Contents
Another look at using fertilizers
pH and micro nutrients graph
Too little or too much nitrogen on corn:  MSU’s new guidelines
Price of corn grain table
Michigan State University corn nitrogen recommendations table
Weeds rob nitrogen from corn
Managing nitrogen fertilizer for greatest efficiency
MSU Nitrogen recommendations for wheat in 2009
Michigan State University recommendations for phosphorus and potassium
Maximum fertilizer rates to band at 2 X 2 placement graph
Phosphorus and potassium fertilization recommendation scheme graph
Potassium recommedation guidelines chart
Phosphorus recommendation guidelines chart
Removal rates of K2O and P2O5 at various yield potentials

A word to the wise: question everything these days. Our reasoning powers that worked in the past are strained today. Farmers used to watch the Chicago Board of Trade for commodity prices; today they watch the price of gas at their local fuel station to gain some sort of rationale for how much fertilizer will cost and what the price of grain will be in the coming year.  While change may be risky, not changing may be even more risky.

In 2008, fertilizer and commodity prices proved how volatile and irrational markets can be. Nitrogen and phosphorus prices have been more stable, but they are still twice what they were several years ago while the price of potash has quadrupled. It’s time to question everything, including the way you have always fertilized crops. Could you be just as successful in harvesting high yields with lower fertilizer inputs? Could even a small change add up to large savings? If you find only one “keeper” idea in this article, it will pay to read on.

When soil structure and pH meet requirements, it is less risky to reduce fertilizer. pH impacts the availability of all nutrients. Low pH can inhibit not only nitrogen (N), phosphorus (P), and potassium (K) but also magnesium (Mg). Use dolomitic lime to increase pH and also supply Mg. Boron (B) can be a concern on alfalfa, especially on sandy soils.  

On high pH soils, zinc (Zn) and manganese (Mn) are generally the first micronutrients that are limited. If you determine a need for micronutrients, purchase only the ones your soil test shows are needed. Generally, soil has abundant supplies of these nutrients for crops; let the soil do its job.

Cutting nutrients based on new additive products
If it sounds too good to be true, maybe it is.  A common sales angle is to suggest that using a particular product will allow a farmer to succeed with reduced nutrient values.  Do you want to swap your known nutrients for unknown when every drop counts? When confronted with new products that are unfamiliar to you, ask for independently verified research results that include statistical analysis. Many of these products induce variable results, so wild swings can be seen in numerical yields. Demand evidence of consistent, statistically significant results. Iowa State University houses a web site for the North Central region on soil additive and growth stimulant products at
www.agronext.iastate.edu/soilfertility/nutrienttopics/addbyproducts.html. Or contact your local extension educator or campus specialist. Many of these products have been tested by universities in the north central region, including Michigan State University.

The economic risk of applying too much nitrogen (N) on corn is just as dramatic as applying too little. In 2004, university agronomists from the North Central region began a project to determine maximum agronomic production of corn at the most efficient economic level. The Maximum Return to Nitrogen (MRTN) approach to corn N recommendations represents the combined efforts of six states to develop the most efficient and profitable system of N management based upon site-year information. The recommendations utilize the latest hybrid genetics, soil productivity qualities, and best N management practices.  

This new method will probably seem radical to many and risky to most. If it does nothing other than make you rethink your N rates and contemplate the price of corn, it will be worth the time to see how your current thinking compares to the following charts.

Michigan’s corn N response database includes 47 site locations over the last ten years, including 30 in the last three years. Statistical modeling is used to fit response curves to each individual site’s corn grain response to N. Response to N is calculated at 1 lb N rates along that curve to find the most economic application rate. All sites are then combined to generate the most profitable N application rate, or MRTN. These trials were conducted with spring, sidedress, or split preplant/ sidedress applied N, and sites were not irrigated. The current MSU Guidelines from these trials is shown in Table 2.

Calculating the N:corn ratio and the price of N per pound
The multitude of possible N and corn grain price ratios make computer spreadsheets an ideal tool to determine specific corn N rate recommendations. . Visiting this calculator, http://extension.agron.iastate.edu/soilfertility/nrate.aspx, will stretch everything you thought you knew about economics and corn’s response to nitrogen. An example of the type of data generated by this calculator is shown in Table 1.

