This test is applicable for situations in which extractable P is likely to exceed 100 ppm with the Bray test and 50 ppm with the Olsen test. The range for the nutrient management P test is 20-250 ppm. This test follows the chemistry of the Olsen P test, but with a dilution of the extract before colorimetric analysis.
A 1-gram scoop of soil and 20 milliliters of 0.5 M sodium bicarbonate (NaHCO3) solution are shaken for 30 minutes. The mixture is filtered and the filtrate is diluted 5X with sodium bicarbonate. Color is developed with the molybdate-blue/ascorbic acid method. The intensity of blue color, corresponding to the amount of P in the extract, is measured with the Brinkman PC 900 Probe Colorimeter at 880 nm. Results are reported as parts per million (ppm) extractable phosphorus (P) in the soil.