Add 5 mL of 2% H3BO3 solution to each 50 mL Erlenmeyer flask or beaker, marked to indicate a volume of 30 mL. Prepare the distillation apparatus (see Note 2 of Method 7A1) and arrange the flask under the condenser outlet so that the end of the condenser is just below the surface of the H3BO3 solution.
Transfer a known aliquot of soil extract into a distillation flask and add from 0.2 to 1.0 g of MgO through a long-stemmed dry-powder funnel (see Note 4). Attach the distillation flask to the steam distillation apparatus and immediately commence distillation, by closing the stopcock on steam by-pass tube of the distillation apparatus. Stop the distillation when distillate reaches the 30 mL mark – by opening the stop-cock on steam by-pass tube – and rinse delivery end of the condenser with deionised water. Temperature of the distillate should not exceed 40°C.
Add 5–6 drops of the Mixed Indicator and titrate the distillate with standardised 0.01 M HCl. The end-point is indicated by a colour change from pale green to faint pink (or as pre-tested by the operator). For a potentiometric end-point, titrate to pH 5.0. A reagent blank should be distilled and titrated in a similar manner.
For a Y mL aliquot of a 10 g/100 mL air-dry soil extract and 0.01 M HCl:
NH4-N = [(sample titre – blank titre) × Y × 100 × 0.1401]mg N/kg.
Report exchangeable NH4-N (mg N/kg) 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.
1. Make periodic checks on recovery of the various mineral-N components by substituting relevant aliquots of the two N standard solutions, diluted as necessary with 2 M KCl extracting solution.
2. Field extraction or use of field-moist soil is necessary for accurate estimates of NO2-N and are often preferred for precise studies of soil mineral-N content. Make due allowances for a variable soil/solution ratio and for the additional moisture added.
3. Unfiltered soil extracts based on 2 M KCl can be stored safely up to 24 h at 25°C and for up to 48 h at 4°C. Filtered extracts can be stored for at least 45 days at 25°C and for up to 4 months at ≤4°C.
4. It is not necessary to use accurately weighed amounts of MgO and Devarda’s alloy. Calibrated glass or stainless steel scoops are satisfactory.
5. Best and Craswell (1985) have shown that 2 M KCl soil extracts containing in excess of 0.02 M Mg can interfere in the determination of NO3-N based on use of Devarda’s alloy and subsequent steam distillation. Recoveries can be as low as 25%. The 1:10 soil/solution ratio used in this method overcame the interference on all soils tested. For narrow soil/solution ratios, as might occur with field extraction of moist soil, 0.4 g of Devarda’s alloy for a 20 mL aliquot of the 2 M KCl soil extract was always sufficient to achieve full recoveries.