of crop rotation. He expressed a belief that the harmful effects observed when the soil is continuously cropped with the same species are due to an accumulation of noxious excretions. According to his theory, the substances excreted by the roots of one order of plants were not usually harmful to the roots of plants belonging to other natural orders; in fact, they might be slightly beneficial. In support of this idea he cited the incontrovertible observation of every husbandman that in the majority of cases good yields of all crops are obtained from the soil when plants belonging to different natural orders are grown in succession.
De Candolle pointed out that there exist, in a state of nature, natural successions of forest trees in the course of which a given species of tree is completely replaced by another species. According to his observations the first species seemed to disappear because its excretions so filled the soil as to render it unsuitable for the longer growth of that species. It is interesting to note in passing that he published an article in the Revue française showing the harm wrought by the decree of Louis XIV. that forests should be perpetually maintained upon forested lands.
De Candolle also made the trenchant observation that when shade trees die from any cause it is very difficult to replace them with trees of the same species.
Using these observations as a foundation, he built up a theory that plants may injure each other by the matter excreted from their roots, and that the success of crop rotation lies in preventing an undue accumulation of excreted material in the soil. He went further and expressed his belief that the excretory matter of the plants of certain families, like the legumes, is not only feebly toxic to themselves, but possesses actual value as a fertilizer for the cereals.
The ideas and observations of De Candolle were not allowed to pass unchallenged, for within a few years controversial articles began to appear.