Soil formation
These alterations lead to the development of layers, termed soil horizons, distinguished by differences in color, structure, texture, and chemistry.
This includes peat and muck soils and results from preservation of plant residues by the low oxygen content of a high water table.
Weathering is usually confined to the top few meters of geologic material, because physical, chemical, and biological stresses and fluctuations generally decrease with depth.
[22] Physical disintegration begins as rocks that have solidified deep in the Earth are exposed to lower pressure near the surface and swell and become mechanically unstable.
Chemical weathering mainly results from the excretion of organic acids and chelating compounds by bacteria[24] and fungi,[25] thought to increase under greenhouse effect.
[38][39] Saprolite is a particular example of a residual soil formed from the transformation of granite, metamorphic and other types of bedrock into clay minerals.
This process is also called arenization, resulting in the formation of sandy soils, thanks to the much higher resistance of quartz compared to other mineral components of granite (e.g., mica, amphibole, feldspar).
[44] If warm temperatures and abundant water are present in the profile at the same time, the processes of weathering, leaching, and plant growth will be maximized.
[50] Soil profiles in arid and semi-arid regions are also apt to accumulate carbonates and certain types of expansive clays (calcrete or caliche horizons).
Wind moves sand and smaller particles (dust), especially in arid regions where there is little plant cover, depositing it close to[55] or far from the entrainment source.
In semiarid regions, the lower effective rainfall on steeper slopes also results in less complete vegetative cover, so there is less plant contribution to soil formation.
[64] In swales and depressions where runoff water tends to concentrate, the regolith is usually more deeply weathered, and soil profile development is more advanced.
Depressions allow the accumulation of water, minerals and organic matter, and in the extreme, the resulting soils will be saline marshes or peat bogs.
Gravity transports water downslope, together with mineral and organic solutes and colloids, increasing particulate and base content at the foot of hills and mountains.
[76] Plants with deep taproots can penetrate many metres through the different soil layers to bring up nutrients from deeper in the profile.
[81] Microaggregates (20–250 μm) are ingested by soil mesofauna and fauna, and bacterial bodies are partly or totally digested in their guts.
[85] Earthworms ingest soil particles and organic residues, enhancing the availability of plant nutrients in the material that passes through their bodies.
[92] In general, the mixing of the soil by the activities of animals, sometimes called pedoturbation, tends to undo or counteract the tendency of other soil-forming processes that create distinct horizons.
[94] Large animals such as gophers, moles, and prairie dogs bore into the lower soil horizons, bringing materials to the surface.
[100] Plants can form new chemicals that can break down minerals, both directly[101] and indirectly through mycorrhizal fungi[25] and rhizosphere bacteria,[102] and improve the soil structure.
[103] The type and amount of vegetation depend on climate, topography, soil characteristics and biological factors, mediated or not by human activities.
[104][105] Soil factors such as density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location.
Accelerated soil erosion from overgrazing, and Pre-Columbian terraforming the Amazon basin resulting in terra preta are two examples of the effects of human management.
[108] It is believed that Native Americans regularly set fires to maintain several large areas of prairie grasslands in Indiana and Michigan, although climate and mammalian grazers (e.g. bisons) are also advocated to explain the maintenance of the Great Plains of North America.
[113] They have a significant effect on the region: the population of snails is estimated to process between 0.7 and 1.1 metric ton per hectare per year of limestone in the Negev desert.
This outcome was not anticipated because the antecedent prairie fire ecology capable of producing these distinct deep rich black soils is not easily observed.
[117] Soil-forming factors continue to affect soils during their existence, even on stable landscapes that are long-enduring, some for millions of years.
Russian geologist Vasily Dokuchaev, commonly regarded as the father of pedology, determined in 1883[126] that soil formation occurs over time under the influence of climate, vegetation, topography, and parent material.
There are two principal methods by which the state equation may be solved: first in a theoretical or conceptual manner by logical deductions from certain premises, and second empirically by experimentation or field observation.
[129] An example of the evolution of soils in prehistoric lake beds is in the Makgadikgadi Pans of the Kalahari Desert, where a change in an ancient river course led to millennia of salinity buildup and formation of calcretes and silcretes.
