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 d i hydrogen 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

Pump

Sampler 40 h-1

Sampler 40 h-1

To sampler wash Heating To waste

2.50 ml min-1 molybdate

2.00 ml min-1 sample

0.32 ml min-1 ascorbic acid

2.50 ml min-1 sod. sulphate

2.90 ml min-1 wash waste

3.40 ml min-1 flowcell waste

To chart recorder

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

1.20mlmin-1 air

2.50 ml min-1 molybdate

2.00 ml min-1 sample

0.32 ml min-1 ascorbic acid

2.50 ml min-1 sod. sulphate

2.90 ml min-1 wash waste

3.40 ml min-1 flowcell waste

To chart recorder

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.

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