Method 55bi Determination of extractable ammoniumN

Reagents.

• Ammonium-N standard solution, 140 pg ml-1 nitrogen - weigh 0.661 g ammonium sulphate (dried at 105°C for 1 h and cooled in a desiccator) into a 100-ml beaker and dissolve in ammonia-free water (distil deionized water acidified with sulphuric acid), transfer with washings to a 1-l volumetric flask and make up to the mark with the ammonia-free water and mix. This should be stored in a refrigerator, but a quantity allowed to warm to room temperature in a stoppered container before use.

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

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

• 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.

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

• Potassium chloride, 2 M - dissolve 745.51 g KCl in water, and make up to 5 l.

Procedure. Steam is passed through the Markham steam distillation apparatus for 20-30 min. Check the performance by pipetting 5 ml ammonium-N standard solution into the 25 ml reservoir of the distillation unit, add 1 drop octan-2-ol, 6 ml magnesium hydroxide suspension and steam distil the released 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. A blank distillation/titration is carried out using 5 ml ammonia-free water and subtracted from the standard titre to give a result which should be 5.00 ml.

Pipette a 25 ml (or y ml, where y ^ 50 ml) aliquot of the soil extract in KCl into the distillation apparatus and proceed as above. If the titre (s) lies outside of the range 0.2-7.0 ml, adjust the volume of extract accordingly. Repeat using a similar aliquot of KCl extractant solution to give a blank titre (b).

Calculation. Subtract the blank from the soil titre, multiply the difference by 700 and divide by y, to get the mg kg-1 ammonium-N, potassium chloride extractable, in air-dry soil. Express results for oven-dry soil as in Method 5.2, Calculation (2).

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