Soil-Forming Factors – Living Organisms
All plants and animals (from microorganisms to humans) affect soil formation. Living organisms add organic matter – a key component of soil – through the breakdown of litter, decomposition of dead roots and the conversion of compounds exuded (i.e. released, from living roots). Microorganisms, especially fungi and bacteria facilitate chemical exchanges between roots and the soil to produce essential nutrients. Both animals and plants allow moisture and gases to seep into deeper layers along burrows and root channels.
Humans can impact soil formation through land management practices that disturb natural processes and change the chemical and physical characteristics of the soil. Cultivation practices and burrowing animals mix soil from different horizons, especially from the organic-rich surface layers. The nature of biological activity in the soil is governed by climate, topography and soil characteristics, such as depth, texture, structure and chemistry (e.g. pH and salinity).
Ecoregions and biomes
Ecoregions can be defined as relatively large units of land or water containing a distinct assemblage of natural communities and species, with boundaries that approximate the original extent of natural communities prior to major land-use change. The limits of ecoregions generally follow continental boundaries or major barriers to plant and animal distribution (such as the Himalayas and the Sahara).
Ecoregions are classified by the presence of biomes, which are major plant communities determined by rainfall and climate. Forests, grasslands (including savannah and shrubland) and deserts are distinguished by climate (e.g. tropical, subtropical and temperate) and water conditions. In addition, forests are divided into conifers, broadleaf or mixed.
Land cover
The term ‘land cover’ is used to describe the physical material at the surface of the planet. While predominantly vegetation, it can also be bare ground, water or artificial surfaces. Depending on the scale of observation and complexity of the cover type, the eventual classification may be a mixture of the above. It is important to distinguish between the terms ‘land cover’ and ‘land use’. For example, a land cover of mixed shrubs and grass could be used as a park, an orchard or savannah. The map below shows the principal types of land cover in 2012 as mapped by satellites orbiting the Earth. The map shows that equatorial regions are covered by extensive forests, which merge to the north and south with open woodland and increasing grasslands or savannah. Mid-latitudes are characterised by aridity giving rise to bare or sparsely vegetated areas. More temperate climates display a mosaic of croplands and forests that indicates the human alteration of natural vegetation patterns. Northern latitudes show mixed and conifer forests, which give way to the open shrubland of the tundra. At this scale, only the largest urban areas are visible.
While trees cover around 27 % of the planet, it is estimated that around three-quarters of the Earth’s vegetated surface have been altered by prolonged human activities.
Share of global land cover
- Bare soil and rock: 15.2 %
- Croplands: 12.6 %
- Grasslands: 13.0 %
- Herbaceous vegetation: 1.3 %
- Inland water bodies: 2.6 %
- Mangroves: 0.1 %
- Shrub-covered areas: 9.5 %
- Snow and glaciers: 9.7 %
- Sparse vegetation: 7.7 %
- Tree-covered areas: 27.7 %
