What is soil?
Soils form the skin of the Earth. Their thickness varies from a few millimetres – there where the soil is very young or scraped-off by external forces (e.g. water, wind, human activity) – to several metres – there where they occur in protected or stable places. They comprise of layers or soil horizons, each with their own characteristics. Soil material consists of a variable and often complex mixture of organic matter, sand, silt and clay particles, or is composed of dominantly organic debris.
Soils are formed under the influence of climate, the material they occur on, the flora and fauna (including human activity), the topography of the terrain, and time. These five factors are commonly known as the Soil forming factors.
Functions of soil
Soil is our life support system. Soils provide anchorage for roots, hold water and nutrients. Soils are home to myriad micro-organisms that fix nitrogen and decompose organic matter, and armies of microscopic animals as well as earthworms and termites. We build on soil as well as with it and in it
Soil plays a vital role in the Earth’s ecosystem. Without soil human life would be very difficult. Soil provides plants with foothold for their roots and holds the necessary nutrients for plants to grow; it filters the rainwater and regulates the discharge of excess rainwater, preventing flooding; it is capable of storing large amounts of organic carbon; it buffers against pollutants, thus protecting groundwater quality; it provides Man with some essential construction and manufacturing materials; it also presents a record of past environmental conditions
Soil functions are general capabilities of soils that are important for various agricultural, environmental, nature protection, landscape architecture and urban applications. Six key soil functions are:
- Food and other biomass production
- Environmental Interaction: storage, filtering, and transformation
- Biological habitat and gene pool
- Source of raw materials
- Physical and cultural heritage
- Platform for man-made structures: buildings, highways
Soils have a life cycle; they are born, mature and become of age. In cases, they may even "die" (disappear or get buried).
Soil formation starts by desintegrating the rock under the influence of climate. Rainwater will dissolve rock elements, temperature fluctations will cause differential expansion and contraction of rock-forming crystals, and the freezing and thawing of water captured in the rock will widen existing cracks and cavities. Pioneer vegetation, at first lichens, will settle and their roots will further loosen the rock. Moreover, decaying plant debris will produce organic acids, which further attack the rock. Organic matter will start to accumulate and be mixed with the mineral material provided by the rock. A soil is born.
Over time, rock minerals will be dissolved or transformed. Elements released from the rock will precipitate and new minerals may be formed. For example, iron will be oxidized and precipitate as iron oxides or hydroxides, giving the soil reddish or yellowish-brownish colours. Soil fauna will settle and mix (‘homogenize’) the soil. The soil will grow in depth through newly formed soil material at the bottom. The soil matures.
Given sufficient time under stable environmental conditions, soils will reach a steady state, whereby soil build-up matches their breakdown. Production of humus from decaying vegetational debris will equal its consumption by soil microbae, fauna and flora. Transformation of rock minerals into soil minerals will keep pace with the removal of earlier formed soil minerals. Slow surface wash of topsoil is matched by new formation of soil material from the bedrock. The soil has aged.
Dramatic events can interrupt the life cycle of soils. A soil may be covered suddenly by a volcanic eruption or by submergence under water. They may disappear in part or altogether by water or wind erosion.
On steep slopes the newly formed loose soil material will be removed fairly easily by runoff of rainwater or gravitational forces. Therefore, soils in mountainous regions are often shallow. On footslopes and in more level areas soil material will accumulate and this leads to deeper soils.