Extracting for Base Cations

By Jared L. DeForest

Reagents

“Freely Exchangeable and Bioavailable”

In 10 l carboy (enough for 40 samples)

• 770.8g Ammonium Acetate (1 M NH₄OAc)
• Adjust to pH 7.0 with acetic acid (CH₃COOH).
• Bring to volume with DI water.

“Bound Elements”

Mehlich III Stock

In 1 l volumetric flask

• 600 ml DI water
• 56.1 g Ammonium Fluoride (1.5 M NH₄F)
• 29.23 EthyleneDiamineTetraAcetic acid (0.1 M EDTA)
• Bring to volume with DI water.

Mehlich III Working Solution

In 10 L carboy (enough for 40 samples)

• 8 l DI Water
• 100 ml Mehlich III Stock
• 115 ml Acetic acid (CH₃COOH)
• 82 ml 10% Nitric acid (HNO₃)
• Bring to volume (10 liters) with DI water.

Procedure

Add the following to 125 ml Erlenmeyer flask

• 10 g Oven-dried equivalent sieved soil (field fresh or air-dried)
• ~75 ml Ammonium Acetate (1 M NH₄OAc)
• or
• ~75 ml Mehlich III working solution

Place on orbital shaker (200 rpm) for 30 minutes

Filter suspension with coarse filter paper in a plastic funnel in a 250 ml volumetric flask.

Wash soil THREE successive ~50 ml aliquots of NH₄OAc or Mehlich III.

Be careful not to pour any sediment into the funnel.

Bring to 250 ml volume.

Pour ~10 ml of the mixed extract into a 15 ml centrifuge tube with proper identification & extraction type. Centrifuge for 5 minutes @ 300 rpm. Using certified standards, analyze on the ICP-OES within a week stored at 4°C or freeze at -20°C.

Note: The use of NH4OAc may overestimate the in situ exchangeable cations and CEC in acidic soils. This method is to estimate “freely bioavailable” cations and CEC, not the effective CEC under “field” conditions.

Created: 09/11/07 Revised: 10/10/11