PROPERTIES OF ULTISOLS AND OXISOLS RELATED TO MEHLICH-3 PHOSPHORUS BUFFER COEFFICIENTS

The accuracy of phosphorus (P) fertilizer recommendations can be impro ved by knowing the soil buffering capacity. Field estimates of the cha nge in Mehlich-3 P per unit of P applied after one year, termed the Fi eld Mehlich-3 P Buffer Coefficient (M3PBC), were available from five U ltisols and five Oxisols. Specific surface area (SSA), clay content, a nd 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 co rrelated linearly with SSA and clay content. Also, Field M3PBC was rel ated well with CBDFe, CBDAl, and their summation. Neither OxaFe, OxaAl , nor their summation was correlated with Field M3PBC. The relationshi p between Field M3PBC and these factors was exponential. Specific surf ace area and clay gave the best relationships, but Field M3PBC could a lso 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 t he Mehlich-3 P to a sufficient level on Ultisols and Oxisols. An estim ate of CBDFe is available from soil survey data and it should be appli cable within a region.