LaMotte STH Series - Manual (Page 6)

Note: The wide range pH test result indicates which narrow
range indicator and color chart should be selected to perform
a more precise pH test. Choose the narrow range indicator
and appropriate chart with a mid-point that is as close as
possible to the value obtained in the wide range test.
Indicator pH Range
Indicator
Code
Color
Chart Code
Bromcresol Green 3.8-5.4 2207 1328
Chlorphenol Red 5.2-6.8 2209 1329
Bromthymol Blue 6.0-7.6 2210 1331
Phenol Red 6.8-8.4 2211 1332
Thymol Blue 8.0-9.6 2213 1335
Example: If the wide range test result is pH 6.0, choose the
Chlorphenol Red Indicator (2209) & the Chlorphenol Red
Color Chart (1329) for Step 5.
5. Add two drops of the chosen narrow range indicator to the
second sample on the spot plate. Compare the resulting color
reaction against the appropriate color chart to obtain a precise
soil pH reading.
Interpretation of pH Reading:
If the pH is Then the soil is
Below 5.5 Strongly Acid
5.5-6.0 Moderately Acid
6.1-7.0 Slightly Acid
Above 7.0 Alkaline
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