With harvest wrapped up across most of Illinois, many growers are starting to consider fall nitrogen fertilizer applications.  Fall applied nitrogen is fairly common throughout Illinois as ammonia is typically cheaper in the fall and there is less of a time crunch than spring applied N, but there are management considerations that growers should be mindful of to avoid excess nutrient runoff during the winter. 

Nutrient runoff is a major concern both in Illinois and throughout the Mississippi River watershed. Nutrient runoff contributes to harmful algal blooms both locally and nationally, with the hypoxic zone in the Gulf of Mexico being one major area of concern.

In response to the issue of nutrient runoff, the Illinois Nutrient Loss Reduction Strategy (NLRS) was developed in 2015.  The primary goal of the NLRS is to reduce nitrogen loads in waterways by 15% and phosphorous loads by 25% by 2025.  The NLRS looks at nutrient runoff coming from point sources (like municipal sewage treatment facilities and industrial wastewater treatment facilities) and nonpoint sources (like agricultural fields). 

Farmers have an important role to play in working towards meeting these goals.  There are many small changes to nutrient management strategies that can be made to help combat the problem of nutrient runoff.  This article will discuss the 4R’s of nutrient management, which is one part of current best management practices in nutrient management.

What are the 4R’s?

The 4R system of nutrient management was developed to ensure that fertilizer is applied correctly to fields to maximize economic returns while minimizing negative environmental impacts. It focuses on applying fertilizer the “right” way – the right fertilizer source, at the right rate, at the right time, and in the right place.

Right Source

Using the right source is all about matching the fertilizer to the needs of the crop.  Different crops have different nutrient requirements. There are also many different fertilizers available with different combinations of N, P, and K.  There are even different forms of nitrogen available with their own unique properties—ammonia and urea being two common forms.  Not all fertilizers are in a form that is immediately plant available and this should be taken into account when choosing which to apply.  For example, urea must undergo a three-step change before it is in a plant available form and how quickly this happens is dependent on soil moisture and temperature.  

Right Rate

Just like using the right source, the right rate should match up with the needs of the crop. Current best management practice is to use the maximum return to nitrogen (MRTN) system to determine the right rate.  MRTN is the nitrogen rate where the economic net return of N application is maximized. It takes into consideration commodity prices, fertilizer prices, geographic region, and crop N response data.  If this sounds complicated, don’t worry.  Cooperative Extension Services at land-grant Universities across the upper Midwest have created an MRTN calculator that makes finding the right rate simple. Illinois has extensive N trial data that makes the MRTN calculator incredibly accurate. 

Right Time

The right time refers to making nutrients available when crops need them.  Many times this is close to planting, but it can also be at certain growth stages.  For example, corn uses over half of its total N supply between V8 and tasseling when the plant is rapidly growing.  Having sufficient plant available nitrogen in the soil during this time is crucial for crop success.  This is why it is often common practice to sidedress N between V4 and V6.  Applying nitrogen in the fall runs the risk of increased N loss from leaching or denitrification, but we can make management decisions to minimize these risks, like using a nitrification inhibitor.

Right Place

The right place refers to both location within a field and location in the soil profile.  Fertilizer should be applied where plants need it and can use it – in the root zone.  Other factors to consider when choosing the “right place” are soil type, slope, and distance to streams/rivers.  Precision agriculture technology has even made it possible for growers to precisely apply nitrogen where crops need it the most based on the characteristics of each field.

Other best management practices for fall applied nitrogen

• Always soil test.  You don’t know what your crops need if you aren’t regularly testing your soils.  It is recommended to test every 3-5 years or before making big changes to management.
• Be mindful of soil type.  Fine-textured soils (like clay or silty clay) have a higher risk of nitrogen loss from denitrification.  Coarse-textured soils (like sand or sandy loam) have a higher risk of nitrogen loss from leaching.
• Only apply ammonia fertilizer in the fall when soil temperatures drop below 50°F.
• Use a nitrification inhibitor to reduce potential nitrogen losses.
• Be mindful of soil moisture.  Fields shouldn’t be too wet or too dry.
• Don’t apply fertilizer if fields are frozen or snow covered.
• Consider what other sources of nitrogen you may be applying later in the season and adjust rates accordingly.  Will you apply MAP or DAP in the spring?  Do you use a nitrogen solution with your herbicide?

For detailed information on best practices in nitrogen management check out the Illinois Agronomy Handbook chapter “Nitrogen Management for Corn” and farmdoc “Considerations for fall application of anhydrous ammonia”