An experiment finished nearly 30 years ago by the United States Department of Agriculture’s Agricultural Research Service, uncovered recently during new study preparations, shows that it’s best to be patient when measuring the movement of nitrates through soil and ground water.
Mark Tomer and Michael Burkart, soil scientists and hydrologists at the USDA’s National Soil Tilth Laboratory in Ames, Iowa, found that nitrate applied during the 1969-74 experiment apparently took nearly 30 years to move through soil and reach a 21-metre-deep water table.
The scientists said this shows that water carrying nitrates can take decades to flow through a watershed’s soil subsurface to a stream and should be studied for longer periods.
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In the original study, conducted on a 74-acre field in western Iowa, fertilizer was applied to soil at three times the normal rate. The resulting soil nitrate concentration was tracked for the next decade.
In 1996, Tomer and Burkart were preparing to monitor ground water for a new experiment when they detected the nitrate 18 m deep in the soil. They confirmed that the nitrate originated from the old experiment by examining ground water flow rates and ages, and by comparing the concentration’s depth with stream flow records.
Leaching of nitrates from agricultural fertilizers has been linked to concerns about drinking water quality and hypoxia, in which water bodies contain low oxygen amounts.
Tomer said farmers are encouraged to use nitrogen more efficiently, but resulting environmental improvements have been difficult to document using studies lasting just two to four years. Conservation practices within a watershed may take several decades to fully effect improvements in ground water quality, he added.
