Every growing crop results in the formation of a substance which is toxic to the growth of other plants, and still more so to itself.

By oxidation this toxin loses its toxic properties and enhances the fertility of the soil. The plants previously poisoned eventually outstrip those which had not been subjected to the poisoning, except in cases where the toxic effect has been sufficient to produce a permanent stunting.

The toxic effect must necessarily vary considerably with the different conditions obtaining, both as to the nature of the soil, the plant affected, and the vigour of growth of the plant producing the toxin.

There is no reason for assuming the excretion of any toxic matter from a plant, the debris from the growing roots is probably sufficient to account for the formation of the toxin.

The heating of a soil produces toxic matter from the organic substances present in it, and in much greater quantities than that produced by the growth of a crop. In both cases the toxin, after oxidation, increases the fertility of the soil.