LaMotte STH Outfit - Manual (Page 6)

6
Test Procedures
pH
pH is a measure of acidity or basicity. Soils can have a pH from 3.5 to
11.0, but plants grow well in the range of 5.0 to 8.5. In soils with a low
pH (acidic), some nutrients can reach toxic levels and the activity of
soil microbes is greatly reduced. Soils with a high pH (alkaline) generally
have a lower micro-nutrient availability and some levels may be defi cient.
PROCEDURE
1. Fill a test tube (0204) approximately one-third full of soil. Use
the Demineralizer Bottle (1155) to add demineralized water to the
tube, until it is fi lled to one-half inch from the top. Cap and shake
until the soil is well dispersed.
2. Add 5 drops of Soil Flocculating Reagent (5643WT). Cap and
shake to mix. Allow contents to settle before proceeding to Step 3.
3. Use a 1 mL pipet (0354) to transfer 1 mL of the clear solution
above the soil to one of the large depressions on a spot plate (0159).
Transfer a second 1 mL sample to the other large depression on the
spot plate.
4. To the fi rst sample on the spot plate, add two drops of *Duplex
Indicator (2221). Compare the resulting color reaction against the
Duplex Color Chart (1313).
5. 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.