Q. Starter fertilizer can shut down the rhizophagy cycle; if you side-dress later in the season, though, would that do the same thing?
A. Side-dressing later in the season can have a similar detrimental effect, but not to the same degree as with starter fertilizer. And there are things that you can do to mitigate how such an application affects the microbial life in the soil and the plant. There are buffers that you can add — humic substances, carbohydrates, enzymes, etc.
Many people talk about stabilizing nitrogen by adding nitrification inhibitors, or urease inhibitors. But from my perspective, that’s going down the wrong road, because these nitrification and urease inhibitors kill biology, and then there’s no biology to metabolize these different forms of nitrate and convert them. I prefer the opposite pathway: when doing a liquid-nitrogen side-dress application, add microbial stimulants and soluble carbon sources to speed up microbial digestion of the applied nitrogen. Whatever nitrogen is being applied — liquid-32, liquid-28, liquified urea, etc. — should be immediately absorbed and incorporated into bacterial cells. When that happens, the nitrogen becomes plant available, but it doesn’t leach. It’s available but not soluble.
The other part of the answer is that when you have a nutrient-rich environment at planting, the plant root system will never be fully colonized with endophytes because the plant doesn’t need those microbes. Five to six weeks later, when the plant wants to establish that microbial community, it has to play catch up, but it never quite can. The root biomass is continuing to grow, and it never fully catches up.
But if you wait to make that nutrient application a few weeks after planting, then that root microbiome is now completely colonized. A nitrogen application will still have a temporary suppressive effect on that microbiome, but as that nitrogen is completely incorporated into the microbiome and is encapsulated in bacterial cells, the microbiome quickly recovers and continues to go. So, there’s less of a net-negative impact when it’s applied later.
From John Kempf’s presentation at the 2024 Acres U.S.A. conference.
