Corn Sidedress Nitrogen Rate Test

Fertilizing corn is one of the largest expenses on our farm, and Nitrogen (N) is the biggest contributor to that cost.  Over that last few seasons we have changed our N program quite a bit with goals of improving corn yield, more effective application timing, and reducing costs.

Corn Nitrogen Test Plot via thefarmerslife.comIn recent years we have stepped away from applying anhydrous ammonia, our chosen form of N, in the fall.  It is not uncommon for farmers to apply a full rate of N for corn in the fall for the following growing season.  When doing this a farmer must add an N stabilizer product to the fertilizer.  This product delays nitrification which is the process than can  eventually lead to N  being lost to the atmosphere or through groundwater.   N-Serve is what we use. Although we have not fall applied any anhydrous for two years we still use N-Serve when applying anything pre-plant through mid-April.

These days we are applying almost 100% of our N as sidedress. This simply means we are applying anhydrous ammonia to an emerged corn crop.  Later applications feed the crop when it needs it most, reducing the potential for N loss. For 2014 we made a sizable investment in our own applicator. Until this year we had always used a standard style applicator from the fertilizer dealer. Our high speed-low disturbance applicator gives us a wider window to sidedress corn before it gets too tall.

With our current plans for application timing and our new applicator we are now looking to reduce the amount of N we are using while maintaining or improving yields.   This year I ran a simple test in one of our corn fields to see how four different rates of N would affect yield.

The Test

Nitrogen Rate Map via

Map of N rates applied to a corn field.

The majority of this field was fertilized at a rate of 145lbs N per acre during sidedress. This rate is indicated by the orange coverage on the map pictures. My simple rate test consists of four full passes through the field at each rate. 200lbs in blue, 145 orange, 175 green, and red at 100lbs. The low rate was the rate suggested for the entire field by the 360 Commander software.  2013’s soybean crop can account for a potential 30lbs\A N credit.  Because the field had a grass cover crop we had 30lbs of N applied via ammonium sulfate ahead of planting to avoid available N being locked up by decaying cover crop residue.

A common rule of thumb used to be to apply 1 pound of N for each bushel of expected yield.  This is now outdated. 0.8:1 is a safer bet these days.

Each pass of the applicator is 24 rows wide. The same width as our planter.  Our corn head is 12 rows so the combine took two passes to cover each fertilizer rate. I simply figured the average yield of each full harvested pass by calculating the average from acres covered and dry bushels from each pass as shown on the yield monitor. It is not the most rigorous experiment ever, but I think it’s a good, practical field size trial in normal growing conditions.

The Results

Yield Chart via thefarmerslife.comHonestly I was hoping the final results would turn out like they did. The lowest rate had the highest yield, and the high rate was the lowest yielding.  I was surprised how much better the low rate was over the high.  Just over 14 bushels per acre better.  Obviously N put out by us wasn’t a limiting factor.  I’m curious why the high rate didn’t do better.  The results from this test tell me we are moving in the right direction by backing off N in corn. However, there are a few factors to consider.

2014 brought nearly perfect corn-growing weather. Our corn was pretty much stress free this year.  We actually had a record corn crop from our farm this year. This whole field averaged 225bu/acre.  If June and July had been really wet the results may have been much different. Heavy rain saturating soils can lead to N loss which may have favored the higher rates. Heat stress or drought could have an effect as well. We of course cannot control the weather, but we can continue to test and fine tune our N program.

Why didn’t we use the rate suggested by 360 Commander considering it had the top yield? Frankly because farmers are skeptical and this was the first year trying the program.  Next year I would like to run a whole corn field at the suggested rate with some check blocks at rates of our own choosing for comparison.

I think we’ve also shown the 1 pound of N for 1 bushel of yield isn’t the best plan anymore.  The 100lb rate combined with 30lbs preplant and an assumed 30lb soybean credit brought a yield of 239 bushels for 160lbs of N.  This doesn’t account for other N in the soil.  There is more than enough N present in soil to grow a fine crop.  The problem is that most of it isn’t in plant available form when you need it most.  That is part of the reason we are incorporating cover crops.  One of their benefits is to improve natural soil fertility.

