<P> The principal climatic variables influencing soil formation are effective precipitation (i.e., precipitation minus evapotranspiration) and temperature, both of which affect the rates of chemical, physical, and biological processes . Temperature and moisture both influence the organic matter content of soil through their effects on the balance between primary production and decomposition: the colder or drier the climate the lesser atmospheric carbon is fixed as organic matter while the lesser organic matter is decomposed . </P> <P> Climate is the dominant factor in soil formation, and soils show the distinctive characteristics of the climate zones in which they form, with a feedback to climate through transfer of carbon stocked in soil horizons back to the atmosphere . 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 . According to the climatic determination of biomes, humid climates favor the growth of trees . In contrast, grasses are the dominant native vegetation in subhumid and semiarid regions, while shrubs and brush of various kinds dominate in arid areas . </P> <P> Water is essential for all the major chemical weathering reactions . To be effective in soil formation, water must penetrate the regolith . The seasonal rainfall distribution, evaporative losses, site topography, and soil permeability interact to determine how effectively precipitation can influence soil formation . The greater the depth of water penetration, the greater the depth of weathering of the soil and its development . Surplus water percolating through the soil profile transports soluble and suspended materials from the upper layers (eluviation) to the lower layers (illuviation), including clay particles and dissolved organic matter . It may also carry away soluble materials in the surface drainage waters . Thus, percolating water stimulates weathering reactions and helps differentiate soil horizons . Likewise, a deficiency of water is a major factor in determining the characteristics of soils of dry regions . Soluble salts are not leached from these soils, and in some cases they build up to levels that curtail plant and microbial growth . Soil profiles in arid and semi-arid regions are also apt to accumulate carbonates and certain types of expansive clays (calcrete or caliche horizons). In tropical soils, when the soil has been deprived of vegetation (e.g. by deforestation) and thereby is submitted to intense evaporation, the upward capillary movement of water, which has dissolved iron and aluminum salts, is responsible for the formation of a superficial hard pan of laterite or bauxite, respectively, which is improper for cutivation, a known case of irreversible soil degradation (lateritization, bauxitization). </P> <P> The direct influences of climate include: </P>

Write 1-1 paragraph on different types of soil