In a University of Minnesota Extension Crop News e-newsletter, Melissa Wilson and Jeff Vetsch posed the question, “Nitrification inhibitors and manure: Do they work?”

Their answer was, “Yes, sometimes, but not always,” and they pointed to some Minnesota research to explain.

Nitrification inhibitors prevent soil bacteria from converting the ammonium portion of manure nitrogen into nitrate. This cuts the risk of nitrate leaching and denitrification.

Two studies used a nitrification inhibitor called nitrapyrin. In the first study, liquid dairy or swine manure was applied in mid-September, mid-October, and mid-April the following year. Nitrapyrin was added to half of the plots. Corn was planted in May, and soil samples were collected when plants were at the V4 state.

In the second study, liquid swine manure was applied in mid-October or mid-November, some with nitrapyrin and some without. Soil samples and yield data were collected after corn was planted the following spring.

In the first study, nitrate concentrations were elevated by nitrapyrin. Yield only improved at two of the seven sites, though. Corn yields were highest when manure was applied in April, with or without a nitrification inhibitor.

In the second study, which took place over four years, corn grain yields were improved in three of the years when manure was applied with nitrapyrin in October. It only improved two of the years when manure was applied in November. Fields that had manure applied with nitrapyrin in October yielded the same as those that received manure in November, whether or not a nitrification inhibitor was used.

The authors indicated that nitrapyrin helped conserve soil nitrogen from fall-applied manure. As for yields, inhibitors seem to help in October. Application may not be as useful in September, as the inhibitors likely don’t last long enough for soil temperatures to drop. They also seem less effective in November, as soil temperatures in more Northern climates are typically below 50°F and nitrification is dramatically reduced naturally.