In 1894 the author contributed to the ‘Journal of the Chemical Society’ (vol. 65, ‘ Trans.,’ March, 1894) a paper 011 “ The Determination of Available Mineral Plant Food in Soils,” in which the use of a 1 per cent, solution of citric acid was proposed as a means of approximately differentiating by means of chemical analysis between the total and the probably available phosphoric acid and potash in soils. The reasons leading up to the tentative adoption of this solution, together with a summary of previous literature on the subject, are given in the original paper, and it need, therefore, now only be said that the method was the result of an attem pt to imitate, in the solvent used, the acidity of root-sap, based on a preliminary examination of the root acidity of 100 specimens of flowering plants representing some 20 natural orders. In order to test the proposed method it was applied to 22 samples of soil drawn from the various plots in Hoos Field, Rothamsted, on which barley under very various manurial conditions had been continuously grown for over forty years. The samples were placed at the author’s disposal by the kindness of Sir John Lawes and Sir Henry Gilbert. The results of this investigation, which are fully set forth in the paper referred to, were of sufficient interest to lead to the undertaking of a similar but much more extended examination of the soils of the Rothamsted wheat plots in Broadbalk Field.