The impact of soil carbon to nitrogen ratios on disease suppression

A foundational goal of regenerative agriculture management practices is to increase the volume of carbon that is cycled through soil systems. Not just statically stored in soils, but cycled through. The more volume of carbon that is cycled, the more robust the soil microbial community becomes, the more efficient plant photosynthesis becomes, and the better the entire ecosystem functions.

When more carbon is cycled in different forms, microbial balance and activity shifts to match, which results in changing the quantity of nitrogen that is sequestered, and the quantity of phosphorus, sulfur, silicon, and trace minerals that are released from the soil mineral matrix.

When abundant carbon is cycled, soil biology has the food sources required to fix all the nitrogen they require from the atmosphere, and no additional N needs to be added. This also results in a change of the dominant direction of N mineralization to be primarily nitrate or ammonium, which influences disease suppression and crop nitrogen sufficiency.

Here are some important thoughts on this topic Don Huber shared:

John: What is the impact of carbon-to-nitrogen ratios on both disease-suppressive soils and also on yield?

Don: The carbon-to-nitrogen ratio depends on the carbon source.

That got me in trouble with my first publication in plant pathology. I challenged the carbon-nitrogen ratio hypothesis. People were saying, “If you have a 12:1 versus a 40:1 ratio, you’ll always have a disease relationship.” And I demonstrated that it’s not the carbon-to-nitrogen ratio. It’s the form of nitrogen that is involved in that ratio.

You can take different crop residues or a different cropping sequence, and it’s the effect of that sequence on the form of nitrogen that determines what the disease reaction is. And, of course, the effect of that form of nitrogen quite often is an effect on manganese or zinc or copper or other nutrients, along with the form of nitrogen.

Carbon-nitrogen ratios work if you’re working with the same nutrient source or crop residue and then varying the nitrogen ratios by either harvesting plants when they’re greener or harvesting plants drier—when you have wider carbon-nitrogen ratios. But the carbon-nitrogen ratio per se isn’t the factor that’s involved there. It’s the effect of that ratio on the form of nitrogen and the other minerals that are involved—such as manganese or zinc or iron or copper—that are critical for particular physiological processes.

P.S. I had an interesting discussion with Koen van Seijen on the Investing in Regenerative Agriculture podcast that just released. Much of our discussion revolved around the question, “How would I invest a billion dollars in accelerating the adoption of regenerative ag?” You can find it here.