Method 55bii Determination of extractable nitrateN


• Boric acid solution, approximately 2% m/v - prepare fresh weekly.

• Devarda's alloy - finely ground powder (^ 53 pm).

• Magnesium hydroxide suspension - heat magnesium oxide (heavy) for 2 h at 800°C. After cooling in a desiccator, make a suspension of 17 g in 100 ml water. Alternatively use MgO powder.

• Mixed indicator - dissolve 0.3 g methyl red and 0.2 g methylene blue in 250 ml ethanol.

• Nitrate-N standard solution, 0.14 mg ml-1 nitrate-N - dissolve 1.011 g potassium nitrate (oven-dry) in water, transfer with washings to a 1-l volumetric flask, make up to the mark and mix.

• Octan-2-ol - antifoam agent: use 1 drop when flasks < 150-250 ml capacity are used.

Procedure. Pipette 5 ml nitrate-N standard solution into the distillation flask, add 1 drop octan-2-ol, approximately 0.5 g Devarda's alloy, 6 ml magnesium hydroxide suspension (or 0.5 g MgO), and steam-distil the ammonia into 5-ml boric acid solution in a 100-ml conical flask. After approximately 40 ml distillate has been collected over a 5-min period, wash the tip of the condenser into the distillate, add 2-3 drops mixed indicator solution and titrate with 0.005 M H2SO4 until the colour changes from green to purple. Carry out a blank distillation using 5 ml water instead of extract solution and subtract from the standard titration to give a difference of 5.0 ml.

After determination of extractable ammonium-N (5.4b.i), add approximately 0.5 g Devarda's alloy and distil as above. Adjust the size of the aliquot of extract if the titre lies outside the range 0.2-7.0 ml.

Calculation. Subtract the blank titre from that of the soil extract, multiply by 700 and divide by y (see Method 5.5b.i) to give the mg kg-1 nitrate-N, potassium chloride extractable, in the air-dry soil. Express results for oven-dry soil as in Method 5.2, Calculation (2). Sum the extractable ammonium-N and nitrate-N to obtain the total mineralizable nitrogen in the soil.

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