Date of this Version
An index to evaluate the phosphorus (P) pollution potential of agricultural fields was proposed by Lemunyon and Gilbert in 1993. Data from three rainfall simulation studies were used to evaluate the relative importance of the variables in the P index. These studies included plots containing sorghum (Sorghum bicolor L.), winter wheat (Triticum aestivum L.), and corn (Zea mays L.) residues on which chemical fertilizer and composted or noncomposted beef cattle feedlot manure were applied under no-till and disked conditions. The factors of erosion, runoff, soil P level P application source, and method, and rate of P addition were weighted and considered in the index calculation. Measured soil erosion accounted for 18% and 88% of the variability in total and particulate P losses, respectively, indicating the importance of erosion in the loss of sediment bound P. Bioavailable and dissolved P losses were unrelated to measured soil erosion but were influenced by tillage, runoff amount, and P source. The P index of Lemunyon and Gilbert was modified based on the results of these experiments. The correlation coefficient value (r) between total P loss and the modified P index values, was 0.74 as compared to 0.52 for the Lemunyon and Gilbert's P index. Additive index values were more closely correlated with total P loss than the index values calculated by multiplying the weight of each factor. If erosion and runoff are accurately predicted, the P index can serve as a useful tool for identifying sites where transport of P to surface water can be a potential concern.