Evaluation of Mehlich 3 as an agri-environmental soil phosphorus test for the Mid-Atlantic United States of America

Laws and guidelines limiting P applications to cropland based on soil P exist in the Mid-Atlantic USA because of water quality concerns. We evaluated Mehlich 3 (M3) as an environmental soil P test using 465 soils typical to the Mid-Atlantic region and found M3-P accurately predicted water soluble P (WSP), desorbable P (Fe oxide strip P [FeO-P]), and total sorbed P (oxalate P). The M3-P saturation ratio (M3 [P/(Al+Fe)]) was linearly related to the well-established oxalate P saturation method (DPSox) and a M3 [P/(Al+Fe)] range of 0.10 to 0.15 corresponded to reported environmental limits for DPSox (25-40%). Rainfall simulation and column leaching studies showed M3 [P/(Al+Fe)] predicted runoff and leachate P concentrations better than M3-P. We suggest consideration of the following approach now used in Delaware for agri-environmental interpretation of M3-P and M3 [P/(Al+Fe)]: (i) Below optimum (crop response likely; M3-P less than or equal to 50 mg kg(-1); M3 [P/(Al+Fe)] < 0.06); (ii) Optimum (economic response to P unlikely, recommendations for P rarely made; M3-P = 51-100 mg kg(-1); M3 [P/(Al+Fe)] = 0.06-0.11); (iii) Above Optimum (soil P will not limit crop yields, no P recommended; M3-P > 100 mg kg(-1); M3 [P/(Al+Fe)] > 0.11); (iv) Environmental (implement improved P management to reduce potential for nonpoint P pollution-in Delaware M3-P > 150 mg kg(-1); M3 [P/(Al+Fe)] > 0.15 is now used). (v) Natural Resource Conservation (no P applied even if the potential water quality impact is low to conserve P, a finite natural resource).