Managing Nitrogen Inputs For South/Central Plains Winter Crops

eKonomics News Team

Managing nitrogen inputs for crop production is a complicated endeavor. One needs to have a working knowledge of the complex soil system, the nitrogen cycle with all the additions and loss pathways, as well as being aware of crop physiological demand, and all the challenges of supplying enough nitrogen at the right time. So, what should be considered when managing nitrogen input into winter crops of the Southern and Central Plains? 

Loss Pathways to Minimize:

1. Ammonia Volatilization

Ammonia volatilization typically occurs when urea-based nitrogen fertilizers (urea or urea-ammonium nitrate) are surface applied. If significant rainfall, irrigation, or mechanical incorporation occurs within a couple days following fertilizer application, the risk of loss is minimal. How much rainfall is required to effectively incorporate the urea-nitrogen is a matter of disagreement. Most studies report rainfall amounts of one-third to one-half of an inch are typically sufficient to minimize volatilization losses. Enough rain must fall to dissolve the granule and move the urea into the soil. 

Factors that influence ammonia volatilization:

• Surface residue – the more residue present, the higher the volatilization risk. 
• Surface soil pH – the higher the pH at the soil surface (above 7.0), the higher the volatilization risk.
• Soil texture – coarser textured (sandier) soils have higher volatilization risk.
• Surface moisture – high moisture environments (heavy dews in the morning) increase volatilization risk.
• Wind – the more wind, the higher the volatilization risk.
• Temperature – warmer air temperatures increase volatilization risk, though significant volatilization can also occur in cold conditions.

2. Leaching

Leaching occurs when nitrate is present, and rainfall occurs at levels sufficient to move water through the soil profile. Leaching is of environmental concern when the nitrate is moved deep enough to reach subsurface water sources. Leaching losses are typically not the greatest risk of nitrogen loss in the South/Central Plains due to the arid nature of the region, but it does occur.

Factors that influence nitrate leaching:

• Rainfall – this is obvious, but periods of high rainfall increase the risk of nitrate leaching. Significant loss can occur with large, single rainfall events.
• Soil texture – coarser textured soils are at greater risk of nitrate leaching.
• Time of year – nitrate leaching is more likely to occur when air temperatures are cold (decreased plant photosynthetic activity and plant water demand).
• Fallow periods – nitrate leaching is more likely to occur during fallow periods when there are no plants actively taking up nutrients from the soil.

3. Denitrification

Denitrification occurs when the soil becomes saturated, resulting in nitrate being converted into a gas form and lost to the environment. Denitrification mainly occurs in heavier textured soils but is a less common loss mechanism in the South/Central Plains than leaching or ammonium volatilization because of the generally drier nature of soils in the region for wheat and canola production. 

Crop Physiological Considerations

Fall fertilization

• Why would anyone want to apply nitrogen fertilizer in the fall when the bulk of the nitrogen uptake is going to occur in the spring? For grass crops the desire to supply some fall nitrogen is to drive tillering. For small grains, the components that constitute yield are total heads per unit area (plants and tillers), kernels per head, and kernel weight. Fall fertilization promotes tillering before winter dormancy, and it is a significant contributor to final yield. For other crops (canola as an example), fall nitrogen fertilization is important to help the crop over-winter.
• It is commonly recommended to supply only a portion of the nitrogen in the fall to avoid excessive fall growth and freeze damage and to minimize over-winter nitrogen loss. The further west one lives in the Plains, the less likely one is to experience significant nitrogen loss of fall applied nitrogen. Typical recommendations promote application rates of about a quarter to a third of the total nitrogen budget be applied prior to seeding. The remainder of the nitrogen budget would then be applied prior to breaking dormancy in the spring or just after. 

Spring fertilization

• Applying the bulk of the nitrogen budget in the spring can have efficiency advantages (at least theoretically). Performance of spring nitrogen applications can be influenced by spring loss mechanisms and soil moisture for crop response. There are several considerations that must be made with spring fertilization. 
• Ammonia volatilization from urea-based fertilizers can still occur, so be mindful of timing and weather for those top-dress applications.
• If supplying nitrogen solutions (urea-ammonium nitrate – UAN), flat-fan nozzles can result in tissue injury if rates are high enough and temperature is warm enough. Streamer bars can decrease the extent of injury by increasing droplet size and decreasing the amount of plant tissue exposed to the fertilizer solution. 
• Warmer temperatures experienced near the time of application can increase tissue injury. Visual symptoms of injury do not necessarily translate into yield reduction, but it can be unattractive to look at for a week or so.
• Top-dress nitrogen effectiveness can be limited if insufficient spring precipitation occurs to move nitrogen into the root zone.

Understanding the loss mechanisms that you can influence, and understanding why you are fertilizing and when, can help you make better nitrogen input decisions.

If you would like more information, check out some additional informational sources:

Great Plains Canola Production Handbook.  Published by Kansas State University.

Fall nitrogen requirements for winter wheat.  Published by Oklahoma State University.