The Atlas of the World Reference Base for Soil Resources illustrates the striking variability of soils in the pedosphere, reminding us that there is a lot of uncertainty in the details of spatial patterns and explanations of soil evolution.
Because soil conditions such as fertility, drainage, and topography can be artificially modified and changed by external activities, it is often assumed that the pedosphere is a renewable resource. However, experience has demonstrated that maintaining soil functions desired by society is not ecologically sustainable; rather, they must be reinforced with external energy and substances. Many ecologically and agriculturally important soil features have their characteristic times of formation and/or resilience much longer than human lives and even longer than some civilizations. The interactions of environmental conditions in natural ecosystems produce modifications much more slowly in soils than needed by modern society to provide expected products and services. During the next 50 years, demand for food crops is projected to grow by 70-85% under the Millennium Assessment scenarios, and demand for water by 30-85%.
The pedosphere with its functional and structural features has its own space and time limitations. Thickness and area are spatial limitations, whereas temporal functions and soil processes vary so widely that incon-gruencies and inconsistencies often make successful management or control very difficult.
Soil thickness is not the thickness of the rooting zone, rather it is the unspecified thickness of the upper layer of the lithosphere involved in regular bio-litho-atmo-hydrosphere interactions. All of the interactions and resulting processes are relevant to defining the functional thickness of soils. This pedosphere thickness strongly controls and regulates the interactions - it is a real biogeomembrane of the Earth. The shallowness of fertile topsoil limits agricultural use and is susceptible to contamination by pollutants, in addition to degradation and destruction due to human-induced erosion.
Assuming the ice-free land area is about 131 Mkm , it has been estimated that about 93 Mkm2 is biologically productive land, of which forests are about 33%, pastures 32%, and cropland 11%. Only about a third of the land surface has pedosphere components that can reasonably be expected to provide sufficient food to support our current human civilization. Major limitations for agriculture include drought, nutrient deficiency, pollution, shallow depth, excess water, and permafrost. Other use limitations involve expansion of urban areas and transportation networks, small isolated tracts of suitable land, traditional parceling of land ownership, and high costs of preparing land for cultivation.
Why are temporal functions a limitation. As mentioned, natural changes of the pedosphere occur at rates too slow to satisfy the desires of modern society. Rates and characteristic times of soil functions, formation, and evolution processes cover at least 9 orders of magnitude (from 10~3 to 10 years). During the Anthropocene, humans have exploited the pedosphere's 'treasure trove' that accumulated over millennia and hundreds of thousands of years of natural soil formation and evolution, creating a modern-day dilemma.
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