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Conservation drainage cuts nitrate loss

Some call it conservation drainage. It’s a relatively new practice, especially in the Midwest, that’s designed to help farmers keep crop yields up and at the same time cut the amount of nitrates leaving crop fields through underground drainage tile.

Conservation drainage cuts nitrate loss

Some call it conservation drainage. It’s a relatively new practice, especially in the Midwest, that’s designed to help farmers keep crop yields up and at the same time cut the amount of nitrates leaving crop fields through underground drainage tile.

Called drainage water management by the USDA Natural Resources Conservation Service, the practice holds more water in the soil within the crop field at certain times of the year. Holding that water reduces nitrate impacts in downstream waters — as tile water volume drops, so does nitrate loss, in similar proportion.

The in-line water control structures, manufactured and sold by Agri Drain near Adair, use stop logs or other ways of regulating water flow. When you want a higher water table, you put in more logs to raise the height of the water within the small structure, and that raises the water table to that level. When you want more complete drainage, you take the logs out to return field conditions to a more fully drained condition.

Key Points

In-line structures control water in the soil profile in tiled fields.

Conservation drainage reduces nitrate loading downstream.

Method controls water levels to maintain or increase yields.

In the typical situation, after harvest, stop logs are put in to raise the water table over the winter, keeping water in the soil in the field and reducing nitrate loss through tile.

The logs are taken out before field operations in the spring — it usually takes two or three days to draw the water down in the soil profile. After planting, depending on moisture conditions, the logs are put back in to raise the water table again and make more water available for the crop.

Before harvest, the logs are taken out again to have soils more dry for field operations. Farmers who have used the structures say it takes only a few minutes to take the small stop logs out or put them back in.

Matt Helmers, an Iowa State University agricultural engineer, says using an in-line structure can result in a 20% to 40% reduction in total water flow from the area controlled by the structure.

“The nitrate concentration in the water doesn’t change with this practice, but when you hold more water in the subsoil instead of allowing it to drain off, you reduce the total nitrate load in downstream water. The percentage of reduction is the same as the percentage of water you keep in the field.”

Not a new concept

The idea of holding water in the soil profile longer at certain times of the year has been studied for more than 30 years. It began in North Carolina, where researchers thought yields could be increased in semi-dry years with the practice.

Field studies there have found average yield increases of about 5%. Monitoring and computer modeling studies in the Midwest show somewhat more limited yield increases of up to 5%.

Leonard Binstock, who recently retired as executive director of the Agricultural Drainage Management Coalition, or ADMC, has years of experience with demonstration sites in the Midwest using the practice.

“In looking at the data from recent conservation innovation grants, I think there’s a yield advantage to using the in-line structures farther up in the watershed,” he says. “That’s where there’s greater need to retain more water for crops. Those lighter soils will respond with more water holding capacity, and that extra water in the profile will give higher yields. Lower in the watershed, the heavy soils already hold more water.”

The structures are little more than a foot square, and about 4 feet long, the same depth as many tiles are set. They take up only a little space in the field — as much as possible, the structures are placed within tile lines in locations that each one can control 10 to 20 acres of drainage, on average.

Costs can range from $500 to more than $2,000 per structure. For a system, the number of structures and field costs can vary from as little as $20 an acre to as much as $110 an acre or more, depending on slope of the land. Systems are most economical on flatter land, where fewer structures are needed.

Binstock figures a system on well-suited land could cost less than $10 per acre per year over 15 years, and yield increases of 2% or 3% could pay for the system. Check with NRCS to see if Environmental Quality Incentives Program or Conservation Stewardship Program cost-share is available, or with the Farm Service Agency for possible payments in Conservation Reserve Program and Conservation Reserve Enhancement Program riparian buffer programs.

Use in existing or new lines

The control structures are most often retrofitted into existing tile lines — usually lateral tile — but they can also be part of the design of a new tile system.

“I really think new tile systems need to be designed with these control structures in mind, so water can be managed in the future,” says Gary Sands, an agricultural engineer for the University of Minnesota Extension.

Known as Dr. Drainage, Sands has worked extensively with controlled drainage. Three years of data from his research experiments show nitrate-nitrogen loads can be reduced by up to 40% with controlled drainage systems.

There are about 20 demonstration sites that were organized in Minnesota, Iowa, Illinois, Indiana and Ohio through a conservation innovation grant from NRCS that included university partners and others in each state.

Harold Reetz, Binstock’s successor at ADMC, is now continuing to gather data from the grant’s three-year, multi-state effort and encouraging more use of the practice. He’s also involved in a three-year Conservation Technology and Information Center project funded by EPA and NRCS that’s offering incentives to 150 Livingston County, Ill., producers to use new practices to adjust nitrogen management to keep nitrates out of Indian Creek.

Reetz and ADMC are gearing up to educate both producers and the public on the benefits of the in-line control structures, bioreactors, treatment wetlands, winter cover crops and other practices that can be used to reduce nitrates in downstream waters.

Betts writes from Johnston.


MANAGING DRAINAGE: In-line structures in tile lines raise the water table after harvest and planting, and lower it before those field operations, reducing total nitrates that leave the field through tile.

This article published in the March, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.

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