A New Method for Turbidity Removal and Decolorization in Determination of Available Phosphorus Using Sodium Bicarbonate Leaching

The determination of available phosphorus in soil is crucial for evaluating soil fertility and guiding rational fertilization. However, traditional sodium bicarbonate extraction methods encounter significant challenges when dealing with soils rich in organic matter which the filtrate of soil was a relatively dark color or turbid, which affected the detection results. This study aimed to develop a novel decolorization and turbidity-removal method for sodium bicarbonate extraction solutions, ensuring accurate determination of soil available phosphorus content in high-organic-matter soils. The new method integrated the ion-exchange enhancement of sodium chloride on soil colloids and the oxidative decolorization of sodium hypochlorite. The effects of sodium chloride concentration in the sodium bicarbonate solution, sodium hypochlorite dosage, decolorization time, and color-development stabilization time were systematically optimized. Results indicated that a 2% sodium chloride concentration in the sodium bicarbonate extraction solution rendered the filtrate clear. When 2 drops of 1∶1 sodium hypochlorite were added to the filtrate and the standing time was within 15 minutes, the color of the filtrate was removed, which did not interfere with the color change of the pH adjustment indicator, and the detection result was accurate. Within 0.75 to 6 hours of color development, the detection result remained stable. When this method was used to determine the content of available phosphorus in three soil available state component analysis reference materials and two quality control reference materials, the results were all within the standard value range, the relative standard deviation (RSD, n=4) was less than 3.40%, and the absolute error was less than 1.5mg/kg. In conclusion, the newly developed method effectively addressed the turbidity and color issues of sodium bicarbonate extracts, eliminating detection interferences. With high accuracy, reliability, and ease of operation, it provided a practical solution.