Fertilizer Recommendation Tables - 2007 Revision

2007 Revision

General Recommended Nitrogen Rates for Corn: Corn N Worksheet

In 2005, the Ontario Soil Management Research and Services Committee approved the revision of the General Nitrogen Recommendations for Corn, based on a review of N response trials from 1961-2004.

¹Manure N Credits can be found in the Soil Management Section of Publication 811, or the Nutrient Management Workbook.

*Eastern Ontario includes Frontenac, Renfrew and counties to the east of them

Approved: Ontario Soil Management Research and Services Committee (June 2007).

Explanation of Factors in Worksheet:

Note: The fertilizer rates calculated in this worksheet are designed to produce highest economic yields when accompanied by good or above average management. Research data show that higher rates will occasionally produce higher yields but that these increases in yield are likely to be small and uneconomical.

A. Base N Requirement

Several factors influence the amount of N available from various soil types, including the normal organic matter content, drainage, aeration and temperature. In most of the province, the medium textured soils (silt loams and loams) provided the greatest amount of nitrogen to the corn crop, as indicated by the lower "Base N Requirement" (Table A). In the coarser textured soils, the organic matter content is normally lower, so these soils would not be able to provide as much mineralized N. These soils are also likely to lose more N through leaching during the winter, with the net result of a higher nitrogen requirement. The finer textured soils also have a higher nitrogen requirement, but in this case it will be due to a greater loss of nitrogen through denitrification during the growing season.

The data showed a significantly lower requirement for nitrogen in Eastern Ontario than in the rest of the province, although the reasons for this are not completely clear. Part of the reason could be that soil organic matter contents in the dominant soils in Eastern Ontario are much higher than in comparable soils west of the Frontenac Axis. The mineralogy of the clays in Eastern Ontario is also different from the rest of the province, with a much greater propensity to hold ammonium between the clay layers in the spring, and then release it later in the growing season. The "Base N Requirement" in Table A clearly show the lower N requirements in Eastern Ontario soils.

Because these values are derived from the average responses on a wide range of sites, they will represent the expected requirements for soils with "average" characteristics. Any soil that varies widely from the average for a particular texture class (e.g. much higher or much lower organic matter content) may differ in the optimum N rates for that particular field.

B. Yield Adjustment

While yield did not explain a large proportion of the differences in optimum N rates, there was a clear trend that fields with higher yields had higher nitrogen requirements. Ironically, the yield factor derived from the N response data (13.6 kg N per tonne of yield) is almost exactly equal to the N removal from the field in the grain portion of the crop.

Since we cannot know what yield will be harvested from the crop at the time of fertilizer, average yields for the previous five years should be used to estimate the productive capacity of the field. Entering an inflated yield goal into this adjustment will not increase the productivity of the field, and may result in environmental harm. To estimate silage yield, multiply grain yield by 5.

C. Heat Unit Adjustment

There is a clear trend in the data that corn in the long season areas of the province require more nitrogen than the short season areas. This may be due to greater moisture stress on the crop in areas with higher average temperatures, which would decrease N use efficiency, or it could be related to differences in soil organic matter content.

D. Previous Crop Adjustment

The crop that was grown immediately prior to planting corn has a significant impact on the nitrogen requirements. Crops like grain corn or cereals with the straw left in the field immobilize a significant quantity of mineral N from the soil as the high carbon residue decomposes, and this is reflected in higher N requirements. Forage legume crops fix nitrogen out of the air, that is released to the corn crop as the residue breaks down, resulting in reduced N requirements.

For simplicity, the N recommendations in this table have been stated relative to the preceding crop with the highest N requirement (grain corn), so all of the adjustments for previous crop are in the form of a credit.

E. Price Ratio Adjustment

The optimum N rate is based on the last increment of nitrogen that produces a yield increment with greater value than the cost of the nitrogen. As the cost of nitrogen fertilizer goes up, or the value of the corn crop goes down, the amount of yield required to pay for a kilogram of nitrogen increases. This means that the nitrogen rate that provides the maximum return to added fertilizer is reduced.

The price ratio is calculated by dividing the price of a kilogram of nitrogen into the value of a kilogram of corn. The corn price will need to be estimated for the year following harvest (unless the corn has been pre-sold at a fixed price), and should include all stabilization payments.

F. Preplant N Recommendation

This figure, calculated by summing values A through E, represents the total N requirements for the crop. This is normally supplied by a combination of starter fertilizer, broadcast or side-dressed fertilizer, and manure.

G. Deduct Starter N

Any N that is supplied at planting should be included here. If part of the N was broadcast pre-plant, with the intent of side-dressing the balance, it should also be included in this line.

H. Deduct Manure N Credits

Available nitrogen from manure (or biosolids) should be included on this line. The more accurate the estimate of available N from manure, based on accurate application rates and manure analysis, the more confidence you will have in the manure N credits.

I. Supplemental N Requirement

The difference between the Preplant N recommendation, and the credits for starter N and manure N, is the amount of nitrogen that will need to be included in a pre-plant broadcast or side-dress application.

J. Timing Adjustment

Nitrogen that is applied just before the crop needs it is utilized more efficiently than N applied pre-plant, because there is less opportunity for loss through denitrification or leaching. This difference is most pronounced in the heavier textured soils. Sandy soils do not normally show a benefit to side-dress N applications, because we seldom receive enough rainfall in the early growing season to leach significant N below the rooting zone in these soils. NOTE: This adjustment does not apply in Eastern Ontario, where the N recommendations are already relatively low.