Bioavailable Base Cations (gravimetrically)

By Jared L. DeForest

This method is preferable over the volumetric method if sample size is high (>36) due to limited number of 250 ml volumetric flasks.

Reagents

In 10 l carboy (enough for 80 samples)

• 770.8g Ammonium Acetate (1 M NH₄OAc; density = 1.0778 g/ml)
• Adjust to pH 7.0.
• Bring to volume with DI water. Store at 4°C.

Ammonium Acetate has a limited shelf-life.

Procedure

Add the following to 125 ml sample cup

• 10 g Oven-dried equivalent sieved soil (field fresh or air-dried)
• 50 ml Ammonium Acetate (1 M NH₄OAc)

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

Filter suspension with coarse filter paper in a plastic funnel in a 125 ml flask.
Wash soil TWO successive ~25 ml aliquots of NH₄OAc.
Be careful not to pour any sediment into the funnel.
Pour into a 125 ml sample cup that is already tarred on a balance.
Add NH₄OAc until weight is 80.0 g (i.e., 74.225 ml).

Pour ~10 ml of the mixed extract into a 15 ml centrifuge tube with proper identification & extraction type. Centrifuge for 5 minutes @ 3000 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: 08/23/10 Revised: 10/10/11