In today’s agricultural media, there is a lot of emphasis on soil health, soil biology, and soil carbon. These topics are all interrelated. Whether your goal is the improve the structure of your soil, increase the natural nutrient cycling from one crop residue to the next, or build your bank of soil carbon, the soil microbes need a steady feed source of carbon-based material to carry out these functions. However, not all materials are equal.
One of the best measurements to determine whether or not a material is easily decomposable is the carbon to nitrogen ratio (C:N). Microbes are most easily able to decompose material with a C:N around 25:1. At this level, the microbes can utilize the carbon converting most of it to carbon dioxide leaving behind soil organic matter that has a C:N of about 10:1. The microbes will continue to decompose the remaining soil organic matter, but at a slower and slower rate due to the complexity of the molecular structures that are formed.
The most common form of carbon inputs is the crop residue that remains after harvest. Soybean residue has a C:N of about 25:1, meaning it can be easily decomposed. Corn and wheat residue can have a C:N ranging from 50:1 to 100:1. While these residues are a great source of building carbon, the microbes will compete with your next growing crop for available nitrogen potentially inducing a nitrogen deficiency. Other common carbon inputs are manures. Most manures have C:N around 5:1 to 20:1 which means that there is more than adequate nitrogen for the microbes to utilize while releasing the excess for a growing crop to use. Manures with a high volume of bedding materials such as straw or wood shaving should be tested to ensure that they will not cause a nitrogen deficiency.
Most cover crops have desirable C:N for easy decomposition. However, grasses will have high carbon content than legumes, brassicas, etc. and may require adjusting a nitrogen program for the following commercial crop.