Comparison of soil Mehlich-3 phosphorus quantified by colorimetry and inductively coupled plasma: A case study for temperate agroecosystems

Phosphorus (P) is one of the most limiting essential nutrients for agricultural production and its availability to crops is assessed by various methods. Mehlich-3, however, remains the most used method worldwide. For decades, the colorimetric method by blue ascorbic acid-molybdate reaction has been used to determine soil P concentration following Mehlich-3 extraction. Since early 1990s, the use of automated methods to quantify soil nutrients including P has expanded rapidly, and the inductively coupled plasma (ICP) emission spectroscopy is becoming one of the most popular instruments in routine soil testing. The main objective of this study was to compare ICP (where M3P is Mehlich-3 P, M3P-ICP) with colorimetric (M3P-Col) methods to estimate soil P using data from soil samples (3020) collected between 2005 and 2021 from 16 experiments conducted under different agroecosystems in Canada and Europe. Five case studies were assessed: (1) laboratory incubation, (2) native lowbush blueberry, (3) soil depth, (4) soil tillage, and (5) annual field crops versus perennial forage. In each study, a regression equation was established between soil M3P-ICP and M3P-Col. Results indicated that the two methods were strongly related in all studies (0.82 < r² < 0.99; p < 0.001), where soil P measured by ICP (2.1–352 mg kg⁻¹) was higher than that measured by colorimetry (0.6–339 mg kg⁻¹) except for the incubation study. Most important P differences were observed with forage and blueberry. Further analysis revealed that large differences between M3P-ICP and M3P-Col occurred primarily due to soil total C content. Soil pH, clay and Fe content, and previous crops also affected the relationship.