Organic matter (or organic material) is matter that has come from a recently 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.
Soil organic matter
Soil is composed of minerals and organic matter, as well as living organisms. The minerals are derived from the weathering of "parent material" - bedrock and overlying sub-soil. The organic matter in soil derives from plants and animals. In a forest, for example, leaf litter and woody material falls to the forest floor. This is sometimes called organic material. When it decays to the point it is no longer recognizable it is called soil organic matter. When the organic matter has broken down into a stable humic substances that resist further decomposition it is called humus. Thus soil organic matter comprises all of the organic matter in the soil exclusive of the undecayed material.
There is a lack of consistency regarding the term soil organic matter. In soil science the term soil organic matter may include both decaying materials and humic substances (humus). However in agriculture, forestry and other disciplines, the term humus is sometimes used to describe both soil organic matter and humic substances (humus). The soil organic matter component of soil is referred to active humus and the humic substances (called humus in soil science) is called inactive humus.
Once living matter
Organic matter may refer simply 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 modelling 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.