Field Calibration for Corn and Soybean of a Weak Organic Acid Soil Phosphorus Test Compared with the Mehlich-3, Bray-P1, and Olsen Methods.

Correlating and calibrating P soil-test methods with crop yield response to fertilization enables use of the methods in production agriculture. Weak organic acid extractants have been used to measure soil P for decades, but a new test (H³A) was developed as a component of a Soil Health Assessment tool. The H³A test is utilized by several private laboratories in the Midwest, and includes citric acid, malic acid, oxalic acid, and lithium citrate. This test has not been correlated with crop yield response to P fertilization in Iowa and many other states. This study correlated the H³A, Mehlich-3 (M3), Bray-1P (BP), and Olsen-P (OP) tests with corn (Zea mays L.) and soybean (Glycine max. L.) grain yield response at 24 sites (96 site-years). Colorimetric determination of extracted P was used for all tests. Sites included several soil series with loam to silty clay loam texture, pH acid to neutral (15-cm depth) and were managed with no-till or chisel-plow/disk tillage. Soil-test P (15-cm depth) measured by H³A, OP, M3, and BP were 2 to 17, 2 to 18, 3 to 41, and 3 to 37 mg P kg^-1, respectively. The H³A test correlated linearly with the other tests, but correlated better with OP (R² = 0.82) than with M3 (R² = 0.72) or BP (R² = 0.66). Critical concentrations ranges defined by fitting linear-plateau and quadratic-plateau models to relationships between soil-test values and relative yield response were 7-10, 7-10, 8-12, and 8-10 mg kg⁻¹ for the H³A, OP, M3, and BP tests, respectively. All relationships were statistically significant (P<0.01). Relationships for the H³A test showed a slightly poorer capacity to predict yield response, but the critical concentration range identified can be used as a preliminary interpretation pertaining to P sufficiency for corn and soybean in Iowa or similar soils.