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

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 P 1 (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 ( R 2 = 0.96–0.97 for annual means and 0.77–0.79 for storm‐flow events) than for BP or WP ( R 2 = 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.