Turco points out that although tile drainage systems are a great benefit to farmers in the spring, they can actually hurt the farmer's crops later in the growing season. "You need tile drainage for three to six weeks in the spring because it allows an aerobic root zone for seed development. Beyond that its effectiveness is limited," he says. "When it rains during the dry summer months, the tile systems don't know that it's July and that water is needed. So the water that the crops need may move out of the root zone."
Turco suggests that farmers consider modifying their systems so that they trap water and hold it. "This would both conserve water in the months when it is needed and prevent the nitrates from entering the surface water by creating an anaerobic zone, which would remove nitrates as nitrogen gas," he says. "If you could store water in the tile system when you wanted to, you could effectively irrigate your field from below ground."
A second key factor in nitrate leaching is bare ground, according to Brouder.
"Nitrogen exists in the soil in several different forms," she says. "Nitrogen that comes from organic matter, such as decomposing plant residue, or nitrogen in the form of ammonium fertilizer does not move out of the soil. However, what happens is, if it is warm enough, microbes in the soil convert these forms of nitrogen into nitrate, which can easily be moved through drain tiles and into surface water."
Brouder says that as long as crops are planted on the ground, the crops will take up the nitrates as nutrition. However, when the ground is bare, nitrates produced by the microbes are carried away with heavy rainfalls.
"The length of time that the crops are in the ground is a key factor in
whether excess nitrate is produc
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Contact: Steve Tally
tally@aers.purdue.edu
765-494-9809
Purdue University
1-Jul-1999