The N rate calculator begins by calculating the N:corn price ratio by dividing the price per pound of nitrogen by the price per bushel of corn.
First calculate the price of N on a per-pound-of-nutrient basis.
To compute the actual cost per pound of nutrient of various fertilizers, use the following formulas:

Material price per ton ÷ (2000 x % analysis of product) = nutrient cost in $ per pound of nutrient
For example: urea at $350 per ton ÷ (2000 x 0.46) = 38 cents per pound of N
Another example: 28% at $200 per ton ÷ (2000 x 0.28) = 35 cents per pound of N

When nitrogen is calculated on a per-pound basis, you can also compare the major N sources on a cost per pound of nitrogen. If anhydrous is $750 per ton and 28% UAN is $250 per ton and urea is $420 per ton, the result for all of them is about $0.46 per pound of actual N.

Next, compute the N:corn price ratio.
Example: Urea at $370 per ton is $0.40 per pound of N ÷ $4/bu. expected corn price = 0.1 N:Corn price ratio.

Once you have the N:corn ratio, use the MSU chart below to see what rate of nitrogen is considered the Maximum Return to Nitrogen, considered the most economical rate of nitrogen to use for the given yield potential.

This is the average N recommendation. For +/- $1 of the MRTN the recommended N rate will range from 10 lbs less to 15 lbs more than the value in the table.

An MSU timing study on post emergent weed control showed that 95 percent of weed control or better can be achieved when weeds are 9 inches tall, but yields were reduced 25 bushels per acre. (Everman, et al. 2008)

Post-emergent herbicides provide a greater window of opportunity to control weeds, but allowing weeds to grow too long can be “robbing” N, and other nutrients, from the intended crop.

In a two-year study in Wisconsin, there was no yield loss when weeds were controlled at the 4-inch stage, but delaying application on 12-inch weeds resulted in an average 9 percent yield loss. Looking at the data from another view point, 2006 data showed the MRTN rate was 96 lbs per acre when weeds were controlled at 4 inches, compared with an MRTN rate of 200 when weeds were controlled at 12 inches.  Timely weed control will ensure valuable nutrients are used for crop production rather than weed production. 
 (Boerboom, et. al. 2008.)

Hopefully you have experienced a year when corn yields exceeded the nitrogen applied. Soil N can provide as much as half of the total N available to the crop. (Camberato, et. all 2008.)  Rain and sunlight are often the two most sought after resources for high yields, and when these two resources are abundant and timely, plants seem to find the additional nutrients to exceed yield expectations.

However, excess rain has an impact on potential N losses; on sandy soils downward leaching can occur and on heavy soils prolonged standing water can cause denitrification (losing N up into the air).  These excessive rains often come early in the season, after N fertilizers have been applied, but prior to maximum uptake by plants.

Timing of N application and N source
The rule of thumb is still true: uptake of fertilizer N is most efficient when applied as close as possible to the time of a corn plant needing it. Some forms of N are more suitable for applying at planting and at sidedress than others. Even when your situation does not allow for the greatest timeliness, there are factors to consider to decrease N losses and increase N uptake by corn.

Urea (46% N)
Several research studies have found the following about broadcast urea at various temperatures:
At 35°F 50 percent can hydrolyze within 4 days and by 10 days, 100 percent hydrolysis.
At 50°F, half of the urea can hydrolyze in 2 days and 80 percent within 4 days.
At 79°F, 90 percent can hydrolyze within 2 days.
When immediately incorporated into the soil, these losses can be reduced to virtually zero.
(Bundy, et al 2001)

Light rainfall can cause urea to move into the soil protecting it from ammonium-N. As little as 0.1 inch of rain can minimize ammonium-N losses and 0.2-0.5 inches within 24 hours after urea application usually prevent volatilization. (Bundy, 2001).  Rainfall or irrigation moves the dissolved urea into the soil where ammonium will be held on the exchange sites of clay and organic matter.  

Urea-N can be converted to nitrate-N (NO3-N) in less than two weeks in late spring. At this point, the NO3-N is susceptible to leaching if excessive rain occurs, especially on sandy soil.

