<P> The scientists who studied the soil in connection with agricultural practices had considered it mainly as a static substrate . However, soil is the result of evolution from more ancient geological materials, under the action of biotic and abiotic (not associated with life) processes . After studies of the improvement of the soil commenced, others began to study soil genesis and as a result also soil types and classifications . </P> <P> In 1860, in Mississippi, Eugene W. Hilgard studied the relationship among rock material, climate, and vegetation, and the type of soils that were developed . He realised that the soils were dynamic, and considered soil types classification . Unfortunately his work was not continued . At the same time Vasily Dokuchaev (about 1870) was leading a team of soil scientists in Russia who conducted an extensive survey of soils, finding that similar basic rocks, climate and vegetation types lead to similar soil layering and types, and established the concepts for soil classifications . Due to language barriers, the work of this team was not communicated to western Europe until 1914 through a publication in German by Konstantin Dmitrievich Glinka, a member of the Russian team . </P> <P> Curtis F. Marbut was influenced by the work of the Russian team, translated Glinka's publication into English, and as he was placed in charge of the U.S. National Cooperative Soil Survey, applied it to a national soil classification system . </P> <P> Soil formation, or pedogenesis, is the combined effect of physical, chemical, biological and anthropogenic processes working on soil parent material . Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits of clay, humus, iron oxide, carbonate, and gypsum, producing a distinct layer called the B horizon . This is a somewhat arbitrary definition as mixtures of sand, silt, clay and humus will support biological and agricultural activity before that time . These constituents are moved from one level to another by water and animal activity . As a result, layers (horizons) form in the soil profile . The alteration and movement of materials within a soil causes the formation of distinctive soil horizons . However, more recent definitions of soil embrace soils without any organic matter, such as those regoliths that formed on Mars and analogous conditions in planet Earth deserts . </P>

What is the most important factor affecting the formation of soil