This discussion focuses on testing garden soil for phosphorus content using DIY methods. Two primary testing methods are highlighted: the Bray-1 Method and the Olsen Method. The Bray-1 Method utilizes a 0.025 normal HCl and 0.03 normal NH4F solution to extract phosphorus, while the Olsen Method employs a 0.5 molar sodium bicarbonate solution for highly calcareous soils. Both methods involve colorimetric analysis to determine phosphorus levels, with results reported in parts per million (ppm).
PREREQUISITESGardeners, agronomists, soil scientists, and anyone interested in assessing soil nutrient levels for optimal plant growth.
Nutrient Management P
For situations involving soil phosphorus and nutrient management decisions where extractable P is likely to exceed 100 ppm on the Bray test and 50 ppm on the Olsen test. The range for the nutrient management P test is 20-250 ppm. This test is basically the same as the Olsen P test, but with a dilution of the extract before colorimetric analysis. In detail: A 1 gram scoop of soil and 20 milliliters of 0.5 molar sodium bicarbonate (NaHCO3) solution are shaken for 30 minutes. The mixture is filtered and the filtrate is diluted 5 X with sodium bicarbonate. A blue color is developed with successive additions of 1) ammonium molybdate-sulfuric acid solution and then 2) ascorbic acid solution. The intensity of blue color, corresponding to the amount of P in the extract, is measured with a fiberoptic probe colorimeter at 882 nm. Results are reported as parts per million (ppm) extractable phosphorus (P) in the soil.
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 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 ammonium molybdate-hydrochloric acid solution and then aminonaphthol-sulfonic acid solution. The color is measured by an absorption spectrophotometer at 640 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 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 successive additions of an ammonium molybdate-sulfuric acid solution and then an ascorbic acid solution and measured with a fiberoptic probe colorimeter at 882 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.