A soil pH of 7 and higher can also accelerate loss of ammonium-N into the air when urea is surface applied. Do not surface apply urea if lime has also been surface applied within the last three months as this can increase NH3 volatilization losses. Incorporation of lime and/or urea will reduce these potential N losses, especially on high pH soils.

In summary, the worst case scenario for urea losses into the air is a surface application with no incorporation onto a high-residue field, when it is warm and sunny, and the soil pH is greater than 7.

Another negative scenario would be supplying urea early, even when incorporated, on sandy soils followed by excessive rains between urea application and time of crop need. In this case urea is applied too early and has time to convert to nitrate-N. 

Anhydrous ammonia (82% N)
Generally, anhydrous ammonia (NH3) is the cheapest source of N per pound of N and must be knifed into the soil. Wet soils may hinder the soil closing behind the knife injection slot, which can result in N losses. Deeper injection (6 inches) is best on both dry and damp soils to prevent losses.

Anhydrous quickly converts to ammonium-N which is held tightly to the soil, eliminating leaching or volatilizing. However, NH4-N then converts to nitrate-N which can be leached. This conversion happens faster in warm temperatures. Fall applications of N are not recommended, but if they must be, it should be as anhydrous and applied after the soil temperatures are below 50°F and predicted to stay cool. Under spring conditions, the conversion generally happens in a week to 10 days when soil temperatures are above 50°F. This is why pre-plant applications are not as efficient as at-planting, which aren’t as efficient as sidedressing. Ammonia’s effects on soil microbes within the application band acts as its own nitrification inhibitor for at least a week.

UAN solutions (28%-32% N)
Urea and 28% UAN will be less cost per ton of material, but more per pound of N. The higher cost may be partially offset by lower application costs.  As a mixture of ammonium nitrate and urea, UAN already has a portion of its N in the nitrate-N form, which could leach if applied early, on sandy soils and if heavy rains occur. The urea portion requires banding, injection or incorporation to keep from losing N into the air and makes UAN a good N source for sidedressing. Again, all the conditions discussed for urea would apply to these liquid solutions.  

Pull out your soil tests and use the following information to assess phosphorus and potassium levels along with your 2009 crop plan to determine where fertilizer will benefit, and where you can save money on nutrients without sacrificing yield. The following charts will help to assess your current soil test levels, showing you where your test values are within the ranges, helping you decide how risky reducing fertilizer will be.

Phosphorus is always most efficient to band 2 X 2 at planting. The price of potash may encourage the use of banding some of it as well. Maximum rates to band at 2 X 2 placement are:

MSU’s recommendations for P2O5 and K2O are based on the Buildup-Maintenance-Draw Down Model, shown below in Figure 1. The lower the level of available nutrients as indicated on a soil test, the greater the probability of a beneficial response to applied fertilizer. The critical level is the soil test level at which 95 percent of maximum yield is expected. This is usually near economic optimum return to fertilizer.

Figure 1.

For potassium, both the Cation Exchange Capacity (CEC) and the soil test value are taken into consideration to determine the critical value.  See Chart 1a. Below the critical level, yields will respond to applied nutrients, providing an economic return. Fertilizer recommendations are based on what the crop needs for the coming year and an additional amount of nutrient for building up the soil to the critical level. Potash and phosphate recommendation guidelines are given in Charts 1a and 1b.
 

Chart 1a.

Chart 1b.

Above this critical level is a plateau range called the maintenance range. In this range, fertilizer recommendations are based on crop removal rates that will achieve economic production and maintain the soil test without expecting test levels to rise or fall. Since this is based on crop removal values, the fertilizer recommendation will vary depending on what crop is grown and the yield potential (see Chart 2).  

If soil test values are in the draw down range, no fertilizer will be needed. In this range, it is possible, profitable, and prudent to utilize the nutrients stored in the soil from past fertilizer and or manure applications. Additional resources are available at www.fieldcrop.msu.edu . Follow the link for a complete PDF copy of the Nutrient Recommendations for Field Crops in Michigan, bulletin E2904.

Sources

Boerboom, C.M., T.L. Trower, C.A.M. Laboski and T.W.Andraski.  2008.  Fertilizing weeds for a profit?  Proc. Of the 2008 WI Fert., Aglime and Pest Mgt. Conf.  47:223-226.