Atkins — Hydrogen Ion Concentration of the Soil. 207 



pH 5-4 and below pH 64, one would expect that they would be washed out 

 in much the same manner. Arrhenius, incleed, in describing the process, classes 

 iron and aluminium together. It is interesting, therefore, to find that, in the 

 two tjTpes of podsol analysed for Frosterus, the iron and aluminium have to a 

 certain extent been separated. The explanation appears to be that the iron 

 present as a ferrous salt is not precipitated at all till over pH 5, and appreciable 

 amounts are still in solution at pll 7. It is, therefore, carried further into the 

 soil before precipitation is complete. 



The explanation of iron podsol formation advanced by Arrhenius may be 

 employed to explain the formation of iron pan in this country. The conditions 

 under which it is formed are described by Hall (1910). At about the level to 

 which the soil is ordinarily aerated, a layer of hydrated ferric oxide accumulates 

 in acid clay and sandy soils, which are apt to be water-logged. The conditions 

 prevailing in certain land in the Ballyhoura Mountains, Co. Cork, may be 

 cited as an example. For these details the writer is indebted to Mr. A. C. Forbes : 

 "The soil consists chiefly of glacial drift on Old Red Sandstone. A thin 

 covering of peat has been more or less removed from this soil for fuel purposes, 

 leaving the surface almost entirely bare of vegetation. A few inches below 

 the surface the soil becomes very compact, water resting on it throughout the 

 greater i^art of the winter, but becoming ver^^ dry in summer. Iron pan is 

 more or less universal. Trees planted on this soil make very little growth for 

 several years, spruce especially showing little power of recovering, while pines, 

 especially Corsican and Maritime, thrive in many places when once established. 

 There is no lack of depth, from six to ten feet of drift lying over the solid 

 rock." 



As already mentioned, peat gives a solution at pH 46 Avith great constancy. 

 Soil from various places where the peat had been removed and trees planted 

 was found to be at pII 505, 525, 5-4, and 56. Here undoubtedly are the 

 conditions for solution of iron salts, which begin to b© deposited as they 

 percolate slowly into the glacial drift. The ferric salts are the first to come 

 down, and, moreover, once pH 5 is surpassed, the ferrous begin to be deposited. 

 Where sufficiently near the air, they may in time become oxidised to the ferric 

 condition. Indeed, according to McBain (1901), it is the minute amount of 

 liydrolysed ferrous salt which is, in all ferrous solutions, the most readily 

 oxidised constituent. Again, the strong buffer action of the clay ensures that 

 the acid water quickly loses its acidity as it percolates; accordingly the pre- 

 cipitation takes place over a relatively narrow vei'tical range. There is, however, 

 a secondaiy result of precipitation through oxidation of ferrous salt to ferric, 

 namely, the regeneration of the acid previously combined with the precipitated 

 iron. This regeneration has been shown (Atkins, 1922, 2) to occur, as ferrous 

 sulphate solutions originally at pH 4-8 become on standing as acid as pH 26, 

 a heavy deposit of ferric hydroxide meanwhile appearing. More ferrous iron 

 may thus be brought into solution in the soil, and in time a small amount of 

 acid may result in the solution and — through oxidation — of the redeposition 

 of much iron. There is thus an additional reason why the region of deposition 

 should be in the zone accessible to oxidation, where in fact processes of oxidation 

 and reduction are both in progress. 



Summary. 



1. The common garden hydrangea, //. Jiortensis, produces blue flowers when 

 grown in soil at pH 57 to 6 or slightly over. In less acid habitats some flowers 

 may be pink and others blue on the same plant, but above about pII 75 phik 

 flowers only are the rule. 



