Organic matter (or organic material) is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds. The definition of organic matter varies upon the subject it is being used for.
Organic matter may refer to matter which was once part of a living organism or produced by a living organism. This definition is synonymous with biotic material, and would include a clam's shell and naturally produced urea, while excluding synthetically produced urea. While this definition is useful for modeling nutrient flows, it is not useful in measuring the organic content of soil.
Organic matter may be defined as material that is capable of decay, or the product of decay (humus), or both. Usually the matter will be the remains of recently living organisms, and may also include still-living organisms. Polymers and plastics, although they may be organic compounds, are usually not considered organic material, due to their poor ability to decompose. A clam's shell, while biotic, would not be considered organic matter by this definition because of its inability to decay.
Measurements of organic matter generally measure only organic compounds or carbon, and so are only an approximation of the level of once-living or decomposed matter. Some definitions of organic matter likewise only consider "organic matter" to refer to only the carbon content, or organic compounds, and do not consider the origins or decomposition of the matter. In this sense, not all organic compounds are created by living organisms, and living organisms do not only leave behind organic material. A clam's shell, for example, while biotic, does not contain much organic carbon, so may not be considered organic matter in this sense. Conversely, urea is one of many organic compounds that can be synthesized without any biological activity.
The equation of "organic" with living organisms comes from the now-abandoned idea of vitalism that attributed a special force to life that alone could create organic substances. This idea was first questioned after the abiotic synthesis of urea by Friedrich Wöhler in 1828.