10,000 lbs
An average Iowa soil contains about 10,000 lbs of nitrogen/acre in organic matter. This nitrogen isn’t susceptible to rapid loss. Farmers apply about 150 lbs/acre of nitrogen to corn fields to increase growth. Farmers do not apply nitrogen to soybean fields. Corn removes about 100 lbs/acre during harvest when the yield is about 175 bushels/acre. Soybeans remove about 175 lbs/acre during harvest when the yield is about 55 bushels/acre.
Plant Uptake
On average, corn or soybeans lose 30 lbs/acre of nitrogen in the form of nitrate to leaching each year when crops aren’t present. More nitrate loss happens in the spring in Iowa because soils are warm and wet, often above water holding capacity and there is no crop present to take up the nitrogen. Soils also are susceptible in fall after harvest.
In field management
Nitrogen fertilizer use is not the primary reason for nitrate loss, nevertheless improved nitrogen fertilizer management is important and can contribute to reducing loss.
- Most nitrate loss to Iowa waterways is caused by a mismatch in timing between nitrate production from soil organic matter and nitrate demand from rapidly growing drops. The majority of nitrate is lost when there is little crop growth (in April- June).
- Nitrate loss from corn and soybean crops is the same despite the fact that fertilizer is applied to corn and not applied to soybean.
Land use/In field
Land use practices can reduce nitrate loss by 10-85 percent
- Add cover crops to corn-soybean rotations
- Extend crop rotations and pasture rotations
- Incorporate energy crops
Land use/In field
Cover Crops
Cover crops prevent soil erosion, improve nutrient cycling, sustain soils, and protect the environment. Adding cover crops to corn/soybean rotations is a long term investment in soil health. Field research has shown that when cover crops grow well they can reduce the nitrate lost in tile drainage with corn and soybean production in the range of 30-50 percent.
Land use/In field
Extended Crop
Rotations
Alternating the crops grown from season to season is known as crop rotation. In Iowa, this is most often a corn-soybean rotation. However, diversifying the rotation by adding a third crop such as alfalfa has been shown to reduce nutrient loss and the benefits can be multiplied; alfalfa supports an increase in livestock production and soil quality can be improved.
Land use/In field
Energy Crops
Energy crops, used to make biofuels, can be planted to restore nitrogen to soils. Incorporating energy crops, such as switchgrass and Miscanthus, can help keep soil, water and nutrients in the ground.
Edge of field
Edge-of-field practices can reduce nitrate loss by 30-50 percent. They include:
- Building wetlands
- Installing saturated buffers
- Constructing bioreactors
- Creating drainage water management
Edge of field
Wetlands
In a wetland, contaminants and sediment are filtered from the water and nitrates are removed through denitrification. Wetlands targeted to intercept water from heavily tiled cropland can improve water quality and provide habitat for wildlife. Research shows that wetlands are very effective in reducing nitrate loads across a broad range of conditions.
Edge of field
Saturated Buffer
Saturated buffers contain underground “plumbing” to intercept water in the field tiles and redirect it into a buffer of grass, trees, and other plantings that can process nitrate and keep it out of adjacent streams.
- Buffers remove sediment, phosphorus and pesticides and provides wildlife habitat above ground
- Nitrate is removed through denitrification and plant uptake by the roots below the surface
Edge of field
Bioreactor
Bioreactors function by routing drainage water through a buried trench filled with woodchips. Denitrification occurs when microbes living on the woodchips convert nitrate to nitrogen gas. Because bioreactors tend to have an orientation that is long and narrow, they fit well in edge-of-field buffer strips and grassed areas.
Edge of field
Drainage Water Management
Drainage water management is the practice of installing a water control structure in a drain tile which allows you to vary the depth of the drainage outlet. Raising the outlet level causes the water table to rise to the level of the outlet, storing water in the soil.
