Copyright © 2009 Regents of the University of Minnesota. All
rights reserved. a. Soil organic matter (SOM): chemical nature
and fractions b. SOM levels in the Northern Great Plains Soil
organic matter (SOM) has been called “the most complex and least
understood component of soils” (22). The Soil Science Society of
America defines SOM as the organic fraction of soil after
removing undecayed plant and animal residues (27). However,
organizations and individual researchers differ in their opinion
about whether undecayed plant and animal tissues (e.g. stover,
dead bugs, earthworms, etc.) should be included in the
definition of SOM. In this chapter, we will adopt a broader
definition of SOM proposed by Magdoff (1992), which consider SOM
to be the diverse organic materials, such as living organisms,
slightly altered plant and animal organic residues, and well-
decomposed plant and animal tissues that vary considerably in
their stability and susceptibility to further degradation.
Simply put, soil organic matter is an