What impact do C:N ratios have on residue management when adopting new soil health practices?

The adoption of new soil health practices will affect the speed and location of residue breakdown and can impact how you handle residue management.

Generally, in soil health systems there is more residue at the soil surface whereas, in conventional systems that residue is buried. Previous wisdom shares that crop residue must be completely buried (for good reason) to help dry fields, reduce disease pressure, and make planting easier. We have since made significant progress in our understanding of disease cycles and genetics, increased technological advancements, and understanding of biological processes in the soil. This understanding of soil biological processes refer to C:N ratios in soil and how microbes consume plant residue.

The C:N ratio is the amount of soil carbon compared to the amount of soil nitrogen. For example, if you have a C:N ratio of 20:1, this means you have 20g of carbon vs 1g of nitrogen found in that organic matter.

Soil microbes are the first to be fed in the soil (yes, even before your crop) and aid in consuming and transforming nutrients into plant-available forms. Because of this, soil microbes require a C:N ratio of about 24:1. If the total carbon in the soil exceeds much more than that ratio, the soil microbes will start to compete with your crop and consume any available nitrogen. Another way to think about this is by comparing these ratios to how you must balance a ration in cattle feed to adjust protein vs. roughage. In a high roughage diet, you may have to supplement the feed with urea to balance the gut microbes in the rumen. Similarly, if the diet is too high in protein, adding roughage is a necessity. This process of balancing cattle feed ration for gut health is very similar to balancing C:N ratio for soil health. 

It is important to keep C:N ratios in mind when planning crop rotations and cover crop blends. Ex. You have a cereal rye cover crop before corn, if terminated late (at anthesis, see Table 1 below) you will have to supplement with additional N to prevent competition between your soil microorganisms and corn.

Well-fed soil microorganisms increase the amount of other nutrients released from the soil as the organic matter and residue continue to decompose. This process of nutrient transformation is enabling growers who have adopted soil health practices to reduce the amount of fertilizer they apply.

When to think about C:N ratios

• Management of heavy crop residue: You can use cover crops with a low C:N ratio to help break down residue in the fall and early spring.
• Fertilizer decisions in the spring: Understand what species you have and when to terminate your cover crop as this will influence how much N will be available to your commodity. This can largely impact your game plan in the spring.
• Cover crop species selection: Be mindful of your overall C:N ratio when making cover crop blends. (See Table 1 for more insight on this).
• Grazing of forage production: If you plan to bale or graze your cover crop knowing the species/blend will help with planning the next crop rotation. If baling takes place, be mindful of the nutrients being removed from the field.

References: 

Table 1. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcseprd331820.pdf