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Journal of Environmental Quality Abstract - Landscape and Watershed Processes

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


This article in JEQ

  1. Vol. 32 No. 6, p. 2140-2149
    Received: Aug 9, 2002

    * Corresponding author(s): apmallar@iastate.edu
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  1. J. G. Klatta,
  2. A. P. Mallarino *a,
  3. J. A. Downingb,
  4. J. A. Kopaskab and
  5. D. J. Wittrya
  1. 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


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 (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.

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Copyright © 2003. American Society of Agronomy, Crop Science Society of America, Soil Science SocietyASA, CSSA, SSSA