<P> The positive impacts of SOM result from a number of complex, interactive edaphic factors; a non-exhaustive list of SOM's effects on soil functioning includes improvements related to soil structure, aggregation, water retention, soil biodiversity, absorption and retention of pollutants, buffering capacity, and the cycling and storage of plant nutrients . SOM increases soil fertility by providing cation exchange sites and acting as reserve of plant nutrients, especially nitrogen (N), phosphorus (P), and sulfur (S), along with micronutrients, which are slowly released upon SOM mineralization . As such, there is a significant correlation between SOM content and soil fertility . </P> <P> SOM also acts as a major sink and source of soil carbon (C). Although the C content of SOM is known to vary considerably, SOM is typically estimated to contain 58% C, and the terms' soil organic carbon' (SOC) and SOM are often used interchangeably, with measured SOC content often serving as a proxy for SOM . Soil represents one of the largest C sinks on the planet and plays a major role in the global carbon cycle . Therefore, SOM / SOC dynamics and the capacity of soils to provide the ecosystem service of carbon sequestration through SOM management have received considerable attention in recent years . </P> <P> The concentration of SOM in soils generally ranges from 1% to 6% of the total topsoil mass for most upland soils . Soils whose upper horizons consist of less than 1% organic matter are mostly limited to desert areas, while the SOM content of soils in low - lying, wet areas can be as high as 90% . Soils containing 12 - 18% SOC are generally classified as organic soils . </P> <P> It can be divided into three general pools: living biomass of microorganisms, fresh and partially decomposed residues, and humus: the well - decomposed organic material . Surface plant litter is generally not included as part of soil organic matter . </P>

In which soil horizons is organic matter found
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