Method 510 Determination of extractable magnesium potassium and sodium

Magnesium, potassium and sodium are extracted from the soil with 1 M ammonium nitrate.

Reagent (extraction).

• Ammonium nitrate, 1 M - dissolve 400 g of ammonium nitrate in water and make up to 5 l.

Procedure (extraction). Transfer 10 ml (scoop filled and struck off level without tapping) of air-dry soil, sieved to ^ 2 mm, into a bottle (e.g. wide-mouth, square HDPE), and shake on a reciprocating shaker (approximately 275 strokes of 25 mm per min) for 30 min. Filter through a Whatman No. 2 filter paper, discard the first few millilitres, and retain the rest for analysis of the required elements. Carry out a blank determination.

Reagents (determination).

• Releasing agent - dissolve 13.4 g lanthanum chloride heptahydrate (LaCl3. 7H2O) in water and make up to 500 ml.

• Magnesium stock standard solution, 1000 pg Mg2+ ml-1 - dissolve 1.6581 g magnesium oxide (previously dried at 105°C overnight and cooled in a desiccator) in the minimum of hydrochloric acid (approximately 5 M). Dilute with water to 1 l in a volumetric flask to obtain a solution of 1000 pg Mg2+ ml-1.

• Magnesium standards, 10 and 0-1 pg Mg2+ ml-1- pipette 5 ml stock solution into a 500-ml volumetric flask and dilute to the mark with M ammonium nitrate reagent to obtain a stock solution of 10 pg Mg2+ ml-1. Pipette 0, 2, 4, 6, 8 and 10 ml of the 10 pg Mg2+ ml-1 stock solution into 100-ml volumetric flasks, add 5 ml releasing agent and make up to the mark with 1 M ammonium nitrate reagent and mix. This will give solutions containing 0, 0.2, 0.4, 0.6, 0.8 and 1.0 pg Mg2+ ml-1.

• Potassium stock standard solution, 1 mg ml-1 of potassium - dry potassium nitrate at 102°C for 1 h and cool in a desiccator. Dissolve 1.293 g of the dried salt in water and add 1 ml of hydrochloric acid (approximately 36% m/m HCl). Dilute to 500 ml and add 1 drop of toluene.

• Potassium working standard solutions, 0-50 pg ml-1 of potassium. Pipette 0, 1, 2, 3, 4 and 5 ml of the potassium stock standard solution into 100ml volumetric flasks, dilute to the mark with 1 M ammonium nitrate solution and mix. These will contain 0, 10, 20, 30, 40 and 50 pg K ml-1.

• Sodium stock standard solution, 1 mg ml-1 of sodium - dry sodium chloride at 105°C for 1 h and cool in a desiccator. Dissolve 0.254 g in water, make up to 100 ml and mix.

• Sodium intermediate standard solution, 20 pg ml-1 of sodium - pipette 10 ml of the sodium stock standard solution into a 500-ml volumetric flask, make up to the mark with 1 M ammonium nitrate reagent and mix.

• Sodium working standard solutions, 0-2 pg ml-1 of sodium - pipette 0, 2, 4, 6, 8 and 10 ml of the sodium intermediate standard solution into a 100-ml volumetric flask, make up to the mark with 1 M ammonium nitrate solution and mix. These will contain 0, 0.4, 0.8, 1.2, 1.6 and 2.0 pg Na+ ml-1.

Procedure (determination). Magnesium is determined by atomic absorption spectrophotometry (see Method 5.2, 'Measurement of calcium and magnesium by AAS').

Potassium and sodium are determined by flame photometry (see Method 5.2 'Measurement of potassium and sodium by flame photometry').

Analyse the standards and adjust the zero and maximum standard readings in the usual way.

• Magnesium: pipette 2 ml sample solution into a 100-ml volumetric flask, add 5 ml releasing agent, make up to the mark with 1 M ammonium nitrate and mix. Nebulize into the AAS and record the readings (computer, chart recorder or manually, as appropriate).

• Potassium: nebulize the extract without further dilution.

• Sodium: pipette 10 ml extract into a 100-ml volumetric flask, make up to the mark with 1 M ammonium nitrate reagent and mix.

Calculations.

1. Magnesium. From the standard graph determine the number of pg ml-1 of magnesium in the sample, subtract the blank value and multiply the difference by 250 (initial extraction ratio of X5 multiplied by subsequent X50 dilution of the extract solution). The result is the number of mg l-1 extract-able magnesium in the air-dry soil. Include any extra dilution factors, and, if required, convert to oven-dry soil using the appropriate factor, as in Method 5.2, Calculation (2).

2. Potassium. From the standard graph determine the number of pg ml-1 of potassium in the sample, subtract the blank value and multiply the difference by 5 (initial extraction ratio). The result is the number of mg l-1 extractable potassium in the air-dry soil. Include any extra dilution factors, and, if required, convert to oven-dry soil using the appropriate factor, as in Method 5.2, Calculation (2).

3. Sodium. From the standard graph determine the number of pg ml-1 of sodium in the sample, subtract the blank value and multiply the difference by 50 (initial extraction ratio of x5 multiplied by subsequent x10 dilution of the extract solution). The result is the number of mg l-1 extractable sodium in the air-dry soil. Include any extra dilution factors, and, if required, convert to oven-dry soil using the appropriate factor, as in Method 5.2, Calculation (2).

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