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

J. G. Klatt, A. P. Mallarino, J. A. Downing, J. A. Kopaska, D. J. Wittry

Research output: Contribution to journalArticlepeer-review

41 Scopus citations

Abstract

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). Moldboardand 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.

Original languageEnglish (US)
Pages (from-to)2140-2149
Number of pages10
JournalJournal of Environmental Quality
Volume32
Issue number6
DOIs
StatePublished - 2003
Externally publishedYes

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