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 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 microg 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.
Klatt
J G
JG
Department of Agronomy, Iowa State University, Ames, IA 50011, USA.
Mallarino
A P
AP
Downing
J A
JA
Kopaska
J A
JA
Wittry
D J
DJ
D003247:Conservation of Natural Resources; D004784:Environmental Monitoring; D005618:Fresh Water; D006801:Humans; D007484:Iowa; D010758:Phosphorus / Q000032:analysis*; D012987:Soil / Q000032:analysis*; D012989:Soil Pollutants / Q000032:analysis*; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*
D012987:Soil; D012989:Soil Pollutants; D014874:Water Pollutants, Chemical; D010758:Phosphorus