Season-long Corn Nitrogen Management
Best practices for nitrogen use efficiency presented by Missy Bauer
Nitrogen is one of the most important nutrients that corn needs to thrive. A lack of nitrogen availability at key points throughout the season can be costly when it comes to bushels in the bin. A season-long nitrogen management program can improve efficiency, increase yields and help to reduce agriculture’s environmental footprint.
Paying the carbon penalty
Paying the carbon penalty requires a breakdown of the previous year’s crop residue. Microbes in the soil will start to digest the previous year's crop residue and consume nitrogen as an energy source in the process. Bacteria that decompose high carbon-low nitrogen residues need more nitrogen to digest the material than is present in the residue. If the plant does not have enough nitrogen, immobilization, which is a temporary reduction in the amount of plant-available nitrogen, will occur. As a result, the grower will see yellow corn in their fields early in the season.
Part of paying the carbon penalty is making sure there is enough nitrogen to feed the microbes and still keep the crop nourished. Carbon-to-nitrogen ratios differ by type of crop residue. For example, it takes more energy or more consumption of nitrogen to breakdown corn residue than it does soybean residue. Therefore, a corn-following-corn rotation has a bigger carbon penalty than a corn-following-soybean rotation.
Carbon to Nitrogen Ratios of Crop Residue
- Soybean = 30:1
- Corn = 60:1
- Wheat = 100:1
Microbes can go to work breaking down residue right after harvest as long as the soils are warm enough for microbial activity. This biological process is slowed when soil temperatures are below 50 degrees and halted under freezing temperatures.
“In Memphis, growers harvested in early fall and started the residue breakdown process, so by spring, the breakdown process is finishing,” says Missy Bauer, independent crop consultant with B & M Consulting in Coldwater, Michigan. “Growers in a corn-following-soybean rotation in Memphis will probably have a pretty small carbon penalty to pay, but at crop emergence, it’s important to ensure there’s enough nitrogen to meet both needs – the microbes and the plant.”
Growers that are further north may have a bigger carbon penalty to pay if there is less time in the fall to start the breakdown process. Once the residue has broken down there is often a net gain of nitrogen released back through mineralization.
Mineralization in fields
Mineralization is the soil's ability to naturally produce nitrogen. Organic nitrogen that is present in soil organic matter, crop residues and manure are converted to inorganic nitrogen through the process of mineralization. In mineralization, bacteria digest the organic material and release ammonium nitrogen. The release of ammonium nitrogen increases as microbial activity increases so each field’s mineralization process is unique. For example, one field may require more than 200 pounds of applied nitrogen fertilizer to produce 200 Bu./A corn, while another field might produce 230 Bu./A corn with only 100 to 120 pounds of nitrogen fertilizer. The difference is the soil’s ability to naturally mineralize its own nitrogen. Higher organic matter soils, such as deep black prairie soils in central Illinois, naturally produce and mineralize more nitrogen than sandy loam soils with little organic matter. Mineralization rates are also related to environmental conditions such as soil temperature and moisture. A hot, dry summer means mineralization rates go down while a summer with good moisture and moderate temperatures will cause rates to increase.
How much nitrogen fertilizer is needed to raise a corn crop? It ultimately depends on the soil's ability to mineralize nitrogen. “We encourage farmers to conduct nitrogen rate studies to help determine the ideal total nitrogen rate for their farm,” says Bauer. “Calculations should be run on these plots to ensure the best economical nitrogen rate.” In-field data can help identify the right nitrogen rate for individual fields.
Nitrogen loss can take a toll
A significant amount of nitrogen loss can occur in fields due to volatilization, leaching and denitrification. Improvements in management, the use of nitrogen managers, stabilizers or inhibitor products, and timing and placement of nitrogen can help reduce these losses. “In order to reduce potential nitrogen losses, it’s important to fully understand how the losses occur,” says Bauer.
Volatilization is nitrogen lost as ammonia gas from the soil surface. Ammonia is an intermediate form of nitrogen when urea is transformed to ammonium. Urease enzymes in the soil and plant residues convert the urea component to the free ammonia gas. Whenever urea is applied to the soil surface and not incorporated, this nitrogen loss could be worse. The same is true for UAN 28% or UAN 32% because half of that product is urea. To manage and reduce volatility for surface-applied urea or UAN, use nitrogen managers, stabilizers or inhibitors, especially in warm weather conditions which can exacerbate losses.
Denitrification occurs when bacteria convert nitrate to nitrogen gases that are lost in the atmosphere under waterlogged conditions. It happens when soils have been saturated for several days. Denitrification rates are typically higher in heavy soils with poor drainage and can occur quickly, causing significant nitrogen loss. Remember to pay close attention to upcoming weather events.
Denitrification is also temperature-based, so the warmer it is under wet conditions, the greater the losses. According to data from the University of Nebraska, if soils are saturated for five days at 55° to 60° F, losses will be about 10 percent, but at 75° to 80° F, losses could be 75 percent. If soils remain wet for two or more days, expect some amount of denitrification to occur.
