Soil health is the capacity of a soil to function in terms of biological productivity, environmental quality, and plant and animal health. As such, it is our best indicator of long-term sustainability in land management. Soil carbon status is one of the most important factors in soil health, and our best opportunity for improvement.
Soil organic matter includes all organic components of the soil system: living and dead plant and animal tissue, excretions, and microbes. While only a small percentage of the soil (less than 3% in most Wyoming agricultural soils), it is very important for soil health, disease suppression, drought resistance, water quality and quantity, and long-term agricultural viability. It is what gives healthy soil it’s dark brown color and rich, earthy smell.
and the activity of worms and microbes.
Soil microbes break down complex carbon-based molecules in crop residues and manure (ie. cellulose, lignin, fat, protein) for food and energy. As a result, nutrients are made available to plants and carbon dioxide is released as a byproduct. Nitrogen and many other nutrients in soil organic matter are not available to plants until the microbes do their job. The microbes responsible for the most rapid organic matter decomposition are aerobic (require oxygen). Tillage introduces oxygen into the soil, stimulating microbial activity. This burst of microbial activity leads to increased rates of organic matter metabolism in the soil, and subsequent loss of soil carbon as carbon dioxide. There many soil benefits to adopting no-till or reduced tillage systems, however, the long-term effect of no-till on soil carbon levels (especially in subsurface soil) is still unclear.
The passive pool of soil carbon turns over in hundreds to thousands of years. It is very stable, and physically protected from soil microbes by forming organic-clay complexes. This pool is the major contributor to cation exchange capacity (ability to hold nutrients), and water holding capacity of the soil. It is very slow to change, and primarily lost through wind and water erosion of topsoil. Humus is part of this pool and has been shown to promote root development and plant growth.
The slow pool of soil carbon is an intermediate pool that turns over in decades and provides some benefits of the other pools. It is especially valuable for its slow release of nitrogen and micronutrients. Changes due to tillage and cropping systems may take longer to manifest than in the rapid pool.
Observing changes in these three basic characteristics (color, smell, and structure) over time can tell you a lot about the effects of your current management on soil health and carbon status. Laboratory soil tests will typically include soil organic matter along with N, P, K and micronutrients. Watching how the level changes over time can be very informative, especially if you are making any changes to cropping or tillage systems. As they say, “if you don't measure it, you can’t manage it.”
The process of observing and recording changes in the soil over time can be facilitated by Soil Health Score Cards. Many states and regions have their own version, including Idaho, Montana, North Dakota, and Nebraska.
As you manage soil N, P, and K for maximum crop production, consider ways to manage C too. The long-term benefits will be well worth the investment.
Addition blog posts on Soil Management in Forage Production and Soil Testing