Soil formation on Earth and beyond: the role of additional soil forming factors

Humankind is an environmental variable whose activities are affecting more and more of the planet and its soils. At present, humans appropriate over 40% of the Earth's net primary productivity (Rojstaczer et al., 2001) and use 60% of freshwater run-off (Postel et al., 1996). It is intuitive that this exploitation of resources results in a massive impact on terrestrial ecosystems. Thus, humankind influences ecological processes on a global scale, sometimes on a par with the role of climate, geological forces and astronomical variations. On this basis, it has been proposed that we are entering a distinct geological era, the ‘Anthropocene’, a period of intensive human industrialization and land change which started in the late eighteenth century, marked by the increase in atmospheric concentrations of carbon dioxide and methane (Crutzen, 2002). It has been estimated that today 83% of the ice-free land on Earth is affected directly or indirectly by humans (Sanderson et al., 2002). In summary, humans can be viewed as a distinct soil-forming factor, apart from other organisms that are not endowed with the ability to reason, due to the magnitude of their impact on the planet, and their cultural characteristics which drive their decision-making process (Amundson and Jenny, 1991). In any natural environment, population growth reaches a limit imposed by factors such as light, space, nutrients, or water. Carrying capacity is the maximum number of individuals a particular habitat can support.