The accuracy of phosphorus (P) fertilizer recommendations can be improved by knowing the soil buffering capacity. Field estimates of the change in Mehlich‐3 P per unit of P applied after one year, termed the Field Mehlich‐3 P Buffer Coefficient (M3PBC), were available from five Ultisols and five Oxisols. Specific surface area (SSA), clay content, and iron (Fe) and aluminum (Al) extractable by both citrate‐bicarbonate‐dithionite (CBD) and oxalate (Oxa) were determined on these soils and the results related to the Field M3PBC. The Field M3PBC was highly correlated linearly with SSA and clay content. Also, Field M3PBC was related well with CBDFe, CBDAl, and their summation. Neither OxaFe, OxaAl, nor their summation was correlated with Field M3PBC. The relationship between Field M3PBC and these factors was exponential. Specific surface area and clay gave the best relationships, but Field M3PBC could also be predicted from the CBD summation of Fe and Al, CBDFe alone, or CBDAl alone, in that order. Knowledge of CBD‐extractable Fe and/or Al can be used to establish the rate of fertilizer required to increase the Mehlich‐3 P to a sufficient level on Ultisols and Oxisols. An estimate of CBDFe is available from soil survey data and it should be applicable within a region.