Field Calibration for Corn of the Mehlich-3 Soil Phosphorus Test with Colorimetric and Inductively Coupled Plasma Emission Spectroscopy Determination Methods

Use of the Mehlich-3 soil extractant with an inductively coupled plasma emission spectroscopy (ICP) determination method (M3-ICP) is displacing the original colorimetry-based test (M3-COL). Current interpretations do not distinguish between these two versions, although the M3-ICP test often measures more P. This study correlated these tests and the Bray-P1 (BP) test with corn (Zea mays L.) yield response at 59 Iowa locations (78 site-yr). The mean P measured by the M3-ICP, M3-COL, and BP tests was 31, 19, and 17 mg P kg^(-1), respectively. The M3-ICP/M3COL ratio decreased exponentially with increasing soil P (P=<0.01) but their difference was not correlated with soil P. Relative or absolute differences tended to decrease linearly (P=<0.01) with increasing soil pH or organic C, but the strength of the relationship was poor (R2 = 0.14-0.32). The BP test measured significantly less P in a CaCO3-affected soil (pH 8.1). The R2 of the relationship between M3-ICP and M3-COL was 0.84, and was 0.89 between the M3COL and BP (0.97 excluding the site with pH 8.1). Critical concentrations defined by Cate-Nelson and linear-plateau models for the M3-ICP, M3-COL, and BP tests were 20 to 32, 16 to 21, and 13 to 20 mg kg^(-1), respectively. The M3-COL and M3-ICP are equally effective for Iowa soils but interpretations differ. The M3-ICP test should be considered a different test and its interpretations should be based on field calibrations rather than conversions based on M3-COL data. A range of 25 to 35 mg kg^(-1) for the M3-ICP test would correspond to the optimum class (16-20 mg kg^(-1)) used in Iowa for the M3-COL and BP tests.