IN THE FIELD
Cover crops and manure application are two ways to build soil organic matter.
ORGANIC MATTER IS A MEASURE OF SOIL HEALTH Soil organic matter metrics and emerging research can help farmers better manage their farm fields. by Amber Radatz, Matt Ruark, and Erica Gentry
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oil health has become a common part of the discussion for cropping and nutrient management systems. While soil health is more conceptual, soil organic matter often enters the conversation as a way to measure progress toward soil health benefits. However, the two aren’t necessarily interchangeable. For farms engaging in soil health practices and discussions, it is valuable to understand the intricacies of soil organic matter and the way it will be used in the future for farm and environmental management. The science of soil health and soil organic matter is in its infancy. By providing a summary of
the state of the science, we hope it will help bring context as you come across new results.
What is soil organic matter? Soil organic matter (SOM) is a key component of both soil health and soil fertility, and there is renewed interest in building SOM for improved soil function. SOM (or soil organic carbon, which compromises the majority of total SOM) is included in nearly all lists of soil health metrics. For example, the Cornell Assessment of Soil Health includes SOM (via loss on ignition) as part of the basic package. Historically, nearly all farmers have at least one measurement
6 | Journal of Nutrient Management | February 2021
of SOM on their fields as it is included in most standard routine soil fertility packages offered by soil test labs. Soil organic matter (ranging from 1% to 5% in most soils) is comprised of dead plants and organisms, as well as animal wastes, in various stages of decay. The majority of SOM exists in a stabilized form, historically referred to as humus. This highly stable material is the carbon that is bound to clay particles or found inside of soil aggregates. Soil aggregates are stable pieces of soil that are a mix of silt, clay, organic material, soil bacteria, and fungal hyphae. Some SOM exists on the outer edges of aggregates or outside the aggregate completely. This part of the organic matter is referred to as the active pool, which “turns over” during a single growing season. In other words, it’s the pool of organic matter from which plant-available nutrients are released. The active pool of SOM consists of permanganate oxidizable carbon (POXC) and mineralizable carbon (min-C). POXC can be measured by jofnm.com