Agricultural Drainage

Bulletin 871-98

Minimizing Nitrate-N Loss

Research has demonstrated a strong linkage between subsurface drainage and nitrate-N losses to surface waters. An obvious, but least economical method to reduce nitrate-N losses is to abandon subsurface drainage systems. The practicality of this method is minimal, however, as crop production would be reduced substantially on millions of acres of productive poorly-drained soils in the Midwest. In addition, sediment and phosphorus concentrations in surface waters would increase. Research conducted in the North Central Region suggests the following strategies would minimize nitrate-N loss to surface waters:

1. Implement wetland restoration areas, denitrifying ponds or managed riparian zones where drainage water could be "treated" to remove excess nitrate-N before discharge into drainage ditches or streams. This may be a cost-effective alternative in portions of the Midwest.

2. Design new subsurface drainage systems or retrofit existing drainage systems to manage soil water and water table levels through controlled drainage or subirrigation, lowering concentrations of nitrate-N in shallow ground water. The cost of retrofitting existing systems for subirrigation can be compared to the benefit of increased yields.

3. The use of alternative cropping systems that contain perennial crops would also likely reduce nitrate-N losses. However, obtaining a market and a satisfactory economic return presents some barriers.

4. The development of improved soil N testing methods to determine the availability of mineralizable N and carryover N from the previous crop would be effective, especially following dry years, legumes, or past manure applications.

5. Fine-tune fertilizer N management. Research shows that applying the correct rate of N at the optimum time would have a substantial effect on reducing nitrate-N losses.

6. Improved management of animal manures would contribute to lower nitrate-N losses in livestock producing areas. Knowing the nutrient content and application rate of the manure, spreading it uniformly, and incorporating it in a timely manner would all lead to better management and confidence in manure N as a nutrient source.