Abstract
This chapter discusses the soil humus and total organic carbon (OC) content. It reviews the results concerning the changes in soil organic matter (SOM) chemical nature regardless of the technical methods used. Information about C cycling and storage in soils is also given. Soil humus is characterized by its amounts and properties such as water retention capacity, aggregation and cementing agent, and the ability to absorb both nutrients and contaminants. The uncontrolled decrease in soil humus content affects soil water regime, aeration, fertility, and purifying capacity. The chapter focuses on the main soil types and the areas of the world where they are found, and considers the distribution of humus in these soils and areas, and the factors that can affect it. It shows that soil humus is the main OC reservoir at the earth surface and that its distribution is not comparable with the surface distribution of taxonomic units. This observation reviews humus characteristics in the four soil units that contribute to more than 80% of total earth OC: podzols, histosols, chernozems, and ferralsols.
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