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TECHNICAL REPORTS

Landscape and Watershed Processes

Soil Phosphorus, Management Practices, and Their Relationship to Phosphorus Delivery in the Iowa Clear Lake Agricultural Watershed

^a Department of Agronomy, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011
^b Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011

^* Corresponding author (apmallar{at}iastate.edu).

Received for publication August 9, 2002. Clear Lake is on Iowa's list of impaired water bodies because of high P concentration. This study assessed soil-test phosphorus (STP), management practices, and P loads from its agricultural watershed. Management practice histories and STP for eight basins were surveyed in 1999. Soil samples (15-cm depth) were analyzed for STP with agronomic [Bray P₁ (BP), Olsen (OP), Mehlich 3 (M3P)] and environmental [iron oxide–impregnated paper (FeP) and water extraction (WP)] tests. Total phosphorus (TP) concentrations in water discharge from five basins were measured during two years, and TP loads were measured for two basins. The agronomic P tests showed that 46 to 83% (depending on the test) of the area tested above optimum for crops. Correlations among tests were high for OP, M3P, and FeP (r > 0.96) and lower for BP and WP (r = 0.88–0.93). Moldboard- and chisel-plow tillage predominated (82% of the area). Applied P (mainly fertilizer) averaged 15 kg P ha^-1 yr^-1, and 40% of the high-testing area (M3P test) was being fertilized. The mean annual water TP concentration across five basins was 275 to 474 µg L^-1. The two-year mean TP loads for the two gauged basins were 1504 and 1510 g P ha^-1 yr^-1. Water TP concentration increased linearly with increasing STP. Relationships were stronger for M3P and FeP (R² = 0.96–0.97 for annual means and 0.77–0.79 for storm-flow events) than for BP or WP (R² = 0.88–0.91 and 0.59–0.69, respectively). Improving P and soil conservation practices in high-testing areas could reduce P loads to the lake.

Abbreviations: BP, Bray-P₁ phosphorus • FeP, phosphorus determined with iron oxide–impregnated paper • M3P, Mehlich-3 phosphorus • OP, Olsen phosphorus • STP, soil-test phosphorus • WP, water-extractable phosphorus • TP, water total phosphorus

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