Exchangeable Acidity & Aluminum

By Jared L. DeForest

Reagents

In 10 l carboy (enough for 65 samples)

• 745.9 g  Potassium Chloride (1 M KCl)
• Bring to volume with DI water.

In 1 l volumetric flask (enough for 100 samples)

• 58.1 g Potassium Fluoride (1 M KF)
• Bring to volume with DI water.
• Phenolpthalein indicator
• Sodium Hydroxide Standard (1N NaOH)
• Hydrochloric acid Standard (1N HCl)

Procedure

Add the following to 125 ml sample cup

• 10 g Oven-dried equivalent, sieved soil (can be field fresh)
• 75 ml Potassium Chloride (1 M KCl)

Place on orbital shaker (200 rpm) for 30 minutes.

Filter suspension (coarse filter paper) in a plastic funnel into a 250 ml Erlenmeyer flask

Wash soil THREE successive 25 ml aliquots of KCl for a total of 150 ml per sample.

Add five drops of Phenolpthalein indicator.

Stir constantly with a magnetic stir plate.

Titrate the solution with 1 N NaOH (or 0.1 N) using a titrating electronic pipette (accuracy 5 µl).

Note the volume to a permanent pink end-point for KCl-exchangeable total acidity.

Repeat this procedure with 150 ml of 1M KCl (accounts for acidity in KCl).

Add 10 ml of 1 M KF & swirl to mix. (Solution should become dark pink.)

Titrate the solution with 1 N HCl (or 0.1 N) using a titrating electronic pipette (accuracy 5 µl).

Note the volume to a permanent clear end-point for KCl-exchangeable Al³⁺.

Adapted from Sims JT. (1996) Lime Requirement. Methods of soil Analysis. Part 3. Chemical Methods-SSSA Book Series no. 5.

Created: 08/01/07 Revised: 10/10/11