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/M3-COL 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 (R 2 = 0.14–0.32). The BP test measured significantly less P in a CaCO3–affected soil (pH 8.1). The R 2 of the relationship between M3-ICP and M3-COL was 0.84, and was 0.89 between the M3-COL 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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