October 2009, Vol. 21, No.10


Researchers Focus on Phosphorus in Soils

When it comes to dairy manure, the difference is in the dryness. Researchers at the U.S. Agricultural Research Service (ARS) Northwest Irrigation and Soils Research Laboratory compared solid dairy manure, commercial fertilizer, and liquid dairy manure and found that the three fertilizers affected phosphorus movement differently.

ARS research showed that soils amended with solid dairy manure had the least amount of phosphorus in the leachate — the liquid that percolates through the soil with irrigation. Liquid dairy manure allowed the most movement of phosphorus through the soil columns tested.

Whether more movement is better depends on the circumstances, said David Tarkalson, an ARS soil scientist and coauthor of the article titled, “Phosphorus Mobility in Soil Columns Treated With Dairy Manures and Commercial Fertilizer.” April Leytem also coauthored the article. In places like Magic Valley, Idaho, where the study was conducted, phosphorus rarely reaches deep groundwater, Tarkalson said. There, surface runoff is phosphorus’s main point of entry into water.

According to Tarkalson, fertilizers, such as the manures, are used to add nutrients, such as nitrogen, to the soil to aid crop development. However, the manure also contains excess phosphorus, which can be conveyed in runoff to waterbodies, especially in confined animal feeding operations such as those found in Idaho’s dairy industry, according to the study.

In water ecosystems, added phosphorus can spur excessive algae growth. Phosphorus can be the limiting nutrient and, when introduced, “it tips the bucket over,” allowing algae to flourish, Tarkalson explained. As the additional algae die, they decompose, consuming the dissolved oxygen found in the water.

The algae also can release other toxic substances while decomposing, he said.

Tarkalson added that there was very little leaching overall in the experiment, even with the liquid manure. Though the study was done with soils native to the Magic Valley region, the findings “definitely have potential for other regions, but more research is needed with other soils,” he said.

The study also found a “strong relationship” between amounts of phosphorus and carbon leaching in the soil. The technical article notes that “the form and quantity of carbon influences phosphorus mobility in soils because of several factors, including microbial activity, organically complexed metals, and coating of phosphorus adsorption sites on clay particles.”

North-Flowing River Flows South for a Few Days

A rare north-flowing river in Florida temporarily flipped direction recently to run the more traditional southerly direction. The St. Johns River, Florida’s longest river, changed direction for several days in late May, according Leroy Pearman, a U.S. Geological Survey hydrologist in Florida.

This event is not uncommon for the distinctive St. Johns. “The St. Johns is one of the laziest rivers in the world,” said Teresa Monson of the St. Johns River Water Management District. The total drop of the river is only 9 m (30 ft) over its 500-km (310-mi) length, dropping about 40 mm/km (1 in./mi). Because it is so “lazy,” the St. Johns actually reverses flow twice a day with the incoming Atlantic Ocean tides.

“This is a common occurrence on tidal controlled streams,” Pearman said, but few rivers feature the “negative flow” as far as 322 km (200 mi) from the source, as the St. Johns does. What made the negative flow event in May noteworthy was its duration, which amounted to several days, according to Pearman.

This event was caused by low pressure that sat for several days off the Florida coast, sending winds and water up the mouth of the St. Johns. “More and more water was pushed upstream each day,” Pearman said.

Monson noted that because the river reverses flow with some frequency, “it is difficult for the river current to flush pollutants.” This can cause concentrations of nutrients, such as nitrogen and phosphorus, to increase, creating disruptive algal blooms in the river ecosystem, according to the river management district’s Web site.

The St. Johns originally was known as “Welaka,” so named by the Timucuan Indians as “river of lakes” because it forms large lakes in central Florida. It is ranked third in length among north-flowing rivers in the United States, starting in marshes south of Melbourne, Fla., and terminating east of Jacksonville, Fla., at the Atlantic Ocean.