Proceed as for 7C1c. Report NH4-N + NO3-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.