51ĀŅĀ×

By Jared L. DeForest

Reagents

ā€œFreely Exchangeable and Bioavailableā€

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.

ā€œBound Elementsā€

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