• 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
    
  • MSU's corn nitrogen recommendations table
    
• Weeds rob nitrogen from corn
  
• Managing nitrogen fertilizer for greatest efficiency
  
• MSU Nitrogen recommendations for wheat in 2009
  
• MSU 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 phosphorus and potassium at various yield potentials
• More MSU internet resources for field crops
  

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.

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 pre-plant, sidedress, or split preplant/ sidedress applied N, and sites were not irrigated. The current MSU guidelines from these trials areshown 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. The Michigan data will be at this site by mid-February. 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, 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 (Table 2) 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.

Soybeans
Soybeans are generally credited for 30 lbs per acre of N to the following corn crop. However, when using the MRTN data base for corn after soybeans (Table 1), no additional N credit should be taken -- that would be double dipping. This N benefit is because soybeans are N fixers and have less residue compared to corn after corn. Therefore, less N is tied up in decomposing the residue and more N is readily available for the following crop.  Corn following soybeans should need less N than corn following corn.

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.

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. al 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 while prolonged standing water on heavy soils 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 application at planting while others are better suited for sidedress applications. 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 losses. 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 potential high loss scenario would be applying urea early, even when incorporated, on sandy soils followed by excessive rains before peak crop demand. In this case urea is applied too early and has time to convert to nitrate-N.

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, and help you decide how risky reducing fertilizer will be. For a complete listing, visit http://www.emdc.msue.msu.edu/mainsearch.cfm type E2904 in the inventory number box to search for Nutrient Recommendations for Field Crops in Michigan. Or click on the link for a complete PDF copy.

Phosphorus is always most efficient to band 2 X 2 at planting. The high price of potash may also encourage the use of banding to maximize the uptake efficiency of that nutrient as well. Maximum rates to band at 2 X 2 placement are in the following table.

Numbers in ( ) are for clayey soils.

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 the maximum 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. When soil tests are below the critical level, yields will respond to applied nutrients, providing an economic return. Fertilizer recommendations are based on crop nutrient needs for the coming year and any additional nutrients required for building up the soil test values to above the critical level. Potash and phosphate recommendation guidelines are given in Charts 1a and 1b.

Chart 1a.

Chart 1b.