Acid-Fluoride extracts for Soil Phosphorus (Bray's):
- Ammonium fluoride (NH4F), 1N; Dissolve 37g of NH4F in distilled water and dilute the solution to 1L. Store this solution in a polyethylene bottle.
- Hydrochloric Acid (HCL), 0.5N: Dilute 20.2ml of concentrated HCL to a volume of 500ml with distilled water.
- Extracting solution: To a 500ml volumetric flask filled about half way with water, add 15ml of 1.0N NH4F and 25ml of 0.5N HCL and bring to volume with DI water. This gives a solution of 0.03N NH4F and 0.025N HCL.
Weigh 5g of sample into an extraction container, and pour in 25ml of extracting solution. Shake for 1 minute and filter the contents through Whatman No. 42 paper. Fill "Alpkem" sample cups with filtrate. Samples are ready for analysis.
If you are in a "field" situation, after shaking you can let the samples settle. Then pipet of a portion from the top of the extraction container into a sample cup.
Note: The amount of soil, extracting solution and shaking time can all vary to suit the P content of your soil. Just be consistant with your samples.
Dickman and Bray (1940)
Bray and Kurtz (1945) Determination of total, organic, and available forms of phosphorus in soils. Soil Sci. 59:39-45