Mehlich 3 Soil Phosphorus as Determined by Colorimetry and Inductively Coupled Plasma

Inductively coupled plasma (ICP) emission spectroscopy is routinely used in laboratory testing to determine soil P extracted by Mehlich 3 (M3P). The effectiveness of ICP, however, has been questioned, particularly when used to detect soil P. Our objective was to compare ICP (M3P-ICP) with colorimetric (M3P-Col) methods of determining soil M3P. Soil samples (488; 0-20cm deep) were collected from five experimental sites in eastern Canada that varied in soil texture, organic matter (OM) content, pH, and previous use of organic or mineral phosphorus (P) fertilizer and were cropped to annual or perennial species. Soil P content ranged from 9.4 to 189.5mgPkg-1 when ICP was used and from 1.9 to 210.3mgPkg-1 for the colorimetric method. Although there was a significant (R2 = 0.91; P 0.001) relationship between the two methods, soil P measured by ICP was, on average, 36% higher. Large differences between M3P-ICP and M3P-Col occurred primarily for soils under perennial forage species, indicating the importance of organic P. More research is required in eastern Canada to investigate other factors (i.e., physical and chemical soil properties, organic or mineral fertilization type) that may further explain the differences between M3P as determined by ICP and colorimetric methods.