206 Scientific Proceedings, Royal Dublin Society. 



Ihc immediate neighboui'liood of the roots, through retention of carbon dioxide 

 in solution ; as a consequence, ferrous salts become more available, and chlorosis 

 disappears. It appears, therefore, that chlorosis in certain plants and the 

 development of hydrangeas with pink flowers are closely related phenomena, 

 since both depend on the same factor, the low availability of ferrous salts in the 

 soil. Ijimestone soils may have pll values from 82 downwards to pH 76, or 

 even less. In these the solubility of ferrous salts are much reduced compared 

 witli soils at pH 6 to 7, and so chlorosis may develop. The work of van Alstine 

 (1920) and of Jones and Shive (1921) is of interest in this connexion, 'and 

 experiments showed that ferric phosphate gave rise to chlorosis at pH 41, 

 though satisfactory growth could be made with ferrous sulphate up to much 

 hi 'J her pH values. Further work in this line has been carried out by Arndt 

 (1922). 



Heretofore the pH values for precipitation of hydroxides have been con- 

 sidered; but in culture solutions, and even in the soil, phosphates may have to 

 be considered in this connexion. It was mentioned previously that M'Call 

 and Haag (1921) found their nutrient solutions free from all traces of ferric 

 iron below pH 314, yet Patten and Mains (1920) give pH 3-5 to 6-0 as the 

 limits for precipitation of ferric iron. The explanation lies in the presence 

 of phosphate in quantity sufficient to precipitate the iron completely. 

 Aluminium, too, has a highly insoluble phosphate, for, according to V. D. Elst 

 (1922), it is precipitated at pH 2-6 to 37, the range-for the hydroxide being 

 given as pH 3 to 5. In the writer's own experience this hydroxide is not 

 completely precipitated till beyond pH 5-4. Conner (1921), too, states that at 

 pH 3-9 aluminium is more completely precipitated as phosphate than it is 

 at pH 60 as hydroxide. Beyond pH 64 the writer could not obtain any 

 further precipitate of aluminium hydroxide. 



The work of Lipman (1921) on the relation of the soil to chlorosis in citrus 

 trees appears to indicate that the lack of available iron may not always be the 

 cause of chlorosis. 



Effects of Excess of Soluble Iron Salts in the Soil. 



There are a number of records of an excess of iron salts being the cause of 

 injury to growing plants, and the subject has been considered experimentally 

 by workers mentioned previously. An effect on the soil itself remains to be 

 considered. Swedish chemists have studied the processes taking place under 

 acid humus soils. The rain-water percolates, and, according to Arrhenius (1922), 

 becomes acidified and dissolves iron compounds, so that in time a bleached 

 earth is found below the humus. The water, as it penetrates, reaches a less 

 acid region, where its iron and aluminium salts are deposited, giving rise to 

 an "ortsten" layer. Since peat gives to aqueous extracts acidity equivalent 

 to pF 4-6, and the subsoil is far less acid and acts as a buffer, the reason for 

 precipitation is clear. Further, under the reducing conditions of the humus, 

 most of the iron is probably in the ferrous condition. Frosterus (1914), in 

 his studv of the soils of Finland, gives illustrations in colour of these changes, 

 as well "as analyses of the different layers. The contrasts are very striking. 

 In a schematic" presentation of his results Frosterus distinguishes between 

 the red iron "podsol" and the brownish humus "podsol." The red earth is 

 low in humus, rich in iron and bases, but poor in aluminium. The brown is 

 richer in humus, containing 3 to 11 per cent., considerably more acid, richer 

 also in aluminium. Now, since ferric salts are precipitated as hydroxides from 

 about pH 3-5 to 60, and aluminium salts from pH 39 to somewhere above 



