Rapid spectrophotometric measurement with a microplate reader for determining phosphorus in NaHCO3 soil extracts

Phosphorus (P) in soil extracted with 0.5 mol L−1 NaHCO3 (pH 8.5) (Olsen-P) is used to evaluate crop P requirement and fertilizer recommendation. A microplate reader with 48- and 96-well plate format was proposed to determine Olsen-P, based on molybdenum blue -ascorbic acid (MA) and malachite green (MG) colorimetric method. The results indicated that the sensitivity of Olsen-P analysis was 2.271 and 3.891 mg L−1 P per absorbance unit and the limit of detection (LOD) was 0.015 and 0.029 mg L−1 for the MA colorimetric measurement with the 48 and 96-well microplate, respectively. The corresponding sensitivity values for the MG colorimetric measurement with the 48 and 96-well microplate were 0.331 and 0.623 mg L−1 P per absorbance unit and the LOD values were 0.002 and 0.007 mg L−1, respectively. Soil Olsen-P concentrations obtained by the microplate reader with the MA and MG colorimetric method were significantly correlated with the conventional spectrometer method with R2 > 0.99 and linear slope near 1.0. The relative standard deviation (RSD) of the MA colorimetric method was 3.0%–23.2% with the 96-well microplate and 0.9%–16.7% with the 48-well microplate. For the MG colorimetric method, the RSD values of the 96 and 48-well configuration were 3.3%–37.6% and 2.3%–17.1%, respectively. The RSD with the 48 -well plate was less than that with the 96-well plate regardless of the MA and MG method. The microplate reader with the MA and MG colorimetric techniques could be used for rapidly measuring P in NaHCO3 soil extracts with high throughput.