Uncertainty resulting from constant bulk density assumption when interpreting soil nutrient concentrations

Soil nutrient concentrations are often expressed as parts per million (ppm) in soil test reports. For incorporation into nutrient management decisions, ppm-based concentrations have to be converted into pounds per acre, and a conversion factor (multiplier) of 2.0 is typically recommended universally to do so. However, this conversion factor stems from an assumed value of bulk density (ρ_b) corresponding to silt loam soil and is invariant to any deviation beyond assumed ρ_b. Here, we quantify and evaluate the potential ramifications of assuming a constant ρ_b value on calculating soil nitrogen credits. A true dynamic conversion factor that is sensitive to variation in ρ_b ranges between 1.28 and 2.68 for soils across US cropland. Failure to account for this dynamic conversion factor was shown to result in an underestimation of soil N credits by up to 40%. In addition to spatial variation, management-induced changes in ρ_b are also important to incorporate into the conversion factor.