By Harold Watters, Ohio State University Extension
Over the past month or so, I participated in three conferences on nutrient loss. While many speakers addressed phosphorus concerns, several mentioned nitrogen as the next target. I focused on the nitrogen talks.
So lets talk about nitrogen management. It leaks, like everywhere. Up and down — up as a gas when the soils are saturated and moves down and out with water movement. By my estimate we mineralized 100 pounds of N per acre in 2017, and probably lost 100 pounds or more in many spots to leaching and to denitrification. Even though 80% of the atmosphere is N, we still have to supply it for our grass crops. And we add more than we need, because we don’t want to be short. That’s an economic concern.
So what can we do about managing nitrogen?
The current tool to make nitrogen recommendations for corn in Ohio is the CNR, which stands for Corn Nitrogen Rate Calculator. The tool is housed at Iowa State University and includes our Ohio data in the model: http://cnrc.agron.iastate.edu. The model includes the neighboring states of Indiana and Michigan. A total of seven states are involved in the tool.
How do you determine your N rate for your crop? The old rule of thumb was 1.2 pounds of N for every bushel of your yield goal. So in the old days for 200-bushel per acre corn, apply 240 units of N. But genetics, economics and environmental concerns have changed that thinking. And we have learned that yield goal is not a factor in setting your N rate. Let me repeat that: Yield Goal is Not a Factor in Setting the N Rate. It comes down to site and weather. And it is very possible that the lowest yielding field on your farm requires the most nitrogen.
Economic based nitrogen calculators have been around since about 2005, they are used to calculate the economic optimum rate of nitrogen. In the recent past with high nitrogen prices and low corn prices, we were looking for the minimum rate to maximize economic yield. Then we went through the high grain prices of 2008 to 2014 and no one worried as much about the economic concerns. Today is different, again.
I made a run on the CNRC with my numbers for next spring — corn at $3.50, N priced at $0.35 per unit N, in Ohio, in a corn/soybean rotation. Our most profitable recommended rate of N would be 175 pounds, with a profitable range from 158 to 191. The “profitable range” is within $1 per acre of the recommended rate. These numbers will change slightly as we add more data to the system from recent nitrogen rate trials. Keep checking back. Why don’t I throw in a factor for the site and weather? That is somewhat covered by selecting Ohio as the state — that will get you to average soils, rainfall, and growing conditions. But because we don’t have enough information from the past to calculate the future, that means you may have to look for a way to incorporate “this” growing season into a nitrogen rate calculation.
Do try out the Corn Nitrogen Rate Calculator on you own, with your prices. It is at http://cnrc.agron.iastate.edu. And perhaps think about how to most efficiently use that rate of nitrogen.
What practices can you use to keep N where you put it, and keep it for crop needs?
What is the right source — NH3 vs. 28% or urea, manure?
N is basically the same, generally it goes into the crop as a nitrate. Do make use of the N in manure — maybe do a PSNT to see what is available. And depending on placement and timing, a stabilizer may be necessary for some sources.
Right rate — use an N rate calculator?
Use the CNRC. Or there are some other models out there, still a lot to learn with these but their goal is to take in-season information into account when selecting the right rate.
Right time — as close to the time for crop needs as possible?
Corn needs little nitrogen early in the season, but by V8, V10 or so there is rapid uptake. Perhaps apply 40 to 60 units of N at planting then delay the remainder until V8. Or apply 100 N with anhydrous ammonia pre-plant then use a crop sensor at V8 to determine the needs for the rest of the season. If you use this method it is really useful to have a N-rich strip to compare. It will also be useful to have a zero rate strip to determine what your soil/field is capable of mineralizing.
Right place — next to the row, incorporated?
Definitely get N into the soil. If we are dry and the nitrogen is on the surface, then it may not get to the crop as quickly as needed but from work I saw from this past summer in western states, this rarely happens. Does N need to be next to the row? Not really, we have enough rain and soil moisture in Ohio that we can put it between the rows and the roots will get to the N.