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Free fertilizer!

It’s true, many of your soils already have plenty of nitrogen in them – you just have to figure out how to unlock it.

One of the amazing things about your soils is that many of them already have enough nitrogen to grow an optimal corn crop this year – and for years to come – without applying any more. 

So why do you have to pay for commercial nitrogen (N) to secure top-notch yields? 

Because most of what is there isn’t available to be taken up by the roots. For that to happen, the process of N mineralization (the natural conversion of organic nitrogen in the soil to the ammonium form) has to take place.

Fabian Fernandez, a professor and Extension educator in nutrient management and environmental protection at the University of Minnesota, is as perplexed about this as you are. 

“The right amount of applied nitrogen can vary from year to year,” he says, “because the amount that’s mineralized varies.”

Exactly why that is and what you can do to get the soil to give up more of its stored N is a head-scratcher even to the experts. What they do know is, if you can get more mineralization, you might be able to reduce the amount and expense of commercial N. Meanwhile, you might also improve the timing of commercial applications if N applications are needed. This can  reduce leaching and runoff into water sources in addition to securing enough N for high yields.

The facts 

  • About 5% of the organic matter in soils is nitrogen, says Fernandez. 
  • About 1% to 3% of that N converts to plant-available nitrogen every year.
  • Soils with 3% to 5% organic matter have about 3,000 pounds to 5,000 pounds of N per acre in the top few inches. If 3% converts to plant-available N in a given year, that’s 90 to 150 pounds of N that you get free! 

The problem is, you don’t know which year or which set of circumstances will see that much mineralization or if there will be none at all. 

“Lots of people are trying to recommend rates of applied nitrogen based on standards of mineralization,” says Fernandez. “We have very little information on that. For example, in one of our studies in several soils across Minnesota, soil organic matter varies from as low as 1.5% to as high as 5.6%. That’s a huge range of nitrogen available to be mineralized.”

Then, there’s the issue of what makes mineralization happen. At least two things stand out, but neither is crystal clear.

One is the application of commercial N. “In our field and laboratory studies, we see that as we apply commercial nitrogen, we get more mineralization,” Fernandez says. “It’s like we sort of prime the pump for mineralization.”

Fernandez speculates that it’s because the applied N enhances the microbes and soil chemistry that allows for more mineralization in the organic matter. 

“The amount of nitrogen we get mineralized far exceeds the amount we get when no supplemental nitrogen is applied,” he says. “We don’t have enough data yet to really get to the reason. 

“Another question is how much nitrogen is needed to get the full priming effect,” he adds. “At this point, I don’t think it is linear – it won’t increase indefinitely with increasing N rates. I’m not sure how much is needed to maximize mineralization in different soils.”

Soil Saturation

The second issue affecting N mineralization is soil saturation with water. “We know mineralization is an aerobic process; it requires oxygen,” he says. “So in theory, well-drained soils should have more mineralization.”

That theory holds up in lab experiments. Yet, in preliminary and limited field trials, Fernandez has documented more mineralization in undrained soils. 

“We’re asking why that would be,” he says. “It makes the point that sometimes there are book values generated in the lab that may or may not relate to what is actually happening in the field.

“A lot of things impact mineralization, but we just have low understanding of them,” Fernandez summarizes. “We are going to figure this out. If we can predict more closely how much nitrogen mineralizes, then we can apply the right amount of fertilizer more precisely than we apply it today. That will translate into greater efficiency.”

What if you don’t apply N?

Fabian Fernandez has reviewed dozens of corn yield studies with varying rates of nitrogen. In virtually all of those trials, there’s a check plot that gets no N at all. In the recent studies, corn yields in the check plots have varied from a low of 52 bushels an acre to a high of 218 bushels. 

“The average is 116 bushels,” says Fernandez. “We get about half a yield with no N fertilizer.”

There is that odd year or field where the check plot yields as well as the plot that got a full rate of nitrogen. Fernandez admits that can be baffling. 

“We’ve just been looking at this seriously for two years,” he says. “I’m optimistic we will find answers and make progress on this.”

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