The salicylic modification to ensure recovery of NO3-N and NO2-N during Kjeldahl digestion was introduced by Cope (1916). Nitro-compounds are initially formed when soils containing NO3-N react with salicylic acid/H2SO4. These nitro-compounds are subsequently reduced to corresponding amino compounds by heating and mixing with sodium thiosulfate (Na2S2O3.5H2O), prior to conventional Kjeldahl digestion/distillation.
Bremner and Mulvaney (1982) reported similar recoveries of NO3-N and NO2-N during Kjeldahl digestion to those obtained by a permanganate-reduced iron procedure (not described). Use of purified KNO3 and NaNO3 standard solutions, however, are recommended as a check on recoveries.
Soils justifying use of this method include those that have recently received significant inputs of N fertiliser and/or sewage sludge, feed-lot manure and the like. Soils used repeatedly for vegetable production are also eligible, because of likely elevated soil NO3– concentrations (e.g. Rayment 1994).
Catalyst Mixture
As for Method 7A1.
Sulfuric–Salicylic Acid Digest Mixture
Dissolve 33.3 g salicylic acid (HO.C6H4.COOH) in 1.0 L H2SO4 (18 M).
Sodium Thiosulfate
Use fine crystals of Na2S2O3.5H2O.
60% Sodium Hydroxide Solution
As for Method 7A1.
2% Boric Acid Solution
As for Method 7A1.
Bromocresol Green–Methyl Red Mixed Indicator
As for Method 7A1.
0.010 M Hydrochloric Acid
As for Method 7A1.
Digest and distil a reagent blank with each batch of assays. Weigh 0.200 g of ground (<0.5 mm) air-dry soil into a Kjeldahl flask (see Note 1, Method 7A1). Add 8.0 mL Sulfuric–salicylic Acid Digest Mixture and allow to stand with occasional shaking for 1 h. Stopper digestion flask (stopper should be contamination free) and leave overnight.
Next day, add 0.5 g Na2S2O3.5H2O through a long-neck funnel and heat gently for about 5 min. Remove from heat and when cool, add 2.2 g catalyst mixture (or the equivalent in tablet form) and complete both digestion and distillation as for Method 7A1.
For a 0.200 g air-dry sample and 0.010 M HCl on the total digest:
%TSN = [(sampletitre – blank titre) × 0.07]
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.