Extractable Phosphorus

Bray-1 Method

The soil phosphorus measured is that which is extracted by a solution consisting of 0.025 normal HCl and 0.03 normal NH4F, referred to as Bray-1 extractant. A 1 gram scoop of air-dried soil and 10 milliliters of extractant are shaken for 5 minutes. The amount of phosphorus extracted is determined by measuring the intensity of the blue color developed in the filtrate when treated with molybadate- ascorbic acid reagent. The color is measured by a Brinkman PC 900 probe colorimeter at 880 nm. The result is reported in parts per million (ppm) phosphorus (P) in the soil. The phosphorus measured does not represent all of the phosphorus that may be available for plant growth; e.g., some fraction of the organic phosphorus not measured may become available upon mineralization. The upper reporting limit for this test is 100 ppm. For situations such as nutrient management decisions where a higher value is needed, see the nutrient management test below.

 

Olsen Method

For highly calcareous soils (pH greater than 7.4), the Olsen sodium bicarbonate method is used. A 1 gram scoop of air-dried soil and 20 milliliters of 0.5 molar sodium bicarbonate (NaHCO3) solution are shaken for 30 minutes. Blue color in the filtered extract is developed with molybdate- ascorbic acid reagent and measured with the Brinkman PC 900 probe colorimeter at 880 nm. Results are reported as parts per million (ppm) phosphorus (P) in the soil. As with the Bray P-1 test, potentially available organic P is not measured by the test. The upper reporting limit for this test is 50 ppm. For situations such as nutrient management decisions where a higher value is needed, see the nutrient management test below.

Our Methods

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