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:
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