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Editor’s Note: This is the 4th part in a 4-part series on the 4Rs and nutrient management.

Nitrogen (N) and phosphorus (P) are the two main concerns when addressing impairments to water quality. In some landscapes nitrogen is the problem and in others phosphorus is the issue. Our Federal Drinking Water Standard is 10 ppm nitrate (N). Yet we are seeing large amounts of nitrate leaching off the landscape or exiting through tile lines and entering surface water bodies from big (the Gulf) to small (Lake Bloomington).

Proper Nitrogen (N) Management:

Right Rate: In the past, Illinois used the 1.2 lbs. per bushel rule when making N recommendations for corn. That approach was changed in 2009 to the Maximum Return to Nitrogen (MRTN) model (http://cnrc.agron.iastate.edu/). This is a collaborative effort by universities in Illinois, Indiana, Iowa, Michigan, Minnesota, Ohio and Wisconsin. As stated in the Illinois Agronomy Handbook (IAH), this model provides “guideline N rates” versus a “fixed N recommendation.” This model gives you an economic N application rate range and offers a little more flexibility than a single “fixed” rate. If you have not seen or used this model, I suggest you go to the website listed above and check it out. This is the N recommendation model being used and promoted by the Illinois NLRS. There is also a table in the IAH for making N rate decisions on wheat.

Much research has been done over the years regarding late-season N applications to soybeans. The results have been inconclusive and the practice is not necessarily recommended in the IAH.

Right Source: When it comes to the nitrogen source to be used, all the different ones have their strengths and weaknesses, but should provide relatively the same yield when used correctly in conjunction with N loss inhibitor technology. These sources can be anhydrous ammonia, ammonium nitrate, ammonium sulfate, urea, urea ammonium nitrate solutions, ammoniated phosphates, or organic N sources (composts/manures).

With respect to the inhibitor technology, there are a host of different products out there. Do your research on these products. Many of them are good products, but there are some that do not necessarily hold up to their label claims. I have also heard comments made that products like nitrapyrin (N-Serve®/Instinct®), NBPT (Agrotain®), and dicyandiamide (DCD®) “stabilize” the nitrogen. These products do not stabilize the nitrogen. They specifically block/inhibit microbial and chemical pathways in the soil to slow down N conversion and potential leaching losses of nitrate. Nitrapyrin and dicyandiamide are nitrification inhibitors that inhibit the conversion of ammonium to nitrate and their efficacy decreases with time. Eventually, ammonium in the soil and soil solution not adsorbed to the soil CEC or utilized by the crop will be converted to nitrate. NBPT inhibits the activity of the urease enzyme. This enzyme is responsible for the conversion of urea to ammonia. With this conversion to ammonia you could experience gaseous losses directly to the atmosphere with surface applications of urea or UAN solutions.

Right Time: With respect to the 4Rs, this “time” refers to when the N is applied. When doing so one should follow some of the basic best management practices (BMPs). You first need to understand the growth habit of your crop and when it is at peak N demands. Peak N demands can vary greatly for corn versus wheat versus soybeans (the Rhizobium produce the N, but soybeans do have a peak demand and the Rhizobia may not provide all the N needed).

In Illinois, a large quantity of the N used on corn is fall-applied and revolves around logistics and economics. By economics we mean that although ammonia is the most economical form of N available, it may not be the most desirable form. Fall application should be made only on those soils north of Route 16 (higher CEC soils) when soil temperature drops to 50⁰ F or less, and in my opinion you should also be using an inhibitor.

Remember, the inhibitor is blocking a microbial transformation and is not changing the N itself. It allows more time for NH4+ to adsorb to the CEC, even at lower temperatures. Warm soils that are slow to cool, sandy soils, organic soils, soils with poor or excessive drainage, soils that rarely freeze, and those soils south of Route 16 should not receive fall N applications. Except (there is always an exception), full or split fall N applications to wheat are allowable. While it is not as common anymore, there are still a few producers who apply the full N rate to wheat in the fall as anhydrous ammonia.

Pre-plant, at-plant, sidedress and topdressing N are our other timings for applying full-rate, split-rate or even rescue applications. These have their advantages and disadvantages. Split application of N for corn and wheat benefits the 4Rs concept by targeting N applications in conjunction with peak crop demands. I have seen tremendous yield benefits/gains with split application and late-season topdressing of N on corn. However, this may not always fit the logistics of producers with a large number of acres to cover in a short application window.

University of Illinois professor Dr. Fred Below has produced several good articles and papers on maximizing yield potential of corn and soybeans. Here is a link to the Crop Physiology home page: http://cropphysiology.cropsci.illinois.edu/. The crop nutrient partitioning charts for corn and soybeans give a good timeline as to a crop’s peak nutrient demands and which plant parts are receiving those nutrients.

Right Place: N placement is the final piece of the 4Rs and ties in directly with rate, timing and source, all of which are dependent upon one another. I personally like to see N placed below the soil surface, whether it be as anhydrous ammonia, a dry source or livestock manures. When left on the soil surface the loss potential is too great via volatilization or runoff with water. However, there are situations where this is not possible. Topdressing dry or liquid N to wheat in the spring, rescue and/or topdressing N late-season in corn, or in No-Till are examples. These tie in to the use of inhibitor technology to protect the N from microbial loss pathways and to the idea of watching the weather. Ideally, we would like to have a light rain to help incorporate N, but not too much to promote surface runoff losses or denitrification. This “placement and timing” has always been the biggest gamble and always will be.

Nitrogen is vulnerable to leaching, denitrification and volatilization. While nature will always cause some losses, following the 4Rs will help reduce this risk and keep more of the N for the crop.

Terry Wyciskalla is an independent crop and soils consultant based out of Nashville, Illinois. He specializes in soil sampling, fertility recommendations, precision ag services and crop problem diagnoses. He serves a 12-county area throughout Illinois. He earned his 4R Nutrient Stewardship certification in 2015 and was part of the 2016 Soy Envoy team.