Cost Analysis

Looking at how cost pans out in this situation is important.  Anhydrous ammonia cost us about $.50/lb of N this year.  The chart below illustrates the gross profit from each N rate based on the achieved yields multiplied by a corn price of $3.50/bushel minus the cost of the N applied.  No other costs are subtracted from the gross because all other costs are the same for this plot.
Corn Profit Chart via thefarmerslife.comIn my test it happens to be the amount of N applied is inverse to profit.  The least amount of N applied not only garnered the most yield, but also the most profit.  Looking at both charts shows the 145 rate, although less productive, was more profitable than 175.

But like I said earlier this is one test in one field during one growing season.  If I had done this in 2013 corn would have been near $7.00/bu and N would have cost around $.10/lb less than 2014.  Replicating this test in years to come would be worthwhile especially as we attempt to become more precise with our N placement.

John Deere 2510H Anhydrous Ammonia Applicator via

Our applicator in the field fertilizing corn.

Going Forward

One of the main features we have on our applicator is the ability to apply N with variable rate technology. The same tech that lets us vary seeding rates based on GPS maps we build ahead of planting can do the same for sidedress.  Unfortunately the electronics never got sorted out enough this year to change the rate quick enough for me to be comfortable doing this. So this year we ran blanket rates as in the past.   I hope we resolve this in 2015 so we can more effectively place N based on crop needs. This can be accomplished in several ways from building our own maps or relying on technology like sensors, software, or UAVs. I’m looking forward to being more profitable and more sustainable going forward with our fertilizer program.

What do you think of our results?


  1. Brian, I’ve been reading your posts since we crossed paths on Facebook a year ago.

    Check strips are good for an idea of what’s going on, but a replicated strip trial gives better information on whether there is a localized issue that is affecting your results to a larger extent than the different rates of N. If you let me get you into On-Farm Network ( we can do the entire statistical analysis and aerial imagery as well as some lab work at no cost to you. It’s supported by checkoff funds, so you’ve already paid for it.

    Please think about it.

    1. Thanks, Meg. I remember a meeting last winter where a speaker mentioned there aren’t enough On Farm trials being done in Indiana.

  2. Did you see any difference during the growing season like colour or hight differences between the rates? It’s a strange result because even if available the plants take the N they need for a certain production. Can anyone explain this result because too much N is harmfull then?

    1. Even with my little quadcopter looking from above I couldn’t discern any difference to the naked eye during the season. I wouldn’t say the the 200lb rate is what hurt the yield, but we did see that more N did not continue to boost yield on this plot.

  3. Brian:

    Neat experiment, and simple enough that even statisticians like Meg and I can understand it. Would you be willing to share your “unaveraged” yields for a slightly more sophisticated analysis? I’m always looking for good field experiments for my biostatistics students to cut their quantitative teeth on.

  4. Brian,

    This could be a much more useful set of data if you include the yields on either side of the plot. Can you calculate the yield for the two passes of 145 rate next to the 100 and 200 rate strips? So you’d have six bars, 145, 100, 145, 175, 200, 145. That way we’d have a better idea how much of the yield variation was due to chance. The way you have it, with just one number for each rate, doesn’t tell you how much variation was due to other factors.

    1. I can come up with those. I also have record of each pass of the combine. So I really have two yield numbers for every rate. When I get time I can add this to the post.

    2. This afternoon I quickly pulled that info from the yield map as best I could with an area selection tool. The strip next to the 200lb test made 224bu. The one next to the 100lb rate made 238. So another way to look at the results is the yield went up from West to East.

        1. Not certain, but we’ve been cover cropping this field for a few years now so hopefully we are getting more available N that way.

          In another field this year in conventional till I forgot to turn on the applicator for one whole pass. That usually gets half yield or lower, but this year they must have been a lot of available soil N because the yield monitor never dropped under 150 through the whole pass.

      1. Thanks for looking up that data. So it really looks as tho the yield differences were due to soil variation, not the nitrogen rates. The best interpretation of this plot would be to say that yields maxed out around 100 lbs/acre and were mostly unaffected by the additional nitrogen.

Comments are closed

%d bloggers like this: