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corollary to his theory of mineral requirements; since plants take their essential constituents in such established ratios they must in time destroy the necessary ratio of these elements in the soil, but when another plant drawing its ash constituents in a different ratio is substituted, it obtains a sufficient supply of nutrients and the soil is thus relieved of exhaustion.

Under the domination of Liebig's theory of mineral requirements, the theory of De Candolle was practically abandoned. Subsequent to his time the mineral matter of both soil and plant claimed paramount attention, and the biological factors connected with soil problems were almost entirely neglected. Unscientific as it now appears, it must be admitted that for several decades Liebig's dictum had more weight than any amount of experimental evidence. It would even appear that his word has been regarded so infallible in certain quarters that further scientific research has been regarded as unnecessary.

The inadequacy of the theory of mineral requirements alone to explain the productivity of soils has been aptly set forth by Coleman in an essay "On the Causes of Fertility or Barrenness of Soils," presented to the Royal Agricultural Society of England. This essay, although written fifty years ago, expresses the status of the problems of the mineral requirements theory as well now as at the time it was written. He says:

The causes which operate in producing the fertility or barrenness of soils have hitherto to a great extent been shrouded in mystery, not from any want of study, but owing to the difficulties which meet the inquirer at every step, and the fact that most important results frequently depend upon causes which have eluded the search of the experimenter. The science of chemistry it was hoped would afford the key wherewith to unlock the mysteries of nature, but though its discoveries have conferred much practical benefit on the agriculturist, it has up to a very recent period effected comparatively little toward settling the causes of fertility or sterility. The theories of scientific men led us to expect that fertility depended upon the presence of certain mineral substances which were found invariably present in the ashes of plants, and the analysis of a soil it was believed would confirm the practical experience of the farmer; these hopes have been falsified except in the few cases of almost simple soils, such as pure clays and sands. In all other instances the analysis presented the existence in varying proportions of those substances supposed to induce fertility equally in the barren as the fertile soil. The proportion of the various ingredients was next proposed as a sign of quality, but researches into the amount of inorganic matter abstracted by each crop have demonstrated that soils of a mixed character contain abundant supplies of mineral food for numerous crops.

From the time of Liebig and the establishment of his theory of the mineral requirements of plants, there appears to have been no serious discussion of the subject of root excretion until recent years. The reports of the Woburn Experimental Fruit Farm of the Royal