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A number o£ experimental results are presented which show the variation of the pH values o£ several soils when samples of each are shaken with CaCl 2 solutions of different concentrations.These results are then interpreted on the basis of the ratio law-derived from the Gouy theory of the electrical double layer -and it is shown that the pH values and electrolyte concentrations show the expected relationship, provided the latter is not too large.The importance of the connection between pH and electrolyte concentration in the routine measurement of soil pH is emphasized, and it is pointed out that such measurements must be carried out using an electrolyte solution of known composition in order to obtain comparable results from different soils.It has been found that 0.01 M CaCl 2 is the most satisfactory for use in normal non-saline soils where the surface density of electrical charge on the colloidal material is high and independent of the solution composition: this electrolyte concentration is such that measurements made with the usual glass electrode/saturated calomel cell give accurate and reproducible values which are largely independent o£ the soil/solution ratio, and is yet sufficiently dilute to allow a satisfactory calculation of the "lime potential" characteristic of the soil sample.

Pure kaolinite washed first with 1 M NaCl containing 0.001 M HCI and then with distilled water remains firmly flocculated when salt free. In this state the kaolinite retains exchangeable Na, which shows that the crystals are negatively charged.

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