How did many people running test plots in 2016 produce over 100 bushels per acre of corn on strips where no commercial fertilizer was applied? In some cases, strips where zero nitrogen was applied stayed green well into the season. Why?

Jim Schwartz of 360 Yield Center believes the answer is related to how well most soils mineralized nitrogen. He notes it was a stark contrast to what happened in 2015.

Here is a primer that explains why Schwartz believes N mineralization varied over the past two seasons.

Agronomists talk about average mineralization amounts. You’ve likely heard an agronomist say that in a normal year, expect the soil to provide 30 to 40 pounds per acre of N through mineralization. While Schwartz believes that’s likely true, he also believes there are years where that amount may be higher, and years where it may be lower. He suggests that mineralization released 80 pounds per acre or more of N in usable form for crops in 2016. In 2015, the amount released was likely below average, he says.

Mineralization of nitrogen defined. David Crohn of the University of California defines nitrogen mineralization as the process by which organic nitrogen in the soil is converted into plant-available in-organic forms. Nitrogen tied up by cover crops, for example, must be released, or mineralized, before it’s available to the crop.

Mineralization of nitrogen requires oxygen. One big factor that can result in higher mineralization rates of nitrogen during the season is the amount of oxygen present in the soil, Schwartz says. The more oxygen that is available, the faster mineralization can occur.

Oxygen content in soil flip-flopped from ’15 to ’16. Where areas received huge amounts of rain in 2015, oxygen became limiting early in the season, Schwartz says. Waterlogged soils have far less nitrogen content than dry soils. And while waterlogged soils contribute to N loss, they also result in less mineralization of nitrogen.

Temperature matters. If you want higher rates of nitrogen mineralization, kick up the average temperature, Schwartz says. Temperatures were definitely warmer on average across the Corn Belt in 2016 compared to 2015.

Deeper rooting finds more N. Corn roots tended to go deeper in 2016 vs. 2015 because overall conditions were more favorable, Schwartz says. When roots go deeper, the amount of nitrogen produced through mineralization increases simply because more soil volume is in play.

Less N needed per bushel of corn produced. The bottom line is that when all these factors are combined, it’s not surprising that some growers produced 220 to 240 bushels of corn on 170 to 180 pounds of total commercial N applied this year, Schwartz observes. That’s far less than 1 pound of N per bushel produced. The difference was that 2016 was a very good year for nitrogen mineralization, he says.