Appendices: Appendix B. Corn Nitrogen Rate Worksheet (Imperial) with Detailed Explanation

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Pub 811: Agronomy Guide > Appendices > Appendix B. Corn Nitrogen Rate Worksheet (Imperial) with Detailed Explanation

Excerpt from Agronomy Guide for Field Crops
Order OMAFRA Publication 811: Agronomy Guide for Field Crops

• Nitrogen Worksheet
• Explanation of Factors in Worksheet

Nitrogen Worksheet:

General Recommended Nitrogen Rates for Corn (Imperial)

The figures in this worksheet are based on a review of N response trials from 1961-2004. The metric version of this worksheet can be found in Chapter 1, Fertility Management. The fertilizer rates calculated here are designed to produce the highest economic yield when accompanied by good or above-average management. Research shows that higher rates will occasionally produce higher yields, but usually not enough to pay for the additional fertilizer.

Table A. Base N Requirements

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

Table D. Previous Crop Adjustments

Table E. Price Ratio (PR) Adjustment for Nitrogen Relative to Corn Price

* Adjustments for these price ratios have not been assessed.

Table J. Timing Adjustments (Southwestern and Central Ontario only)

Explanation of Factors in Worksheet

A. Base N Requirement

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 both coarser and finer textured soils, the nitrogen requirements are higher.

The data showed a significantly lower requirement for nitrogen in the Ottawa Valley than in the rest of the province, although the reasons for this are not completely clear. This appears to apply in all of the counties east of the Frontenac Axis.

Since 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 from the average (e.g., higher or lower organic matter (OM) content) may differ in the optimum N rates.

B. Yield Adjustment

There is a weak but consistent relationship between fields with higher yields at optimum N rates and higher nitrogen requirements. The yield factor derived from the N response data (0.77 lb N per bushel of yield) is almost exactly equal to the N removal from the field in the grain portion of the crop.

Use average yields for the previous 5 years to estimate the productive capacity of the field. Entering an inflated yield goal into this adjustment will not increase the productivity of the field, will cost money for wasted N and may result in environmental harm. To convert silage yield to an estimate of grain yield, divide the silage yield by 5 for grain yield in tonnes/hectare or tons/acre, or multiply tons per acre by 7 to estimate bushels per acre.

C. Heat Unit Adjustment

Research shows that corn in the long-season areas of the province requires more nitrogen. 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 OM content.

D. Previous Crop Adjustment

The crop that was grown immediately prior to planting corn has a significant impact on the nitrogen requirements. Crops such as grain corn 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, which is released to the corn crop as the residue breaks down, resulting in reduced N requirements.

E. Price Ratio Adjustment

The optimum N rate is the point where the yield increase from the last pound of added nitrogen just pays for the extra N. 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 pound of nitrogen increases. This means that the nitrogen rate that provides the maximum return to added fertilizer is reduced. The amount of reduction in N rates for various combinations of corn and nitrogen price is found in Table E.

For prices outside of the ranges provided, calculate the adjustment to fertilizer rates by following these steps:

• Determine the price of a kilogram of nitrogen. Divide the price per tonne of fertilizer by the number of kilograms of nitrogen in each tonne (the %N multiplied by 10). Calculate the price per pound by multiplying the price per kilogram by 0.45. For example, urea (46% N) at $865 per tonne will have an N price per kilogram of $865/460 kg = $1.88/kg N, or $0.85/lb N.
• Estimate the value of a kilogram (or pound) of corn for the year following harvest (unless the corn has been pre-sold at a fixed price), including all stabilization payments, minus costs for drying, trucking and elevation. Price the value of corn to be fed on-farm at the replacement cost for the corn if it had to be purchased from off farm. The price for a kilogram of corn is the expected price per tonne, divided by 1,000. The price for a pound of corn is the expected net price per bushel divided by 56.
• Calculate the N:corn price ratio, by dividing the price of a kilogram (or pound) of nitrogen into the value of a kilogram (or pound) of corn.
• Subtract 5 from the price ratio, because the N recommendations were developed for a price ratio of 5.
• Multiply the resulting figure by 6.7 (6 for Imperial calculations), and enter this figure into the price ratio adjustment.

F. Total N Recommendation

This figure, calculated by adding 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

Include any N that is supplied at planting here.

H. Deduct Manure N Credits

Include available nitrogen from manure (or biosolids) on this line. Available N from manure, based on accurate application rates and manure analysis will give more reliable N credits. For estimates of available nitrogen from manure, see Table 9-8, Typical Amounts of Available Nitrogen, Phosphate and Potash From Different Types of Organic Nutrient Sources.

I. Preplant Additional N

The difference between the Total N recommendation, and the credits for starter and manure N, is the amount of nitrogen to be included in a pre-plant application.

OR

J. Sidedress Additional N

Nitrogen that is applied just before the crop needs it is utilized more efficiently than N applied preplant (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.

NOTE: This adjustment does not apply in Eastern Ontario, where the N recommendations are already relatively low.