'Targeted' solution may be best for phosphorus

oil must be kept in field

By Jean Caspers-Simmet

NASHUA, Iowa -- Phosphorus movement into lakes and rivers could be reduced if efforts were targeted at the areas causing the greatest problems, said John Downing, Iowa State University animal ecology professor.

"A lot of people think that we have to limit phosphorus everywhere in watersheds,'' said Downing at the recent Northeast Iowa Agricultural Experiment Association annual meeting at the Nashua research farm. "It's not true.''


Downing has a good idea where the greatest problems are thanks to a geographical information system survey his Limnology Laboratory has conducted of 132 Iowa watersheds. The survey uses computer modeling to look at tiny parcels of land to quantify where the greatest siltation, phosphorus and nitrogen loading occur.

"In most cases, a lot of the loss is concentrated in small areas,'' Downing said.

Studies have shown that 60 percent to 80 percent of phosphorus coming into lakes is from less than 5 percent of the land.

Small solutions

"We're usually talking about very small pieces of land -- highly erodible soils, eroding streambanks and places getting way too much fertilizer,'' Downing said.

Targeting one acre is more cost effective than treating 10 acres upstream. Placing eroding streambanks in permanent vegetation and building small nutrient retention ponds can be effective at slowing sediment and phosphorus loading.

"Not everyone may be comfortable with this because it may be their land that is eroding and causing the problem,'' Downing said.

Downing said phosphorus moving off farmland into water is the problem. Eroded soil particles contain high concentrations of phosphorus. Land in grass keeps soil and phosphorus in place.


"We need to keep soil in the field,'' Downing said.

Iowa has some of the most productive land in the world. Due to soil erosion Iowa lakes are some of the most nutrient rich in the world, Downing said. High levels of phosphorus and sediment cause algae blooms and poor water clarity.

Agricultural runoff isn't responsible for the entire phosphorus problem, Downing said. Urban areas leak phosphorus and rainwater contains increasing levels of phosphorus as does ground water. In fact, 30 percent of the phosphorus problem in Clear Lake comes from particulates in rainfall and from ground water. A study of Iowa water clarity records shows that quality continues to decline, Downing said. In 1896 water clarity was 10 feet at Clear Lake. It declined to 5 feet by 1950, 3 feet by 1970 and was 1 foot in the late 1990s.

Iowa's lakes are important resources, Downing said. Clear Lake generates $200 million in economic activity annually. That's $54,000 for each of the lake's 3,684 acres.

Citizens willing

According to research conducted by ISU's Center for Agriculture and Rural Development, Clear Lake visitors would be willing to pay $150 per year over the next five years to improve the lake.

People living in the watershed would be willing to pay $600 to $700 per year over the next five years.

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