Pioneers of hot KCl extraction to obtain a useful laboratory index of PMN include Øien and Selmer-Olsen (1980), Gianello and Bremner (1986a) and Wang et al. (1996). The method is attractive analytically, as many laboratories are already set up to measure NH4-N in KCl extracts of air-dry soils.
2 M KCl Extracting Solution
As for Method 7C1. Ensure any stored solution is well stoppered to prevent NH3 absorption from the environment.
Other Reagents
As for 7C2a – NH4-N only.
PMNKCl(Ammonium) Standard Solutions
PMNKCl (Ammonium) Primary Standard
1 mL contains 0.20 mg of NH4-N.
Prepare as for 7C2a.
PMNKCl (Ammonium) Secondary Standard
1 mL contains 0.05 mg of NH4-N.
Pipette 125 mL of NH4 Primary Standard into a 500 mL volumetric flask and make to volume with 2 M KCl extracting solution. Freshly prepare when working standards are required.
PMNKCl (Ammonium) Working Standards
Pipette 0, 1.6, 3.2, 4.8, 8.0, 12.0, 16.0, 24.0, 32.0, 40.0 and 60.0 mL of PMNKCl (NH4) Secondary Standard into separate 200 mL volumetric flasks. Make to volume with 2 M KCl extracting solution. These solutions contain 0, 0.4, 0.8, 1.2 … 15.0 mg N/L as NH4, equivalent to soil concentrations of 0, 6.7, 13.3, 20.0, 33.3, 50, 66.7, 100, 133.4, 166.7 and 250 mg/kg PMNKCl -N for a soil/solution ratio of 1:16.67.
Extraction
Weigh 3.0 g of air-dry soil (<2 mm) into 75 mL stoppered digest tubes suitable for use in a precise, temperature-controlled Al digestion block. These digest tubes should be prior calibrated to 50 mL. Add 25 mL of 2 M KCl Extracting Solution and quickly heat at 95°C for 16 h. Cool the extracts to room temperature (≈25°C) as soon as possible then dilute with 2 M KCl Extracting Solution at the same temperature to 50 mL. After dilution, shake each digest tube manually for 15 sec, then filter through a Whatman No. 42 paper in an NH3-free environment. The final soil/solution ratio is 1:16.67.
Analysis
Set up the automated colour, SFA equipment (only for NH4-N) as described for Method 7C2a and as recommended by the manufacturer of the equipment, noting the 1:16.67 final soil/solution ratio. ‘Condition’ the manifold before use, make a final check on instrument settings, then determine NH4-N concentrations in the PMNKCl extracts from relevant working standards run on commencement and as required throughout the batch. Dilute the PMNKCl extracts with 2 M KCl extracting solution (same batch) if N concentrations exceed the top standard. Also use 2 M KCl or deionised water as the wash solution during autoanalysis.
PMNKCl = (Measured PMNKCl – Mineral N from Method 7C2a/b, (or equivalent), with all results expressed as mg N/kg, air-dry.