Determination of extractable organic and inorganic soil

Method 11.1. Determination of extractabie organic and inorganic soii P 

Procedure (extraction). Transfer 2.5 g air-dry soil, 2 mm mesh size, into a 250 ml polypropylene screw-cap centrifuge bottle/tube and add 100 ml acetic acid - 8-hydroxyquinoline reagent. Cap the tube and shake overnight (17 h) on a reciprocating shaker, at approximately 275 strokes of 25 mm length per minute at a constant temperature (20°C). Centrifuge for 15 min at 2800 rpm and remove an aliquot for the determination of acid extractable inorganic phosphorus (a). 

Procedure (determination of inorganic phosphate (a) in the acetic acid extract). The 8-hydroxyquinoline forms a precipitate in acidic ammonium molybdate solution, which will interfere unless the aliquot is 5 ml. It should therefore be removed by ignition as follows. Transfer 10 ml acetic acid extract to a 45-ml silica basin, add 0.5 ml 1 M magnesium acetate and evaporate to dryness on a water-bath. (Note do not use magnesium nitrate, which reacts adversely on heating with 8-hydroxyquinoline.) 

Procedure (determination of inorganic phosphate (b) in the alkaline extract). Pipette 25 ml of the alkaline extract into a 50-ml volumetric flask followed by 3.5 ml 1 M HCI from a dispenser. This will neutralize the sodium hydroxide and leave 1 ml excess of acid, causing precipitation of organic matter. Make up to 50 ml and mix. Filter through a dry Whatman No. 44, 9 cm, filter paper 

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