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Soil pH

Soil pH is determined on all samples using a 1:1 (weight:volume) soil to water ratio.

pH 6.86 buffer standard - purchased at pH 7.0 (0.5M Phosphate)
Dissolve 3.40 + 0.01 g of dried reagent-grade potassium dihydrogen phosphate (KH2PO4) and 3.55 + 0.01 g of dried reagent-grade anhydrous disodium hydrogen phosphate (Na2HPO4) in sufficient deionized water to make 1000 mL.

pH 4.01 buffer standard - purchased at pH 4.0 (0.05M Potassium hydrogen phthalate)
Dissolve 21 + 0.05 g of dried reagent-grade potassium hydrogen phthalate (KHC8H4O4) in sufficient deionized water to make 1000 mL.

Procedure

  1. Transfer 12 mL scoop of soil (assume 15 g) to paper cups arranged in numbered trays. After 100 samples (1 set) is transferred, scoop every 20th sample an additional time as rechecks and include at the beginning of the set.
  2. Add 15.0 mL of deionized water by automatic pipette with enough force to mix thoroughly and allow to stand at least 1 hour.
  3. An AS-3000 Dual pH Analyser is used to measure pH. Check the filling solution level of the electrodes daily.
  4. Follow instrument procedures for calibrating with buffers; pH 7.0 and pH 4.0.
  5. Set lab number parameters accordingly.
  6. Allow the meter to stabilize at least 15 minutes.
  7. Measure the pH of the check soils. The pH should be read to 1 decimal place (0.1) and should be +0.2 pH units from the standard Check value. If the Check soil is not within tolerance, recalibrate the meter and/or seek assistance.  Make sure both checks read properly before leaving the analyser.
  8. After the set is completed, compare the additional recheck readings with the original readings and record on notebook.
  9. Store electrodes in storage solution or buffer standard solution with meter in Standby mode.
  10. Clean electrodes weekly.