Pedosphere

The primary conditions for soil development are controlled by the chemical composition of the rock on which the soil will be. Rock types that form the base of the soil profile are often either sedimentary (carbonate or siliceous), igneous or metaigneous (metamorphosed igneous rocks) or volcanic and metavolcanic rocks. The rock type and the processes that lead to its exposure at the surface are controlled by the regional geologic setting of the specific area under study, which revolve around the underlying theory of plate tectonics, subsequent deformation, uplift, subsidence and deposition. Metaigneous and metavolcanic rocks form the largest component of cratons and are high in silica. Igneous and volcanic rocks are also high in silica, but with non-metamorphosed rock, weathering becomes faster and the mobilization of ions is more widespread. Rocks high in silica produce silicic acid as a weathering product. There are few rock types that lead to localized enrichment of some of the biologically limiting elements like phosphorus (P) and nitrogen (N). Phosphatic shale (2O5) and phosphorite (> 15% P2O5) form in anoxic deep water basins that preserve organic material. Greenstone (metabasalt), phyllite, and schist release up to 30–50% of the nitrogen pool. Thick successions of carbonate rocks are often deposited on craton margins during sea level rise. The widespread dissolution of carbonate and evaporites leads to elevated levels of Mg2+, , Sr2+, Na+, Cl− and ions in aqueous solution. ==Weathering and dissolution of minerals==

The process of soil formation is dominated by chemical weathering of silicate minerals, aided by acidic products of pioneering plants and organisms as well as carbonic acid inputs from the atmosphere. Carbonic acid is produced in the atmosphere and soil layers through the carbonation reaction. The Mg is soluble in water and is carried in the runoff, but the Fe often reacts with oxygen to precipitate Fe2O3 (hematite), the oxidized state of iron oxide. Sulfur, a byproduct of decaying organic material, will also react with iron to form pyrite (FeS2) in reducing environments. Pyrite dissolution leads to low pH levels due to elevated H+ ions and further precipitation of Fe2O3 ultimately changing the redox conditions of the environment. ==Biosphere==

Inputs from the biosphere may begin with lichen and other microorganisms that secrete oxalic acid. These microorganisms, associated with the lichen community or independently inhabiting rocks, include blue-green algae, green algae, various fungi, and numerous bacteria. Lichen has long been viewed as the pioneers of soil development as the following 1997 Isozaki statement suggests: However, lichens are not necessarily the only pioneering organisms nor the earliest form of soil formation as it has been documented that seed-bearing plants may occupy an area and colonize quicker than lichen. Also, eolian sedimentation (wind generated) can produce high rates of sediment accumulation. Nonetheless, lichen can certainly withstand harsher conditions than most vascular plants, and although they have slower colonization rates, they do form the dominant group in alpine regions. Organic acids released from plant roots include acetic acid and citric acid. During the decay of organic matter phenolic acids are released from plant matter and humic acid and fulvic acid are released by soil microbes. These organic acids speed up chemical weathering by combining with some of the weathering products in a process known as chelation. In the soil profile, these organic acids are often concentrated at the top of the profile, while carbonic acid plays a larger role towards the bottom of the profile or below in the aquifer. As the soil column develops further into thicker accumulations, larger animals come to inhabit the soil and continue to alter the chemical evolution of their respective niche. Earthworms aerate the soil and convert large amounts of organic matter into rich humus, improving soil fertility. Small burrowing mammals store food, grow young and may hibernate in the pedosphere altering the course of soil evolution. Large mammalian herbivores above ground transport nutrients in form of nitrogen-rich waste and phosphorus-rich antlers, while predators leave phosphorus-rich piles of bones on the soil surface, leading to localized enrichment of the soil. ==Redox conditions in wetland soils==

Nutrient cycling in lakes and freshwater wetlands depends heavily on redox conditions. Decomposition in anoxic or reduced soils is also carried out by sulfur-reducing bacteria which, instead of O2 use as an electron acceptor and produce hydrogen sulfide (H2S) and carbon dioxide in the process: In most freshwater wetlands there is little sulfate () so methanogenesis becomes the dominant form of decomposition by methanogenic bacteria only when sulfate is depleted. Acetate, a compound that is a byproduct of fermenting cellulose is split by methanogenic bacteria to produce methane (CH4) and carbon dioxide (CO2), which are released to the atmosphere. Methane is also released during the reduction of CO2 by the same bacteria. ==Atmosphere==

In the pedosphere it is safe to assume that gases are in equilibrium with the atmosphere. the high concentration of CO2 and the occurrence of metals in soil solutions results in lower pH levels in the soil. Gases that escape from the pedosphere to the atmosphere include the gaseous byproducts of carbonate dissolution, decomposition, redox reactions and microbial photosynthesis. The main inputs from the atmosphere are aeolian sedimentation, rainfall and gas diffusion. Eolian sedimentation includes anything that can be entrained by wind or that stays suspended in air and includes a wide variety of aerosol particles, biological particles like pollen, and dust particles. Nitrogen is the most abundant constituent in rain (after water), as water vapor utilizes aerosol particles to nucleate rain droplets. ==Soil in forests==

Soil is well developed in the forest as suggested by the thick humus layers, rich diversity of large trees and animals that live there. Forest soils can form a thick soil carbon sponge. In forests, precipitation exceeds evapotranspiration which results in an excess of water that percolates downward through the soil layers. Slow rates of decomposition leads to large amounts of fulvic acid, greatly enhancing chemical weathering. The downward percolation, in conjunction with chemical weathering leaches Mg, Fe, and aluminium (Al) from the soil and transports them downward, a process known as podzolization. This process leads to marked contrasts in the appearance and chemistry of the soil layers. ==Soil in the tropics==

Tropical forests receive more insolation and rainfall over longer growing seasons than any other environment on earth. With these elevated temperatures, insolation and rainfall, biomass is extremely productive leading to the production of as much as 800 grams of carbon per square meter per year (8 tons of C/hectare/year). Higher temperatures and larger amounts of water contribute to higher rates of chemical weathering. Increased rates of decomposition cause smaller amounts of fulvic acid to percolate and leach metals from the zone of active weathering. Thus, in stark contrast to soil in temperate forests, tropical forests have little to no podzolization and therefore do not have marked visual and chemical contrasts with the soil layers. Instead, the mobile metals Mg, Fe and Al are precipitated as oxide minerals giving the soil a rusty red color. ==Soil in grasslands and deserts==

Precipitation in grasslands is equal to or less than evapotranspiration and causes soil development to operate in relative drought. Furthermore, carbon buildup in the soils is decreased due to slower decomposition rates. As a result, the rates of carbon circulation in the biogeochemical cycle is decreased. ==References==