All fields are not at the same risk for denitrification, so manage each field independently. Consider which fields are poorly drained or need tile drainage. Even though Mother Nature is out of our control, denitrification can be reduced by good drainage, limited compaction and uniform soil density.
Leaching is the loss of soluble nitrate as it moves with excess soil water below the root zone. A greater potential for leaching occurs in sandier soils that have a lower water holding capacity. However, leaching of nitrate can occur in any soil if enough rainfall moves water through the root zone. It is important to monitor events that lead to denitrification and leaching losses so informed management decisions can be made. According to Bauer, growers should weather-proof their system by applying nitrogen closer to when the crop needs it and by splitting up the applications. There are many nitrogen manager, stabilizer and inhibitor products that may help reduce denitrification and leaching as well.
Timing and placement make the difference
Timing and placement are keys to managing the plant’s nitrogen needs through the entire season. Breaking up applications of nitrogen can also help weather-proof the system and limit nitrogen lost to the environment. The environment sets the tone for timing of nitrogen, timing sets the tone for placement, and placement sets the tone for picking the sources. When laying out a plan for nitrogen timing and placement it is important to consider what the goals are as the growing season progresses.
The front side of nitrogen management is based on the carbon penalty. Part of the nitrogen applied should be to help break down residue, feed the microbes and reduce immobilization. The carbon penalty will depend on crop rotation, as well as tillage and location (north to south). The initial carbon penalty can be paid by applying some broadcast nitrogen in the fall or early spring. Ammonium nitrogen present in MAP, DAP and AMS (ammonium sulfate) is a good source for fall or early spring applications to start the breakdown process, but only 25 or 30 pounds of nitrogen is needed at this timing to pay the carbon penalty. Depending on how big the carbon penalty is, a pre-plant or pre-emergence broadcast application of UAN or urea can also help. Keep in mind that deep-banded nitrogen, such as anhydrous ammonia, does not pay the carbon penalty well.
It is also important on the front side to keep young plants nourished by making sure enough nitrogen is present for early uptake and growth. To keep young plants fed, the ideal placement is banded nitrogen close to the plant. A starter fertilizer band on a corn planter typically two inches to the side and two inches below the seed (2x2) provides a placement that young plants can take up early in the season. Depending on the soil types and salt index of the starter fertilizer, it may be possible to apply 25 to 35 pounds of nitrogen per acre in this 2x2 placement with the corn planter with minimal risk of injury to newly germinated seedlings.
An alternative to starter fertilizer applied with the corn planter would be a shallow strip-till application. A strip-till machine ahead of planting could apply nitrogen in bands, however it would be important to keep the bands shallow (less than four inches deep). If the bands are too deep, young corn roots cannot take advantage of the early uptake. Banded nitrogen gains a lot of efficiency at this time, but rates need to be closely monitored. Too much nitrogen too close to the seed can burn both the seed and its roots. Figure 1 shows an example of how managing the carbon penalty and early plant growth can be improved when using a combination of broadcast UAN pre-emergence and starter nitrogen fertilizer in a 2x2 banded placement.
It is important to apply in-season nitrogen to manage the back end of crop and ear development. Peak uptake of nitrogen occurs from vegetative stage 10 (V10) through tassel emergence. Adequate nitrogen will need to be available during this time. Applying a sidedress band of nitrogen between V4 to V8 growth stage will help ensure there is enough nitrogen available. Nitrogen uptake will continue through black layer. Sidedress bands of nitrogen help improve ear tip fill and kernel depth. This timing and placement improves overall nitrogen efficiency and reduces potential losses because its timing is closely aligned with plant uptake. A larger portion of the total applied nitrogen should be saved for managing this back end. Figure 2 shows how important in-season sidedress rates of nitrogen are to ear tip fill and kernel depth. The total nitrogen rates are the same for all treatments, but the amount that was applied up front or at sidedress varied based on the treatment. Applying too much nitrogen up front and not saving enough for sidedress can reduce yields significantly, especially on less fertile soils. Growers who apply 100 percent of their nitrogen for the corn crop in the previous fall or early spring are subject to greater risk of environmental or yield loss.
In summary, a nitrogen program should be designed to keep the crop’s needs met season-long until black layer. Apply enough nitrogen on the front side to pay the carbon penalty, keep young plants nourished and manage the rest through the back end with sidedress applications. Applying nitrogen at different timings and placements also helps reduce potential loss.
“Managing both the front end and back end can lead to improved nitrogen efficiencies and increased yields,” says Bauer.
Source: University of Minnesota, AG-FO-03770-B Revised 2014, Understanding Nitrogen in Soils
This white paper is a summary of a presentation at the "Plant Smart, Grow Wise" seminar brought to you by Farm Journal Media and Verdesian Life Sciences in Memphis in June 2016. Missy Bauer, Farm Journal Associate Field Agronomist spoke on nitrogen management to deliver yield. The information in this white paper is an extension of this event and focuses specifically on nitrogen management for corn crops.