Farm nitrogen (N) fertilization practices are receiving major attention across Canada, stimulated by the combination of very high prices for N materials and concerns over fertilizer-linked nitrous oxide (N2O) emissions – N2O being a powerful greenhouse gas.

The 4R approach promoted by the North American fertilizer industry (reference here) – Right Material, Right Timing, Right Placement and Right Rate – has been featured in many discussions, and rightly so. It’s a good approach.

Of the four Rs, we have good guidance from research on three. The use of urease and nitrification inhibitors and polymer-coated urea means better N-use efficiency in many cases and less N2O. Soil incorporation is usually better than non-incorporation. And split applications, with some N applied near planting time followed by more 4 to 10 weeks later for corn is usually better than all applied at planting.

But on rates of N application for corn, especially in Ontario, there is a lot of uncertainty – too much, in my view.

(A quick note: With apologies to international readers, the following is presented almost entirely using pounds, bushels and acres – for the reason that these remain the main units of fertilizer usage in Canadian farming. My target audience for this column is farmers; soil specialists can find much more detailed information elsewhere.)

The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) has provided an on-line N calculator for corn for many years. There are, in fact, two versions on the web as of early 2023 – the first version here and a newer one here. The two are similar but not the same. The first version considers the comparative prices of nitrogen fertilizer and grain corn in making the recommendation, and calculates significantly lower rates when the N is applied at time of side-dress application versus planting. The newer version ignores prices and does not distinguish between at-planting versus side-dressing. However, it does make N recommendations on a township-by-township basis. Recommended N application rates rise as yield expectation increases with both versions.

Calculations using version one for our farm near Guelph for loam soil, corn after soybeans, average yield of 190 bushels/acre and a price ratio of 8:1 (price per unit of weight for urea versus corn), give recommendations of 128 pounds of N/acre (144 kg/ha) pre-plant applied, and only 102 pounds/acre if the N is all side-dressed in June. Calculator version two gives a recommendation of 116 pounds/acre.

I don’t think our farm situation is unique. Most Ontario farmers would likely consider these rates too low.

At a couple of winter 2023 farm meetings, a respected industry agronomist asked two audiences of at least 200 farmers how many were using the OMAFRA calculator recommendations. About a dozen hands went up each time.

It would be easy to conclude, therefore, that the OMAFRA calculators underestimate N application needs for most farms – “need” being defined as the most economical (profitable) rate while taking into account judgements about the large and largely unpredictable uncertainty that exists in seasonal crop needs.

However, those OMAFRA rates – at least for version one – were calculated by Ken Janovicek (research analyst, University of Guelph) and Greg Stewart (then OMAFRA corn specialist), both known for their thoroughness, based on a detailed analysis of hundreds of N-response research trials across Southern Ontario.

The field trials were mostly pre-2000 and it has been suggested that newer corn hybrids are more responsive to higher N rates than what the calculator recommends. However, there are few research data to support this conjecture. In fact, there is good evidence that newer hybrids are more efficient in their use of fertilizer N. See, for example, this study by Ciampitti and Vyn .

A check with US states to the west/south-west of Ontario shows that seven of them, from Michigan to Iowa, use a common approach to corn nitrogen calculations (see here). A review of their recommended rates shows that these are also mostly lower than what many/most Ontario corn growers use. The recommended rates for the seven states do vary according to N/corn price ratios but, surprisingly, are independent of projected yield.

For those interested, an excellent discussion about the rationale behind the Midwest recommendations is available here. (A more comprehensive discussion is available here.)

It would be easy to conclude, based on the above, that the difference between public recommendations and farmer application rates is attributable to farmers applying more than economically needed/justified.

But there is good evidence the other way too. In a well-run, 10-year experiment at the Elora Research Station, Dr. Bill Deen, Dr. Joshua Nasielski, Caleb Niemeyer and colleagues found that the economically optimum N rate was always higher than that recommended by the OMAFRA calculator – often by a substantial amount, and with the gap being greater as amount of pre-silking rainfall increased. The same has been seen in other long-term corn-N-response research experiments at Elora and Ridgetown ON. (There is more discussion on this research below.)

A closer look at the Midwest recommendations also shows some important variances. Here is a graph showing the range in most economical N application rates for Michigan using current new-crop prices for corn and urea, and corn after soybeans.

The average recommended rate is 139 lb/acre of N, but this ranges from about 50 to near 250 lb/acre.

So what’s going on here?

Perhaps it is best to start by taking a look at the total amount of N required to produce a corn crop – almost always substantially higher than the amount recommended for application. I have seen estimates from 200 to 275 pounds/acre of N in the grain, stover and roots of a 200-bushel/acre (12.6 t/ha) corn crop, with the discrepancies generally reflecting differences in measured or assumed protein/N content in all three components.

Let’s say this is 240 pounds/acre on average. Nitrogen will be contained in root exudates as well though there is a good chance that this N becomes available again for seasonal crop uptake after digestion by root-associated micro-organisms.

Also, the crop cannot use the available N with 100% efficiency. If the efficiency is only 2/3, then the total amount of potentially available N needed becomes 360 pounds/per acre. Since nearly all of the N supply comes from fertilizer application and the breakdown (mineralization) of soil organic matter plus previous crop residues, that means that the latter is at least as important as the former.

