The Neuse Agricultural Rule mandates that all persons engaging in agricultural operations in the Neuse River Basin shall collectively achieve and maintain a 30% net nitrogen loading reduction. This reduction is to be achieved by a combination of standard best management practices that include riparian buffers, nutrient management plans, and water control structures.

Water Table Management and Nitrogen

Water Table Management

• Drainage is an important component of land management in North Carolina.

• On flat, poorly drained Coastal Plain soils, drainage is necessary for seedbed preparation and planting. Plants stressed by waterlogged soil produce lower yields.

• Crops suffer from drought stress in some years even on traditionally wet soils, with substantially reduced yields in those years.

• Water table management systems provide drainage during wet periods, but utilize water control structures with flashboard risers to manage the water level in the drainage outlet, making it possible to reduce overdrainage and provide subirrigation during dry periods.

Controlled Drainage

• Water control structures allow the water in the drainage outlet to be raised or lowered as needed.

• When the flashboards are lowered or removed, subsurface drainage occurs more quickly.

• When flashboards are added to the riser, the subsurface drainage rate is decreased and the height of the water level in the ditches and surrounding fields rises.

• Managing the field water through the use of controlled drainage allows timely drainage but also maximum storage of water within the field for utilization by the crop.

Nitrogen

• Plants do not use all soil nutrients (whether they are added by natural processes or as fertilizer).

• Excess nutrients (such as some forms of nitrogen) are transported from fields with drainage water.

• Nitrogen transported from drained fields can be minimized by managing the drainage system such that only the minimum drainage water necessary is allowed to exit the field.

• Drainage control reduces the annual transport of total nitrogen at the field edge by ~ 45%.

• Controlled drainage reduces nitrogen transport by three processes.

• It reduces the volume of drainage water leaving a field.

• It provides a higher field water table level that promotes denitrification (conversion of nitrate nitrogen to harmless nitrogen gas) within the soil profile.

• It provides a more optimum moisture environment for the crop that leads to slightly higher yield. Higher yield results in higher nutrient uptake and removal in the harvested crop.

• (Controlled drainage has also been documented to reduce phosphorus transport by roughly 35%).

Management Recommendations

Objectives of successful management of controlled drainage systems:

• Achieve optimum crop production efficiency and maximum nutrient utilization.

• Attain maximum water quality benefits.

• Do not leave water control structures unattended for more than 24 hours during the growing season if the risers are set closer than 18 inches to the surface.

• Maintain the water level within 30 inches of the ground surface along at least 50% of the upstream ditch reach all year to achieve maximum water quality benefit

• Drainage control to reduce nitrogen transport is most effective during winter and early spring periods. Maintain water levels within 12-18 inches of the ground surface in winter.

• Check structure and remove debris after major storms.

• To minimize sloughing of saturated ditch banks, lower water levels one board per day to reach desired water levels for field work (will take approximately 3 to 7 days to reach lowest levels).

General management guidelines to promote water quality while enabling timely field work and optimizing crop yields.

Control setting* management timetable.

*Control setting is the depth of the top of the flashboard risers below the average surface elevation.

• in an unusually dry season, control can be 3 to 6 inches higher;

• in an unusually wet season and during wet periods, control should be adjusted 3 to 6 inches lower;

• in coarse-textured soils, trafficability can be provided with control 6 inches higher.

Prepared by

C. William Zanner, Department of Soil Science Extension Associate,

North Carolina Cooperative Extension Service

Developed from information in these resources:

Evans, R.O., J.W. Gilliam, and R.W. Skaggs. 1991. Controlled drainage management guidelines for improving drainage water quality. North Carolina Cooperative Extension Service. AG-443.

Gilliam, J.W., D.L. Osmond, and R.O. Evans. 1997. Selected Agricultural Best Management Practices to Control Nitrogen in the Neuse River Basin. North Carolina Agricultural Research Service Technical Bulletin 311, North Carolina State University, Raleigh, NC.

Natural Resources Conservation Service. 1998. Conservation Practice Standard: Structure for Water Control.