Method 59c Determination of resin extractable phosphorus automated method

The extraction method of Hislop and Cooke (1968), has been outlined in Chapter 4, 'Phosphate extractants'. A blank determination without soil should be carried out. The autoanalysis manifold is shown in Fig. 5.4. Some adjustments to dilution and/or readout sensitivity may be necessary to handle both

Table 5.1. Shoulder colour code for peristaltic pump tubing.

Colour code

Orange red

Orange blue

Orange green

Orange yellow

Orange white

Black

Orange

White

Grey

Yellow

Yellow blue

Blue

Green

Purple

Purple black

Purple orange

Purple white

Delivery (ml min-1)

0.03 0.05 0.10 0.16 0.23 0.32 0.42 0.60 0.80 1.00 1.20 1.40 1.60 2.00 2.50 2.90 3.40 3.90

agricultural (lower P) and glasshouse (higher P) soils. The above authors referred to P2O5, but we have converted values to P.

Reagents.

• Ammonium molybdate - sulphuric acid stock reagent - dissolve 10 g powdered ammonium molybdate in approximately 70 ml water and dilute to 100 ml. Carefully add 150 ml sulphuric acid, 98% m/m H2SO4, to 150 ml water in a 600/800 ml beaker while stirring with a glass rod, and allow to cool. Add the molybdate solution with careful stirring and allow to cool.

• Ammonium molybdate - sulphuric acid autoanalysis reagent - dilute 100 ml of the ammonium molybdate - sulphuric acid stock reagent to 1 l with water and mix.

• Phosphorus stock standard solution, 1 mg P ml-1 - Dry potassium dihy-drogen orthophosphate at 102°C for 1 h and cool in a desiccator. Dissolve 0.879 g of the dried salt in water and add 1 ml of hydrochloric acid, approximately 36% m/m HCl. Dilute to 200 ml and add 1 drop of toluene to the solution.

• Phosphorus intermediate standard solution, 100 pg ml-1 - pipette 50 ml of the phosphorus stock standard solution, 1 mg ml-1, into a 500-ml volumetric flask, make up to the mark with sodium sulphate extractant and mix. Add 1 drop of toluene to the solution.

• Phosphorus working standard solutions, 0-35 pg P ml-1 - prepare fresh daily solutions by pipetting 0, 5, 10, 15, 20, 25, 30 and 35 ml of the phos-

To sampler wash

Heating To waste Bath

To chart recorder

Heating To waste Bath

To chart recorder

Fig. 5.4. Manifold for the automated determination of phosphorus in soil resin extracts.

Sampler 40 h

Pump

Sampler 40 h air molybdate sample ascorbic acid sod. sulphate wash waste flowcell waste

Fig. 5.4. Manifold for the automated determination of phosphorus in soil resin extracts.

phorus intermediate standard solution, 100 pg ml-1, into 100-ml volumetric flasks, make up to the mark with sodium sulphate extractant, and mix. These will contain 0, 5, 10, 15, 20, 25, 30 and 35 pg P ml-1 respectively, and are suitable for glasshouse soils that are approximately x8 higher in P than agricultural soils; a lower range of 0, 1, 2, 3, 4, and 5 pg P ml-1 should be prepared for the latter.

Calculation. The 2-ml scoop of soil was extracted via resin into 50 ml sodium sulphate extractant; therefore the concentration must be multiplied by 25 to give the pg P ml-1 in soil by resin extraction. Hislop and Cooke (1968) classified the soils with respect to mg P l-1 air-dry soil as follows:

• agricultural soils: low, <28; medium, 28-65; high, >65

• glasshouse soils: low, <305; medium, 305-436; high, >436.

ADAS have indexed resin P values as follows:

0, 0-19; 1, 20-30; 2, 31-49; 3, 50-85; 4, 86-132, 5, >132 mg P l-1.

Method 5.10. 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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