Manure is a valuable asset to livestock farmers who grow crops. It provides nutrients — nitrogen, phosphorus and potash — and helps maintain or increase soil health. One of manure’s challenges is maintaining its nutrients in the rootzone and available for crop uptake. Nitrogen is the least stable of the major crop nutrients and therefore, after it is field applied, manure’s N is the most difficult nutrient to maintain within the crop rootzone.

Manure provides N for crop utilization in two forms, both as ammonia N and the N within organic matter. The N within organic matter is made available for crop uptake over time as the organic material breaks down and the N is mineralized, generally over a 3-year period. The bacteria actively breaking down organic matter and releasing N are more active during the warm summer months, when the need for N is greatest. Because organic N is slowly released over an extended time period and when plant uptake is the highest, there is less concern with organic N availability. 

The ammonia in manure is quite different. In soil, ammonia is in the ammonium form (NH₄). The positive charged NH₄ securely binds to the negative charged soil particles but this bond may be rather temporary. When soils are warm (above 50 F) the nitrification process quickly converts NH₄ to nitrite (NO₂) and nitrate (NO₃). Nitrite and nitrate no longer bind to soil particles and are subject to leaching as rain and/or irrigation water moves through the soil profile. In addition, if soils become saturated during extremely wet periods NO₃ then becomes subject to denitrification and may be lost to the atmosphere as atmospheric nitrogen (N₂) and nitrous oxide (N₂O).

For more information on the N cycle see the Michigan State University Extension news article: What is reactive N and why should I care?

Nitrification inhibitors act by slowing down the nitrification process with the goal of maintaining a higher percentage of the soil N as securely bound NH₄. N-Serve and Instinct are both products of Dow Agrisciences LLC containing the nitrification inhibitor nitrapyrin. When mixed with commercial fertilizers containing ammonia, or with manure, nitrapyrin acts by controlling the population of the bacteria responsible for initiating the nitrification process. N-Serve is an oil based product and is not easily mixed with manure. Instinct on the other hand is a water-soluble micro encapsulated product that may be easily used with liquid manure. The question then becomes does nitrapyrin maintain additional N in the rootzone and available for crop uptake?

In 2009 and 2010 Iowa State University researchers Kyveryga and Blackmer used precision agriculture technologies to compare fall applied swine manure with nitrification inhibitor to swine manure without a nitrification inhibitor. Using GPS site specific technologies along with combine yield monitors at harvest, the Iowa State researchers were able to use field length treatment plots at multiple locations across the state. In 2009 there were 11 cooperating sites and in 2010 there were 15 sites. Manure was applied from early November to early December each year with most applications taking place in the last two weeks of November.

Kyvenyga and Blackmer report that in 2009 the addition of nitrification inhibitor had no effect on grain yield or late season N status of the stalk. In 2010 the researchers report there was a 50% chance of the nitrification inhibitor providing a positive economic return (increasing yield equal to or greater than the additional cost of the nitrification inhibitor), an effect they recognize as possibly due to random chance. They also report that in 2010 nitrification inhibitor had no effect on early season soil nitrate test or late-season stalk N status. The Iowa State researchers also report an unusually high amount of rainfall across Iowa in June and July 2010.

It appears that a positive yield response to the addition of nitrification inhibitors during manure application may be weather dependent. In a 15-year study comparing fall applied anhydrous ammonia to fall applied anhydrous ammonia with nitrification inhibitor and spring applied anhydrous ammonia, Minnesota researchers Randall and Vetsch (2001) report the addition of nitrification inhibitor had a positive yield response in 7 out of the 15 years. In the Minnesota trial a positive response to nitrification inhibitor was associated with years in which there were either a warm November or warm and wet April, May and June. While the Minnesota study involves fall applied anhydrous ammonia rather than manure, it does indicate conditions where a positive response to nitrapyrin added to fall applied ammonia, either manure ammonia or commercial anhydrous ammonia, may be expected.

Farmers considering the use of nitrification inhibitors should consider the products as insurance against unfavorable weather conditions and not as products with a positive yield response under normal conditions.