Notes from the Nitrogen Summit: The unseen nitrogen

The most common problem for farmers and crop consultants when determining an optimal nitrogen (N) rate for corn is that they never truly know if and by how much they “over” applied their N fertilizer. Under-application of N is readily seen in corn plants through classic N deficiency symptoms of yellowing of the midrib on lower leaves, likely resulting in reduced yields (http://www.agronext.iastate.edu/soilfertility/photos/photossdef.html). There are a few metrics used to determine if N was over-applied, such as the corn stalk nitrate test (https://store.extension.iastate.edu/Product/pm1584-pdf) and end-of-season soil nitrate test. These tests require extensive sampling, only pertain to corn, and the relationship between stalk or soil nitrate values and crop yield are not well documented in Wisconsin. What I suggest here is the use of nitrogen use efficiency (NUE) calculations as simple metrics that can be used to understand how efficient you are (and can be) with N fertilizer.

The partial nutrient balance. One way that farmers and crop consultants can “see” the unused nitrogen is through nitrogen use efficiency calculations. There is no one specific calculation that is the true NUE, but instead the term NUE represents a suite of calculations that each have a specific meaning. The simplest calculation is the partial nutrient balance (PNB), which is the amount of N removed in the grain divided by the N applied (http://go.wisc.edu/1467hc). A PNB of 1 (or slightly less) is considered ideal (i.e. system sustainability). If the value is much less than one, then this indicates there are opportunities for improvement. The PNB is a calculation that can be done by all growers on a field-by-field basis as long as they know their yield and the amount of N applied. Based on average N concentrations from corn samples analyzed at the University of Wisconsin Soil and Plant Analysis Laboratory, 0.70 lb-N is removed with 1 bu of corn grain (at 15.5% moisture). Therefore, multiplying 0.70 by the yield (bu/ac) gives an estimate of the amount of N removed. This calculation can also be used to “see” a simple N budget for the field (N input vs. N output) by subtracting the N removal from the N applied; if it is negative, more N was removed than what was applied. If it is positive, this is the amount of N remaining in the soil system. However, a lot of this N can still be tied up in the crop residue and for many systems there is a limit on how close we can get this value to 1. More accurate PNB values can be determined if the actual N concentration of the grain is known.

The value of the zero-N check strip. To get a sense of how much N is available from the soil, we can use the amount of N taken up by the corn plant (ears, stalks, and leaves – the entire above ground biomass) when no fertilizer is applied. This will involve cutting a whole plant at ground level, drying, and analyzing for total N; this is obviously not a standard sample practice. This value (total N uptake in unfertilized corn) can then be subtracted from the N in the above ground biomass from fertilized corn and then divided by the amount of N applied to give us the apparent crop recovery efficiency (RE). It is called the “apparent” crop recovery efficiency because we aren’t using the N taken up in root biomass and we are making assumptions about the fate of the applied N. In this case a value of 1 would indicate that the increase in N uptake from unfertilized to fertilized was the same as the amount of N applied (this is an unrealistic system). Snyder and Bruulsema (2007) provide some context, stating that values of 0.5 to 0.8 represent corn systems under best management practices. So, the value of the zero-N check strip allows you to know how much of your N was “needed” by the crop. This is in contrast to the partial nutrient balance calculation, which doesn’t indicate anything about crop need of the fertilizer, only the N balance of your soil and cropping system.

Recovery efficiency verifies the need of the fertilizer. Different soils will supply different amounts of N. For example, in an N rate trial conducted in 2011, corn fertilzied with 150 lb/ac of N at the Lancaster Agricultural Experiment Station had a RE of 63% while corn grown at the Arlington Agricultural Experiment Station had a RE of 28%. This indicates that would want to thing about altering our N rates at Arlington because less than 30% of the N that was applied was taken up. At Lancaster, we may not be able to improve much as nearly 70% of the N applied as take up by the plant.

Comparing the RE and the PNB. A stark contrast between these two calculations can be seen with irrigated sweet corn in the central sands in 2011. Yields without N were incredibly low, leading to RE of 88%, while the PNB was only 43% (150 lb/ac of N was used in this example). The 43% PNB is quite low, indicating a lot more N is applied to the system than is removed. However, a large percentage of the N applied ended up being taken up by the crop. Thus, for sweet corn on sandy soil, the PNB alone doesn’t tell the whole story regarding efficiency and fate of the N applied.

Overall, you’ll notice that I’ve used a lot of terms like “estimate” and “apparent” throughout the past few paragraphs. These calculations are not meant to be the last word on how efficient you are with your N fertilizer, but simply provide a tool for farmers and crop consultants to assess the current state of their N management. These measures need to be conducted across many growing seasons to get a range of efficiency values across different rainfall patterns. But, if we can make the “unseen” nitrogen “seen”, this will give both farmers and consultants more confidence with continuing their current N management plan or identify fields on which improvement strategies are warranted.

For more information on nitrogen use efficiency calculations see http://go.wisc.edu/1467hc. For more general information on nitrogen in soil check out Soil and Applied Nitrogen (http://www.soils.wisc.edu/extension/pubs/A2519.pdf).