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Nitrification inhibitors slow conversion of ammonium to nitrate. This helps reduce nitrogen losses because ammonium is stored safely in the soil but nitrate is susceptible to significant losses under excessively wet conditions. Nitrate is mobile in soil and can move below the root zone with excessive moisture, especially on sandy soils. In water-logged soils, nitrate is susceptible to being converted to gaseous forms that escape to the atmosphere.
The nitrification inhibitor dicyandiamide (DCD) has been evaluated in the United States since the 1970’s. The National Fertilizer Development Center at TVA led the first coordinated effort to evaluate DCD in the United States and held a symposium in 1981 to review early results from DCD evaluations. DCD and nitrapyrin (N-Serve®) are now considered the old reliable nitrification inhibitors for slowing the conversion of ammonium nitrogen, the form in anhydrous ammonia, to nitrate nitrogen (1). Examples of delayed conversion of ammonium to nitrate by DCD and nitrapyrin are presented in Figure 1 below. The nitrogen rate was 150 lb per acre. The DCD rate was 11 lb per acre and the nitrapyrin rate was 0.5 lb active ingredient per acre. DCD contains 66 percent nitrogen so DCD supplied 5 out of the total of 150 lb nitrogen per acre.
Figure 1. Percent ammonium remaining in soil at Monmouth and Brownstown, IL (2)
So if DCD is an old reliable what is the innovation? Current formulations of DCD are for use with liquid and granular fertilizers and most evaluations of DCD have been with liquid and granular fertilizers. Exactrix is testing a new system for co-injecting DCD with anhydrous ammonia. Concentrating DCD in ammonia bands is expected to delay conversion ammonium to nitrate more than when DCD is used with liquid or granular fertilizers. This is partly due to the DCD concentration effect and partly due to conversion of ammonium to nitrate being relatively slow in ammonia bands, even without a nitrification inhibitor.
Benefits from Nitrification Inhibitors
The obvious benefit of nitrification inhibitors is that nitrogen losses are reduced when excessive moisture conditions occur. If excessive moisture conditions do not occur, then nitrogen losses are not reduced. That means that nitrification inhibitors are more beneficial in more humid areas where the probability of excessive moisture conditions is higher. Nitrification inhibitors are more beneficial in highly permeable soils with rapid downward movement of water and in soils that tend to get waterlogged. Nitrification inhibitors are also more beneficial when no “insurance N” is applied to compensate for when excessive moisture conditions occur. With no “insurance N”, yields can be reduced significantly when excessive moisture conditions cause nitrate losses.
Generally, the potential for nitrogen losses due to excessive moisture are greater with increasing time between application and when the plant takes up the nitrogen. For example, nitrate losses tend to be higher with fall and early spring application than late spring or sidedress application for corn. Nitrification inhibitors can significantly reduce nitrate losses and associated yield reductions from nitrogen applied in late fall or early spring. An example of a nitrification inhibitor helping avoid a large yield reduction due to nitrogen losses from preplant nitrogen is illustrated in Figure 2 below. These data were obtained on a sandy irrigated soil in Minnesota. Much more nitrogen was required to achieve optimum yield with the preplant than sidedress application. The nitrification inhibitor greatly alleviated yield losses due to nitrogen losses from the preplant application. A nitrification inhibitor will have less pronounced effects on yield when the potential for nitrogen loss is less.
Providing more flexibility for time of nitrogen application is important to many crop producers. The timing of nitrogen application has to be balanced with numerous other considerations such other competing field operations. For example, timely planting is such a critical factor that nitrogen may be applied earlier than optimum for reducing nitrogen losses in order to increase the probability of achieving timely planting. Early spring and fall applications reduce competition with other field operations. Also, nitrogen prices are also often lower in the fall and soil compaction often is less of a problem in the fall than spring. Providing more flexibility for time of nitrogen application is an important benefit of nitrification inhibitors. However, in situations with large potential for nitrogen loss due to excess moisture, nitrification inhibitors often are not a total substitute for applying nitrogen close to the time of crop need.
Figure 2. Effects of nitrogen application time and nitrification inhibitor on corn yield response to nitrogen on a sandy irrigated soil (3).
Nitrification inhibitors provide environmental benefits. In situations with significant potential for movement of nitrate below the root zone, nitrification inhibitors can reduce the amount of nitrogen that ends up in the ground and surface waters. Another benefit of nitrification inhibitors is that they reduce nitrogen loss in the form of nitrous oxide gas (N2O) which is roughly 300 times more potent than carbon dioxide as a greenhouse gas. This is likely to become a more important consideration as more emphasis is placed on reducing the level of greenhouse gases in the atmosphere in an effort to slow global climate change.
DCD is marketed in the United States under the trade name Guardian®. A maximum of 5 lb Guardian® per acre is recommended for banded applications in general (4). Less than 5 lb DCD per acre may be required when DCD is co-injected into anhydrous ammonia bands with the Exactrix system. This is because the Exactrix system injects the anhydrous ammonia stream at approximately 350 psi into a tight band well below the opener outlet. We will be testing this concept.
The rate of Guardian® to use is largely an economic consideration although the environmental benefits from reduced nitrogen losses are also important. The web-based Guardian spreadsheet provides a framework for evaluating economics (4). In the example provided, the Guardian® cost is $6.55 per acre. That means that with corn prices at current levels, an average yield increase of at least one bushel per acre over years would cover the Guardian® cost. Anything over that is profit. Requirement for less applied nitrogen also factors into the economics. The average yield increase over years likely will be significantly more than one bushel per acre for early spring and fall applications in the more humid part of the United States. That assumes that significant “insurance N” is not applied. Savings from application of less nitrogen also need to be considered.
The bottom line is that soil is a terrible place to store nitrogen but practical considerations often dictate that nitrogen be applied significantly in advance of nitrogen uptake by the crop. We believe that DCD co-injected with anhydrous ammonia using the Exactrix system will be a practical option for providing greater flexibility of time of nitrogen application and at the same time avoid significant yield losses due to nitrogen losses associated with excessive moisture.