observed. Without appreciating the additional factors which would have to be considered in such an experiment, e. g., absorptive power of the soil, action of microorganisms, oxidation, etc., Mr. Gyde concluded that the excretions of plants are not harmful to their kind, but that the necessity for a rotation of crops arises from the depletion of the soil of the mineral plant food constituents. He thus appears to turn, mirabile dictu, from a proposition which was partially proved to one for which he had no proof; neither has conclusive proof been afforded by any subsequent investigator.
Professor Johnson in "How Crops Grow" has justly remarked that Mr. Gyde's results are not to be regarded as conclusive proofs for or against the existence of root excretions.
The curiously regular growth of fungi in continually widening circles, known as "fairy rings," presents many questions of scientific interest and it is not surprising to find that the theory of deleterious excretions was called in to explain this phenomenon. If one assumes that harmful excretions are left in the soil by these plants, it is easy to understand how the new and thrifty growth would continually arise on the outer edge of the ring, and thus give rise to the phenomenon observed.
The subject of "fairy rings" appears to have been studied by Way, who admitted, in a paper published in 1847, "that by far the most scientific and intelligent solution of the question is that which was based upon De Candolle's theory of the excretions of plants." But on account of objections which appeared insuperable to him, he was unable to accept it as a final satisfactory explanation.
In connection with the decadence of the De Candollian theory, special mention must be made of Liebig and of his attitude toward the question. At first he pronounced this theory of crop rotation to be the only one "resting on a firm basis." He regarded the experiment of Macaire-Prinsep as positive proof that the roots, probably of all plants, expel substances which can not be utilized in metabolism.
In addition to his extensive investigations upon the chemistry of the soil, Liebig made numerous studies upon the chemistry of the ash constituents of plants. He found that the essential elements were present in the ash of all plants, in quantities which formed a more or less definite ratio for a given plant. Reasoning from these facts, Liebig developed the idea that each plant requires a certain ratio of mineral constituents in the soil, as well as a certain minimum amount. He held firmly to the idea that plants could no more attain their maximum growth in the absence of a proper ratio of these mineral nutrients than when the total quantity was too small. This theory became known as Liebig' theory of mineral requirements.
Liebig's explanation of the benefits of crop rotation followed as a
