EDAPHIC OR SOIL FACTORS: CHEMICAL 169 



For similar reasons, in the determination of the acidity of peat a 

 prepared suspension and not an extract from fresh peat should be used, 

 since the latter is subject to considerable variations. 



Significance of Hydrogen Ions. — The H ion, a simply constructed 

 and most active ion, affects the physiological processes within plants to 

 a high degree. It greatly influences enzyme actions so important in 

 the life cycle of a plant. The concentration of the cell sap of a plant 

 must maintain a certain relation to the ionic concentration of the soil 

 solution. When this relation is interfered with, by change of the con- 

 centration of the solution, morbid phenomena soon appear in the plant, 

 and even death may occur. Certain pathogenic bacteria can grow 

 only upon media of very definite H ion concentration. Ulehla (1923) 

 proved that the apical cells of certain algae (Cladophora, Basidioholus) 

 burst when they are transferred into an acid medium. Apparently 

 adsorptive effects of H ions upon the colloidal material of the cell 

 membrane are involved, and not osmotic phenomena. 



The harmful effects of parasitic fungi can be diminished or com- 

 pletely counteracted by changes of pH values. According to Fischer 

 (1925), the effect of the potato-scab organism {Actinomyces scabies) 

 at pH 8 is strong, at 7.2 weak, at 6.8 near zero, and at pH 5.2 to 5.0 the 

 organism is entirely destroyed. Gaumann (1925) showed that an 

 infection of garden beets by Phoma hetae produces the heart disease of 

 the roots only in strongly alkaline soils. Attempts to induce the infec- 

 tion in weakly acid soils failed. The decisive influence of pH values 

 upon the vitality of mycorhizal fungi, and thereby upon the vigor of 

 conifers, has recently been demonstrated by Melin (1924). 



The higher plants have a more or less wide pH tolerance, with a 

 distinct region of optimum, which may be taken to express the soil 

 preference of the species. The pH optimum of wild species, examined 

 in natural habitats, results in a more or less steep, one-peaked 

 (unimodal) or possibly occasionally in a two-peaked (bimodal) 

 curve. 



Pearsall (1926) and more recently Volk (1931) have criticized the 

 two optima of Salisbury and others. These optima seem to have 

 resulted from a very unequal distribution of samples of the soils with 

 respect to their chemical composition, or else they may have resulted 

 from the fact that the species studied belonged to different ecotypes. 

 When associations are concerned, the causes of errors are evidently 

 less, and there are probably no bimodal curves for one and the 

 same association. 



Perfect unimodal pH curves were obtained from soils of the root 

 layer of Ammophila arenaria at Blakeney Point, England (Salisbury), 



