This is an alternative to Method 15I1 when the laboratory has suitable autoanalysis equipment. The method is based on that described by Blakemore et al. (1987). It involves the automated colorimetric determination of NH4+ in the 1 M NaCl leachates from Methods 15D1 and 15D2.
Dichloro-S-triazine Solution
As for Method 7A2.
Sodium Nitroprusside Solution
As for Method 7A2.
Sodium Salicylate Solution
As for Method 7A2.
Citrate-Tartrate Reagent
As for Method 7A2.
Wash Solution
Combine 1 part 60% Aqueous Ethanol (Method 15A2) with 24 parts of 1 M NaCl Displacement Solution (Method 15D1) and mix well. Reagents should be from the same batches used during the extraction process.
Auto CEC Ammonium Primary Standard
1 L contains 200 mmolc of NH4-N.
Dissolve 16.009 g ammonium nitrate (NH4NO3; previously dried at 100°C for 1 h) in wash solution and make volume to 1 L in a volumetric flask.
Auto CEC Ammonium Working Standards
Dispense 2.5, 5.0, 7.5, 10.0, 15.0 and 20.0 mL Auto CEC Ammonium-N Primary Standard into 500 mL volumetric flasks and bring to volume with Wash Solution. These solutions contain 1.0, 2.0, 3.0, 4.0, 6.0 and 8.0 mmolcNH4-N/L. For 5.0 g soil made to 250 mL, direct soil strengths (CEC) of standards are 5.0, 10.0, 15.0, 20, 30 and 40 cmolc/kg.
In Method 15D1 and 15D2 the exchange complex of the soil was saturated with NH4+ that was then extracted by 1 M NaCl. The NaCl extracts were retained for determination of CEC.
Transfer an aliquot of each NaCl leachate into a sample tube and load the sampler. Avoid contact of these solutions with NH3 fumes.
Set up the autoanalysis apparatus using the manifold and the procedure described in Method 7A2. The heating bath is essential as variations in temperature can affect the sensitivity of the analysis.
Prior to commencement of every run, circulate reagents and wash the solution for at least 20 min, ‘condition’ the manifold and check instrument settings by pumping the highest working standard required for several minutes. Run standards and unknowns at a constant rate (usually 60/h) with a 5:1 sample/wash ratio. Samples containing >8 mmolc/L should be diluted with wash solution as necessary.
Determine the concentration of NH4-N by comparison with calibration curves (or from a regression equation) prepared from standard solutions run on commencement, after at least every 30 soil leachates, and upon completion.
CEC (cmolc/kg = [NH4-N measured – NH4-N blank]
Report CEC (cmolc/kg), expressed on an oven-dry basis. Use the air-dry moisture to oven-dry moisture ratio to convert to an oven-dry concentration. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.