It has long been the accepted theory that plants obtain the mineral constituents they require from the soil through the intermediary of a solution that is formed in the water with which the soil particles remain in contact. As, however, the amounts of phosphoric acid and potash revealed by analysis are always far in excess of the requirements of the crop, and as the variation in these quantities in no way determines the need or otherwise for a further supply of the constituents in the shape of fertilisers, Daubeny suggested a distinction between dormant and available plant food in the soil, the latter being the more readily soluble compounds of phosphoric acid and potash, which in virtue of their solubility determine the amount of each constituent obtainable during the short season of a plant’s growth. This point of view was revived by B. Dyer in 1894, and has been subjected to considerable examination, without, however, revealing any constant correspondence between the quantities of easily soluble plant food and the response of the soil to particular fertilisers. A new aspect of the problem was set forth in 1903 by M. Whitney and F. K. Cameron, who maintained that as all soils contain practically the same compounds of phosphoric acid and potash possessing a very low solubility, the soil solution must become saturated with these constituents to the same low degree of concentration in all soils, irrespective of the actual amounts of phosphoric acid and potash there present.