Hydrogen ion activity of a soil solution at a pH of 6.5 is a hundred times less acidic than a pH of 4.5.
The determination of the hydrogen-ion activity (or pH) of soils is by far the most commonly made soil test.
Within the entire range of soil water content from the moisture equivalent to the soil-to-water ratio of 1:5, the effect of dilution is to increase the pH of suspension of soil, regardless of the initial pH value of the soil or the distilled water used in preparation of the suspensions.
A vacuum tube voltmeter may be either a potentiometric type or a direct-reading pH meter can be used.
Measurements with the potentiometric type of pH meter are independent of the vacuum tube circuit characteristics, and the accuracy of the results depends only on the linearity of the slide-wire.
Both types of pH meters are, however, capable of giving satisfactory results provided that the meter is standardized with two different reference buffer solutions differing in pH by about 2 or 3 pH units.
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