51ĀŅĀ×

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 4Ā°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 4Ā°C or freeze at -20Ā°C.

Note: The use of NH4OAc may overestimate the in situ exchangeable cations and CEC in acidic soils. This method is to estimate ā€œfreely bioavailableā€ cations and CEC, not the effective CEC under ā€œfieldā€ conditions.

Created: 08/23/10 Revised: 10/10/11