Prediction of Corn and Soybean Grain Yield Response to Phosphorus in Minnesota Using the Haney H3A and Mehlich-III Tests.

Phosphorus, an essential nutrient for crop growth, is supplemented in agronomic systems with various P fertilizers. Various soil tests exist to assess P levels in soil. In Minnesota the Bray-P1 is suggested for use when soil pH is <7.5 and the Olsen P test is suggested when pH is >7.5. The Mehlich-3 test is not suggested for use in Minnesota but has been gaining more attention. In addition, the Haney H3A test is being offered by labs to assess inorganic P availability in soils. The objective of this study was to compared the Bray-P1, Olsen, Mehlich-III, and Haney H3A and determine if the tests correlate to corn (Zea mays L.) and soybean (Glycine max var. Merr.) response to P fertilizer in Minnesota. Replicated strip trials were conducted in farmer fields across Minnesota. Phosphorus fertilizer strips were applied within each field. The strips consisted of 0 or 98 kg P ha-1 and were replicated 3-4 times within each field. Soil samples were collected from an area representing every 12 m from the 0P strip to a depth of 15 cm. Corn and soybean harvest samples were collected every 12m along each strip. Relative yield of the 0 P control was calculated based on the yield of the adjacent strip where P was applied. All soil tests extracted differing levels of P when soil pH was <7.5. When soil pH was >7.5, the Bray-P1 and Haney H3A tests correlated well but tended to underestimate availability due to neutralization of the acid in the extracts from free soil carbonates. The best correlation at high soil pH was between the Olsen and the Mehlich-III test measured by color or ICP. The data indicate that soil pH should be a factor when deciding which soil test to use.