Soil formation

Soil is a dynamic medium where chemical, physical and biological activities go on all the time. It is a product of decay and a medium for growth, changing with the seasons.

How soil forms (pedogenesis): it starts with weathering, whose mantle is the basic input.

• Bacteria, mosses and lichens colonise the weathered material or transported deposits; small organisms shelter in it.
• Dead remains build up humus. Grasses and ferns grow, then bushes and trees from seeds brought by birds and wind.
• Roots go down, burrowing animals bring particles up, and the material becomes porous and sponge-like, holding water and letting air through: a mature soil of mineral and organic matter.

Five soil-forming factors (they act together):

• Parent material (passive): weathered rock in place (residual soils) or transported deposits (transported soils); its texture, structure and minerals matter. Young soils show strong links to their parent rock.
• Topography (passive): soils are thin on steep slopes, thick on flat uplands; gentle slopes with slow erosion and good percolation are best; flat areas may build thick, dark, clayey soils.
• Climate (active): moisture and temperature. Heavy rain carries soil components down (eluviation) and deposits them lower (illuviation). In wet equatorial areas even silica is washed out (desilication). In dry climates, evaporation draws groundwater up and leaves salts as a crust, a hardpan. In tropical and intermediate-rainfall areas, calcium carbonate nodules (kanker) form. Heat speeds up chemical activity (except carbonation), so tropical soils have deeper profiles; tundra soils are mostly mechanically broken material.
• Biological activity: plants add organic matter and humus. In cold climates bacteria are slow, so humus builds up and peat forms (sub-arctic, tundra). In hot, humid climates bacteria are intense and dead plants are oxidised fast, leaving little humus. Bacteria fix nitrogen from the air; Rhizobium lives in the root nodules of leguminous plants. Ants, termites, earthworms and rodents rework the soil.
• Time: decides maturity and profile development. Soils on fresh alluvium or glacial till are young, with no or poor horizons; a mature soil has a full profile. No fixed number of years can be given.

Example (new): farmers who grow gram or peas between wheat crops are using the Rhizobium bacteria in those plants' roots to put nitrogen back into the soil without buying as much fertiliser.