Tunsisa Hurisso and Steve Culman, School of Environment and Natural Resources, Ohio State University
In short, the answer is usually both.
Soil organic matter is a mixture consisting of various compounds (e.g. simple sugars, cellulose, proteins, etc.) derived primarily from plants and microbes. It represents 1-6% by mass of agricultural soils, but plays a disproportionate role in soil function. Organic matter in soils acts like a sponge, holding nutrients (sequestration or stabilization) that become available to plants when organic matter is broken down (mineralization) by the collective action of the soil food web (mainly by bacteria and fungi). In addition to nutrients, organic matter also enhances the soil’s water holding capacity, making farmlands more resilient to periods of drought. In contrast, when organic matter is depleted due to repeated plowing and/or removal of crop residues from the field, the ability of a soil to hold water and nutrients will be greatly diminished. Therefore, growers should strive for both organic matter stabilization and mineralization processes to ensure short-term crop productivity and to build long-term soil resilience.