Farmers Have Tools to Limit Nitrogen Loss

Nitrogen is often the most yield-limiting factor in non-leguminous crops.

The fertilizer is relatively expensive, but the yield penalty for underapplying can be even worse for profitability.

“Facing uncertainty, farmers are going to choose to overapply nitrogen fertilizer,” said Charlie White, a Penn State professor of soil fertility.

He spoke at the Penn State Climate Solutions Symposium on May 15 at The Penn Stater.

White and colleagues recently updated a fertility calculator that accounts for soil organic matter, which is an important source of nitrogen for the crop.

Fertilizer only needs to make up the difference between soil-available nitrogen and the requirements for the yield goal. It doesn’t need to provide nitrogen for the whole yield, White said.

Adding in data such as soil texture, the tool creates a response curve predicting the effect of various nitrogen application rates.

Another tool, the pre-sidedress nitrate test, is used when the corn is 12 inches tall. Its predictions can give farmers the confidence to cut back sidedress nitrogen that is unlikely to improve yield, White said.

At the end of the season, farmers can use the corn stalk nitrate test to check whether they applied the right amount of nitrogen.

All of these tests need to be updated every 20 to 30 years to reflect changes in farming practices.

White and his team are revamping the corn stalk nitrate test now, having recently updated the other tests.

When farmers apply nitrogen, they can use products, such as nitrification inhibitors or polymer coatings, designed to reduce nitrogen loss.

The polymer is a plastic that can leave tiny residue in the field, so the manufacturer is looking at alternatives, White said.

Farmers can also shift when manure is applied in the spring.

Farmers often kill their cover crop and then apply manure before planting corn.

But this late spring slug of nitrogen from the decomposing cover crop and the fertilizer may be more than the plants can use, said Heather Karsten, a Penn State crop production professor.

Penn State researchers see promise in applying manure first, in early April, and killing the cover crop later.

During an early manure application, cool temperatures are limiting microbial activity and the cover crop is growing rapidly.

These conditions limit the risk of nitrogen being lost as nitrous oxide, a potent greenhouse gas, Karsten said.

By the time the cover crop is killed, it will have stored up elevated levels of nitrogen, which could be released over time through decomposition at a rate more suitable to crop needs, she said.