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Soil moisture managed with tiles

For an additional $50 per acre, farmers installing subsurface drainage tiles can manage the level of subsurface moisture in a field and reduce the amount of nitrates escaping into rivers and streams.

Drainage tile systems, which cost about $500 an acre to install, “aerate the soil to provide a proper root environment and allow producers to access the field,” noted Don Pitts, agricultural engineer, National Resources Conservation Service in Illinois. Pitts spoke at InfoAg 2005 in Springfield, Ill., in July. The conference featured talks on site-specific crop and soil management as well as computer systems for agriculture.

The problem with current tile system designs is that they drain fields 12 months of the year, noted Pitts. “In most of the drain tiles in the Midwest, there is no mechanism for managing the flow of water through the drain tiles. They flow whenever there is moisture above the outlet for the drain line.

“To improve management, we need to control when it drains. Drainage occurs at times of the year when there is no need for it.”

The latter could have a negative impact on the environment, according to Pitts.

“Nitrates can enter the drinking supply in local areas. In lakes and reservoirs, nitrogen is a contributor to atrophic conditions. There is also the national concern over Gulf hypoxia.”

Pitts noted that in a typical Illinois stream “the flow-weighted concentrations of nitrogen seem to be increasing over time. The amount of the rain that falls in this area determines the size of the hypoxia zone in the Gulf. Excess nitrogen is the primary cause.

“The primary pathway for water to leave central Illinois is by tile line, and these tile lines have been associated with high nitrate levels in streams,” Pitts said. “This relationship exists in the other Midwest states also. So the problem is that we need drainage for crop production, but it has a significant negative environmental impact.”

Studies show nitrates in tile water at concentrations of 10 to 25 parts per million, according to Pitts. “Surface water is about an order of magnitude less. So whenever we can create conditions where we have surface runoff rather than tile flow, we have lower nitrate rates in streams.”

According to studies on nitrate load based on the month of the year, half of the nitrate load in streams comes during the months from October through March “when we don't really need to have the tiles functioning. So for half of the year, we have the opportunity to manage or reduce the water leaving tile systems and reduce the nitrate loss in fields and leave it in the fields for crop production.”

In 1998, a demonstration project sponsored by NRCS, local water conservation districts and the University of Illinois studied a new design using a control structure in the tile line. The structure consists of boards that are placed at various elevations. Water cannot leave the field until it goes over the top of the board and out.

“This translates into maintaining a depth of water in the field. So instead of water leaving by the tile line, we have more surface runoff which has low nitrate concentration.”

At 50 sites where the boards are being used, nitrate load was reduced by at least half, according to Pitts.

Here's how the system could work for farmers, according to Pitts. “After harvest, the boards are placed in the control box. As the water rises, the water table in the field tends to come near the surface. Usually, growers in Illinois don't want water on the surface, so the elevation of the water table is set just below the surface. Then sometime in March, the boards are removed and water is allowed to drain internally from the field so planting can occur.

“In the beginning, growers were worried that they might not be able to get into the field in time. But they've learned that we can get in the field within a week to 10 days with a patterned tile system.

“After planting, you can place the boards back in the box and allow the water table to come up (with rainfall). Then we can lower the water table as crop roots develop. If a situation occurs later where there is no rain, we have reserved some water.”

The new design requires a flat topography and intensively-drained fields that have a lot of tile. “If we have a minimum amount of tile, we run the risk that if we hold the water too high, too long, we can't remove it fast enough.”

The more grade, the less influence the control structure has and the higher the cost “because more control structures will have to be installed in the field.”

Many new tile systems being installed today are low-cost alternatives, according to Pitts. “They run the lateral lines down grade. These can't be managed as well.”

Pitts suggests that lateral lines be laid on the contours. “Think about how flood irrigation systems are designed in the Mid-South. We also have to be able to access the tile structures. That's becoming easier because we have automated control devices that can be programmed to move four times a year at three elevations or on any elevation based on rainfall and other factors.”

Pitts said the control structure, if managed properly, will not restrict root development, compact soil, blow out or silt-in tiles, which were concerns of cooperating growers. The researchers also conducted research on the design's impact on earthworms.

“We did a study with Purdue University on two fields, one managed for high water, the other not managed. We counted earthworms for three years and found that the managed field had more earthworms.”

The bottom line is that once water leaves a field, “you can't get it back,” Pitts said. “So don't let it leave the field until you need it to leave the field.”


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