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TECHNICAL REPORTS

Waste Management

Evaluation of Phosphorus Source Coefficients as Predictors of Runoff Phosphorus Concentrations

^a USDA-ARS, Environ. Management and Byproducts Utilization Lab., Beltsville, MD 20705-2325
^b Dep. Environ. Sci. and Technol., Univ. of Maryland, College Park, MD 20742-5821

^* Corresponding author (Matt.Smith{at}ars.usda.gov).

Received for publication December 27, 2007. Many states have adopted a P site index (PSI) as a risk assessment tool to determine when P-based nutrient management is required for a given agricultural field. Some PSIs use a weighting factor, the phosphorus source coefficient (PSC), to account for differences in P solubility between organic P sources. Information relating to appropriate values of PSC for various organic P sources is limited. The objectives of this study were to determine PSCs for organic P sources and to examine the relationship between PSCs and P concentrations measured in simulated rainfall runoff. An incubation study was used to calculate PSCs based on the extractability of P from organic P sources (separated and unseparated liquid dairy manure, digested dairy manure, dairy manure solids, poultry litter, and compost) relative to P from triple superphosphate fertilizer. The PSCs from the 14-d incubations were the best predictors of runoff P after 14 d soil equilibration in the runoff boxes. The values for iron-oxide strip phosphorus (FeO-P) PSC ranged from 78% for compost to 28% for poultry litter and were significantly related to runoff DR-P (r² = 0.80***) and FeO-P (r² = 0.76***) during the 14-d runoff event. Mehlich 3 PSCs ranged from 59% for compost to 30% for unseparated dairy manure and were better predictors of DR-P and FeO-P during the 56-d event (r² = 0.73*** and 0.65***, respectively). The results of this study indicate that PSCs based on soil incubations may improve the ability of PSCs to predict the risk of runoff transport, particularly after manure incorporation.

Abbreviations: BARC, Henry A. Wallace Beltsville Agricultural Research Center • DI-H₂O, deionized water • DR-P, dissolved reactive phosphorus • FeO-P, iron-oxide strip phosphorus • M3-P, Mehlich 3 phosphorus • PAN, plant-available nitrogen • PSC, phosphorus source coefficient • PSI, P site index • TK-P, total Kjeldahl phosphorus • TSP, triple superphosphate • TS, total solids • WS-P, water soluble phosphorus