This modified Kjeldahl digestion method can achieve quantitative recovery of all forms of soil N. It is similar to Method 7A3 except it involves an automated colour continuous SFA finish based on the Berthelot indophenol reaction reviewed by Searle (1984). The analytical finish is that of Method 7A2a but with different working standards and wash solution, noting the SFA system outlined is an example based on AutoAnalyzer I/II technology. Slight changes to reagents, flow rates, etc. may be necessary if the laboratory’s SFA system is amenable to faster rates of analysis, lower reagent volumes, micro-bore tubing, speedier reagent change-overs, sample dilution, automatic data handling and the like. See the preamble to Method 7A2a for background information on SFA. Alternatively, FIA technology, outlined in the preamble to Method 7A2b, can be used.
To accommodate the automated colorimetric finish, the modified Kjeldahl digest described in Method 7A3 is diluted to a known volume. The most relevant soils include those recently fertilised with N and those that have received significant applications of sewage sludge, feed-lot manure, etc. Soils used repeatedly for conventional vegetable production are also candidates for inclusion (e.g. Rayment 1994).
Sulfuric–Salicylic Acid Wash Solution
Take 53 mL Sulfuric–salicylic Acid Digest mixture and 3.3 g sodium thiosulfate (Na2S2O3.5H2O) and heat gently for about 5 min, then remove from heat. When cool, add 14.7 g of catalyst mixture and digest to dissolve. Cool and dilute carefully to 1.0 L with deionised water.
Nitrogen Primary Standard
1 mL contains 0.20 mg of N.
As for Method 7A2a, but make to volume (500 mL) with Sulfuric–salicylic Acid Wash solution.
Nitrogen Working Standards
Dispense accurately 0, 1, 2, 3, 4, 6, 10, 15, 20 and 30 mL of N Primary Standard into separate 500 mL volumetric flasks and make to volume with Sulfuric–salicylic Acid Wash solution. These solutions contain 0, 0.4, 0.6 … 12.0 mg N/L. For 0.2 g soil sample in 150 mL of solution (i.e. a 1:750 soil/solution ratio) these standards are equivalent to soil concentrations of 0, 0.03, 0.045 … 0.9% N.
Weigh 0.200 g of ground (<0.5 mm) air-dry soil into a calibrated (e.g. 150 mL) Kjeldahl flask. On completion of digestion as described in Method 7A3, make to volume with deionised water.
Set up and operate autoanalysis equipment as described in Method 7A2a but use the Sulfuric–salicylic Acid Wash solution and the N Working Standards described above. Dilute digest solutions that test in excess of the upper working standard with Sulfuric–salicylic Acid Wash solution or redigest soil using a reduced sample size. If necessary, make similar adaptations for FIA, guided by Method 7A2b.
Report TSN (%) on an oven-dry basis. Use the air-dry moisture to oven-dry moisture ratio to make the oven-dry conversion. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.