Whereas
soil is formed from the rock below, it is eroded away from the top. A
cover of plant life slows down erosion, allowing the soil layer to build
up, but there is more going on. Just above the base rock, is the C-horizon, containing the recently weathered and still weathering soil. It is rich in nutrients. The A-horizon is where most plant roots are found and all soil organisms. Its nutrients have been used by plants or leached downward, so it is relatively poor in nutrients, but rich in life. By comparison, the B-horizon is the zone where new material from below and nutrients from above accumulate. Sometimes an impermeable layer or pan is formed above it (podsol), denying plants to access this rejuvenating source of new nutrients. On the surface of the soil, often a thin layer is found, rich in leaf litter and other organic material.
| horizon | description of detailed soil horizons |
| O | consists mainly of organic matter from the vegetation, which accumulates under conditions of free aeration. |
| A | eluvial (outwash) horizon consisting mainly of mineral matter mixed with some humified (decomposed) organic matter. |
| E | strongly eluviated horizons having much less organic matter and/or iron and/or clay than the horizons underneath. Usually pale coloured and high in quartz. |
| B | illuvial (inwashed) horizon characterised by concentrations in clay, iron or organic matter. Some lime may accumulate, but if the accumulation is excessive, the horizon is named K. |
| K | horizon containing appreciable carbonate, usually mainly lime or calcium carbonate. |
| G | gleyed horizons which form under reducing (anoxic) conditions with impeded aeration, reflected in blueish, greenish or greyish colour. |
| C | weathered parent material lacking the properties of the solum and resembling more the fresh parent material. |
| R | regolith, the unconsolidated bedrock or parent material. |
Soil and top soil are produced naturally at a rate of 1mm in 200-400 years, averaging at about 1 ton/ha/y. A full soil profile develops in 2,000 - 10,000 years, a period which is long for humans but short for the planet. World-wide, agricultural soil is lost at a rate 10-40 times faster than its natural replacement. The USA lost 80mm since farming began, 200 years ago. This amounts to some 18 t/ha/y. China appears to lose 40 t/ha/y. World-wide loss of agricultural land is 6 million ha per year, from a world-wide total of 1200 million ha (0.5%/y). These are compelling reasons for improving the way humans manage their soils.
Information provided by: http://www.seafriends.org.nz