Anything you read about healthy garden soil and how to improve your soil mentions organic matter. While only a small fraction of agricultural or garden soils—between 3 and 6 percent1—consists of organic matter, it plays a crucial role in soil productivity. Being knowledgeable about organic matter is important even if you have “good” garden soil because all soil needs to be replenished with organic matter from time to time. Amending soil with organic matter is not a one-time project but an ongoing process.
This article explains what organic matter is, what differentiates it from organic material and fertilizer, and why organic matter is beneficial for soil productivity in many ways.
What is Organic Matter?
Generally speaking, organic matter comes from living materials that fix and store carbon and deliver it as a source of energy to the soil. More specifically, organic matter is divided into three types, depending on the time it takes for the organic matter to fully decompose. Active organic matter consists of fresh plant and animal residues that take from a few months to a few years to decompose. This type of organic matter is soil that is very much alive because it is filled with lots of active microorganisms. At the other end of the spectrum is passive organic matter, also called humus. It is the stable form of organic matter where the decomposition has already been completed and there is no longer any microbiological activity. Slow organic matter is somewhere between active and passive—it is organic matter that takes decades to decompose2, such as bones.
Organic Matter vs. Organic Material
Organic material is the source of organic matter: leaves, compost, manure, plant residues etc. As organic material decomposes, it changes its form and mass; it is an unstable material. About 90 percent of the organic material disappears during the decomposition process into organic matter. That’s why soil formation, of which the integration of organic matter is an initial step, takes so long—up to 100 years to form one inch of topsoil.3