Is Nitrogen Necessary for Decomposition?
Q. What’s your opinion on applying small amounts of nitrogen in the fall, along with a microbial inoculum and biostimulants, to help expedite the conversion of plant products into microbial biomass?
A. It depends a bit on the soil context, and it depends on how you define a small amount. My historical distaste for fall applications of nitrogen is primarily in regard to large amounts!
We know that there is a chemistry balance that needs to happen to form optimal levels of mineral-associated organic matter — there is an optimal balance of carbon to nitrogen to phosphorus and to sulfur. It’s very common for us to have conversations about carbon-to-nitrogen ratios or nitrogen-to-sulfur ratios, but we often don’t look at that full combination — the full spectrum. But if we want to take this stoichiometry approach to balancing carbon, phosphorus, nitrogen and sulfur levels, and to do that accurately, we also have to take into consideration what the soil is capable of delivering, because during the winter months, the fungal population is particularly good at accessing those other elements — N, P and S — if they are present in the soil profile.
So, I wouldn’t be immediately dismissive of the idea of a small fall nitrogen application — no more than three to five pounds per acre — but what is probably going to happen is that you’ll have a narrower carbon-to-nitrogen ratio, and you will stimulate bacterial populations more, relative to fungal populations, and that could conceivably lead to more mineralization and more nutrient loss in the fall and over the winter months compared to the spring.
Overall, if the idea of adding nitrogen is just to get more rapid residue decomposition, I’d say you can do that with a good biostimulant alone.