Of course, a substantial portion (perhaps 1/3 or more) of the total N contained in grain corn plus crop residue (‘stover’) and roots is returned to the soil after harvest. That reduces the net amount of N used in growing the crop. However, all of that N is needed to grow the crop, even if a substantial amount is credited back to soil afterwards.

Note that this is not simply a case of crediting the N in non-harvested parts of the crop in year one as input for year two. For if that was the case, we’d need less N fertilizer for corn when grown after corn than after soybeans – the latter producing much less residue. In reality, it’s the reverse, with less N needed for corn after soybeans. This is a standard consideration with most public N recommendations for growing corn.

The agronomic research literature has plenty of papers on efforts to estimate the amount of N to be supplied by decaying (or ‘mineralized’) soil organic matter and crop residues, and I’ll make no pretense to understand it all nor to summarize it here. However, this information is critically important if farmers and their advisors are to more accurately know how much N will be available from these sources for our current crop – and how much has to be provided by added fertilizer ingredients.

Pre-side-dress soil nitrogen testing (PSNT) is offered as a means for estimating plant-available nitrogen amounts in June in Ontario and nearby states, but I find agronomists to be cautious in its use. There can be major variability in the readings, both spatially and from week to week – as well as in interpreting the results.

Among the public sources of recommendation, as of 2023, there is no adjustment for soil organic matter content in Ontario or the seven states to the west/southwest. Cornell University in New York State provides very detailed recommendations of soil supply for both drained and undrained land for each of more than 600 named soils in the state (available here), but no makes no adjustment for differences in SOM within each soil class.

Greg Stewart has recently introduced an adjustment for differences in soil organic matter in his “Maizex N Tracker” calculator (available as a download here). This is a modified version of the version one of the OMAFRA N calculator described above. Greg’s adjustment is about 12 pounds/acre removed from, or added to, the recommended N application rate for every 1% increase, or decrease, in soil organic matter content.

For a much larger SOM effect, you can check this on-line calculator from Pennsylvania State University (PSU). For our farm’s loam soil (~40% sand, 20% clay) and no cover crops, the calculator says that N need increases by 117 pounds per acre with a decrease in top-soil SOM from 3% to 2%. (PSU cautions that the calculator is still under development. More on the PSU corn N program here.)

I’ll add one more issue – the need/opportunity to adjust the seasonal nitrogen application rate for annual differences in expected yield. In practice, this means adjusting the amount of nitrogen fertilizer to be applied in a late split application (early to mid July) according to the accumulated rainfall to date.

Fortunately, for our farm located near the Elora Research Station with a similar loamy soil, I have the results of an MSc thesis completed recently by Caleb Niemeyer at U of Guelph to use as a guide (available here).

Caleb found high correlations between seasonal rainfall between corn development stages V5 and V12 and both seasonal yields and N required for the full growing season.  This graph shows results for a 10-year study involving continuous corn at Elora. A similar pattern was found in a much larger array of corn-yield/N-response trials at Elora after the year 2000 and at the U Guelph Ridgetown campus from 2009 to 2017. MERN stands for Most Economical Rate of Nitrogen.

Adjustment of seasonal N requirements for this rainfall effect increased the precision of the version-one OMAFRA corn yield calculator from R2 = 0.19 to 0.49. See these graphs below.

Fortunately, late N application is still possible at about stage V12 with modern high-clearance equipment, and Ontario data show no effect of the late application on yield if a sufficient amount of N is also applied at planting time (perhaps 70 to 100 pounds/acre).

The lack of a measured effect of V5-to-V12 rainfall effect on MERN in years before 2000 at Elora is a puzzle. Caleb Niemeyer suggests that it might be caused by greater rainfall variance after 2000 or higher yield levels and vulnerability of higher yields to drought stress.

Unfortunately, we don’t have the same detailed information for most farm soils and locations in the province

A number of models have been developed – and some commercialized – which using soil traits and real-time weather data, to better predict current crop needs and N supply from SOM mineralization. I’ll not attempt to provide an overview, though will note that they mostly get mixed reviews in reports available on the Internet.

One of these, Adapt-N, was developed at Cornell and is in use in Western New York State (including farms a few km from Niagara in Southern Ontario) and the US Midwest. Some references are here and here. Yara now owns the rights to Adapt-N technology. Some quite differing results have been reported as to its possible superiority in predicting best rates of N application in US research (see, for example, here versus here). It is being adapted for use in Southern Ontario by the Ontario-based Deveron company.

It’s not possible for me to give any opinion as to the utility of recommendations provided by Adapt-N or any other modelling approach, but I am trying to learn more, including a 2023 evaluation on our own farm. I am convinced that this is the direction in which grain-crop agriculture must move if we are to do a better job in managing the fourth R for fertilizer N usage in Canada. An interesting new paper by Sulik et al at the University of Guelph also describes the attractiveness of a modelling approach for improving N recommendations for corn.

At a farm information session this winter, a respected Ontario agronomic consultant said that choosing a nitrogen rate for corn is as much about art as science.

She’s right. And we need to change that.

Discussions with many agronomists and soil scientists contributed to the thought process outlined above. However, I would like to acknowledge, especially, comments and advice from Greg Stewart (Maizex Seeds), Caleb Niemeyer (Woodrill Ltd and PhD candidate, University of Guelph) and Dr. John Sulik (Assistant Professor of Plant Agriculture, University of  Guelph). However, none of them bears any responsibility for comments made.