Sharpening Your Nitrogen Pencil
Greg Stewart, OMAFRA Corn Specialist

A million acres of wheat
Some (oh, okay maybe just one) of my colleagues within OMAF get very excited about the prospect of a million acres of winter wheat in the province. For me, the real excitement is a million acres of improved soil structure, a million acres of red clover, a million acres with excellent options for manure management, and a million acres of higher yielding, less weather-sensitive corn that can be grown the following year in many cases with significantly reduced nitrogen. That much red clover is a bit of a stretch, especially for those who struggle with non-uniform stands across too many fields. However, I still challenge most growers to consider the costs of red clover seeding, the options for taking out crop insurance on the underseeding, and the potential benefits that accrue to the subsequent corn crop. Factor these three things in and red clover can still be viable even if some stands fail to catch uniformly.

Table 1. Average corn yield across 13 sites, and N recommendations based on actual yields and nitrogen applied, yield goals and credits, and the PSNT.
Corn Yield (bu/ac) Recommended
N RateRecommended
N Rate
by Yield Goal
less N Credits
Mean 157

N credits, soil nitrate testing, and environmental responsibility
Back in the mid-1990s we did some work evaluating the most suitable nitrogen rate for corn after a red clover plowdown.

At the time we were interested in examining how well a pre-sidedress nitrogen test (PSNT) could predict the most economical N rates and we also monitored the nitrate left over in the soil after corn harvest. This residual nitrogen is most vulnerable to leaching during the overwinter period. Of the 13 demonstration sites in this project, 10 were tilled the previous fall (6 were moldboard plowed, 1 chisel plowed, 2 disced, and 1 tilled using an Aerway); 2 were spring moldboard plowed; 1 was no-till. At ten of the thirteen sites, co-operating farmers described red clover stands to be either average, good or excellent.

Table 2. Post-corn harvest soil nitrate levels (NO3 in ppm) at the 0-30 cm soil depth interval at three side-dress nitrogen rates averaged across 13 sites where corn followed red clover.
Nitrogen Rate Applied to Corn
(lb N per acre)
Soil Nitrate Nitrogen PPM
Source: T. Vyn et al. 1996, U of Guelph

Table 1 indicates the average yield and various nitrogen recommendations based on the actual yield response to applied nitrogen as well as the rates from general recommendations (yield goal less N credits) and the PSNT. In this study the PSNT did an excellent job of predicting that required nitrogen fertilizer would be quite low across these sites.

Table 2 indicates that in the situations where nitrogen was applied at rates heavier than required, there was significantly more nitrogen remaining after the harvest. This not only means money spent on nitrogen fertilizer that was not required, but also leaves more nitrogen at risk of moving into surface and groundwater.

Soil N Testing
Work with the PSNT has shown some potential for it as a decent indicator of nitrogen needs within a cash crop system.

However, corn producers who have cropping systems that include legumes and manures can realize a much greater benefit. Here the potential reductions in nitrogen use can be flagged by PSNT values. Within NMAN (OMAF nutrient management software program) you will also be able to input soil nitrate test values to serve as a tool to refine nitrogen rate applications.

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