From Des Moines to the Gulf of Mexico, water pollution from nitrates leaving agricultural fields has become a big problem for growers. According to the University of Illinois Extension, national goals set to reduce the size of the Gulf’s hypoxic zone require a 45% reduction in both nitrogen (N) and phosphorus (P) loads across the Mississippi River Basin.
To help growers meet that goal, the university has released a booklet for those with tile drainage: “Ten Ways to Reduce Nitrogen Loads from Drained Cropland in the Midwest.”
The 10 steps are broken down into three categories: practices that reduce nitrate in the plant root zone, practices that reduce delivery of nitrate to the field’s edge, and practices that remove nitrogen at the edge of the field or downstream.
Here’s a brief summary of the 10 steps and some action steps growers can take:
Practices that Reduce Nitrate in the Plant Root Zone
1. Improved Nitrogen Management
Also known as the 4Rs, by using the right N source and applying it at the right rate, the right time and in the right place, these steps help reduce nitrates by either reducing the amount of N available and/or by increasing the plant uptake.
2. Winter Cover Crops
Cover crops reduce nitrate loss by increasing N and water uptake outside the annual cropping season. While the amount of nitrates reduced by cover crops largely varies by location, establishment of the cover and the amount of growth it achieves, covers are more likely to reduce nitrate loads for high organic matter, poorly drained soils.
3. Increasing Perennials in the Cropping System
The university says that farm ground planted with annual row crops have much higher nitrate losses, with continuous corn generally seeing the greatest nitrate concentrations and losses. Multiple years of alfalfa and grass sod crops, on the other hand, see the lowest nitrate concentrations and losses. “Perennial grass crops are the least ‘leaky’ cropping system because they absorb N whenever soil mineralization is occurring,” the authors say.
Practices that Reduce Delivery of Nitrate to the Field’s Edge
4. Drainage Water Management (Controlled Drainage)
By using adjustable, flow-retarding structures placed in the drainage system that allow the outlet level to be adjusted, tile drainage is able to hold more water in the soil profile. Nitrate loads can be reduced anywhere from 15-75% through this practice.
5. Reduced Drainage Intensity
Installing tiles closer to the surface or spaced wider apart helps reduce drain flow, thereby reducing the delivery of nitrate to the field’s edge. Research shows nitrate loss increases with higher drainage intensities, so decreasing the spacing between tiles typically results in higher nitrate losses, the university says. And by using a shallower drainage depth, the soil profile will be wetter but still provide for crops needs. See the illustration on page 26 of the booklet for a visual example.
6. Recycling Drainage Water
This is the practice of storing water in a pond or reservoir, then returning it to the soil through irrigation during dry periods. The benefits of this system appear promising: researchers say it has the potential to completely reduce drainage flow to surface waters, thus completely reducing nitrate loads.
Practices that Remove Nitrogen at the Edge of the Field or Downstream
These are trenches filled with a carbon source, usually woodchips, where drainage water is routed through for denitrification. Sources report that bioreactors have been able to remove anywhere from 12-98% of nitrates, but the average expected reduction is 30-40%. Iowa no-tiller Rob Stout discovered his bioreactor was removing 97% of nitrates in its first few months. You can learn more about his experience with bioreactors in the article, “Halting Nutrient Runoff While Perfecting Fertility.”
These ecosystems consisting of plants, soil, bacteria and water primarily remove nitrates through denitrification, with plant uptake and flow reduction providing additional benefits. They’re generally more efficient at nitrate removal during warm months and under lower flow conditions, with the average annual nitrate removal at 20-50%.
9. Alternative Open-Ditch Design
An open-ditch design combines the benefits of natural ecosystems with the ability to convey drainage waters from fields. An example would be a two-stage ditch, which creates a zone of plants and soil within the ditch that has the capacity to absorb part of the nitrate load through both uptake and denitrification. An illustration of a two-stage ditch is available on page 37.
10. Saturated Buffers
These buffers consist of shallow, perforated drain pipe that extends laterally along riparian buffers and is connected to the main drainage. Drainage water seeps from the perforated pipe through the riparian zone where existing vegetation can uptake both water and nitrates.
For more information on these 10 practices, you can download the booklet or order a printed copy of it. To learn how to reduce P loads in tile drainage, see the article, “6 Tips for Keeping ‘P’ in Fields and Out of Local Water Sources.”