REAGENTS

1. Extraction
   1. Ammonium Fluoride: NH₄F
   2. Hydrochloric Acid: HC1
2. Color Development
   1. Ammonium Molybdate: (NH₄)₆ Mo₇O₂₄4H₂O
   2. Antimony Potassium Tartrate: SbKO₇C₄H₄
   3. Sulfuric Acid: H₂SO₄
   4. Ascorbic Acid: C₆H₈O₆

EXTRACTING SOLUTION

1. 0.03 N NH₄F in 0.025 N HC1:
   1. Weigh 1.11 g of ammonium fluoride into a 1 liter volumetric flask containing 500 mL deionized water. Swirl to dissolve.
   2. Add 4.16 mL of 6N HC1.
   3. Dilute to volume with deionized water and mix well. Store in a plastic bottle.

COLOR DEVELOPING SOLUTION

1. Sulfuric Acid, Molybdate, Tartrate, Ascorbic Acid Solution.
   1. Weigh 1.50 g ammonium molybdate into a 1 liter volumetric flask containing 500 mL deionized water.
   2. Add 0.04 g antimony potassium tartrate.
   3. Add 17.5 mL sulfuric acid and swirl to dissolve.
   4. Add 1.32 g ascorbic acid. Dissolve, dilute to volume with deionized water and mix well. Prepare fresh daily.

EXTRACTION PROCEDURE

1. Scoop 2 g soil (1.70 mL) into a 50 mL extraction bottle.
2. Add 10 mL extracting solution.
3. Shake for 5 minutes on a shaker at high speed (250 oscillations per minute.)
4. Filter using Whatman #1 paper.

COLOR DEVELOPING PROCEDURE

1. Pipet 2 mL of the filtrate into a test tube.
2. Add 8 mL of color developing solution and mix well.
3. Let stand for 10 minutes. The color is stable for 2 hours.
4. Analyze on a colorimeter or spectrophotometer set at 882 nm.

CALCULATION

ppm P in extract × 10 = lbs/acre

REFERENCE

1. Bray, R. H., and L. T. Kurtz. 1945. Determination of total, organic, and available form of phosphorus in soil. Soil Soc. 58:39-45.