Soil organic matter is anything in the soil of biological origin. Carbon is its key component comprising about 58% by weight. Simple assessment of total organic matter is obtained by measuring organic carbon in soil. Living organisms (including roots) contribute about 15% of the total organic matter in soil. These are critical to operation of the soil carbon cycle. What follows refers to the remaining 85% of the soil organic matter - the non-living component.
As shown below, non-living organic matter in soils can be grouped into four distinct categories on the basis of size, behaviour and persistence. These categories are arranged in order of decreasing ability to decompose. Each of them contribute to soil health in different ways.
Dissolved organic matter (DOM): is the organic matter which dissolves in soil water. It comprises the relatively simple organic compounds (e.g. organic acids, sugars and amino acids) which easily decompose. It has a turnover time of less than 12 months. Exudates from plant roots (mucilages and gums) are included here.
Particulate organic matter (POM): is the organic matter that retains evidence of its original cellular structure, and is discussed further in the next section.
Humus: is usually the largest proportion of organic matter in soil, contributing 45 to 75%. Typically it adheres to soil minerals, and plays an important role structuring soil. Humus is the end product of soil organism activity, is chemically complex, and does not have recognisable characteristics of its origin. Humus is of very small unit size and has large surface area in relation to its weight. It holds nutrients, has high water holding capacity and significant cation exchange capacity, buffers pH change and can hold cations. Humus is quite slow to decompose and exists in soil for decades.
Resistant organic matter: has a high carbon content and includes charcoal, charred plant materials, graphite and coal. Turnover times are long and estimated in hundreds of years. It is not biologically active but contributes positively to soil structural properties, including water holding capacity, cation exchange capacity and thermal properties.