Soil organic matter (SOM) refers to material in soil that originates from living organisms, both plant and animal, that undergo decomposition. The decomposition of organic matter occurs in three stages: living, dead, and very dead. The living stages comprise about 15% of the total SOM, where living things break down organic matter into chemicals. The dead portion also makes up 15% of SOM, consisting of recently deceased organisms that begin the decomposition process. The final stage, the very dead stage, comprises up to 70% of SOM. This fraction is more resistant to decomposing and includes organic matter in the form of humus. Humus is the most resistant to decomposition yet is the most stable. It is this fraction that plays a key role in binding the soil and enhancing overall soil stability. 

SOM can originate from various sources such as crop residues, animal manure, compost, cover crops (green manure), and perennial crops. As decomposition occurs, this material releases nutrients into the soil. On average, each percent of organic matter in the soil can produce 20-30 pounds of nitrogen, 4.5-6.6 pounds of P2O5, and 2-3 pounds of sulfur over one year. While SOM alone isn’t enough to support a growing crop, conducting a soil test can provide a good direction in regard to fertilizer application.  

How does soil organic matter correlate with agriculture? 

In a typical agricultural field, SOM can range from 1-6%. The presence of SOM influences the chemical, physical and biological properties of the soil. SOM benefits the soil in several ways, including minimizing leaching and erosion, increase in biological function and diversity, nutrient supply through decomposition, increase to soil structure and overall stability and an increase to the overall water holding capacity. In sandier soils, the presence of SOM can be beneficial, allowing for water to be held for longer periods within the soil. In clay soils, SOM can also be beneficial, by reducing the cohesion between clay particles. SOM can also contribute to higher crop yields by improving water and nutrient availability within the soil. Studies have shown that increased SOM levels can lead to higher yields, particularly in crops like corn.  

In the context of herbicide applications, SOM plays a role in its effectiveness. As SOM increases, it can absorb and bind to the herbicide, making it less readily available within the soil. Therefore, with a higher SOM percentage, adjustments to herbicide rates should be considered.  

How to properly manage soil organic matter? 

Understanding SOM is essential for maintaining soil health. SOM changes slowly over time, therefore, any changes to your farming operation should be viewed as a long-term investment. A conservative tillage approach can increase SOM limiting residue burial. An alternative strategy to increase SOM is through the rotation of annual crops (if possible) and including higher residue crops and cover crops (green manure), which add additional organic matter to the soil. For example, chopping and leaving straw on the soil surface and not just leaving the roots or stubble can help to increase SOM. Implementing cover crops into the rotation as well, can increase SOM levels from 4-114% depending on the mix. Additionally, applying manure, if available, is a great source of organic matter and nutrients for the soil.  

Having intensive tillage and simple crop rotations can reduce SOM over time, leading to soil degradation through reduced stability, increased erosion and poor drainage. By prioritizing organic matter management, as a result you can see an improvement in your overall soil health, leading to long-term benefits such as improved fertility, structure, and productivity. These improvements not only support sustainable farming practices but also ensure that the soil can support a healthy crop.