Climate influenced soils

Some of the early classifications, even those in use before 1900, recognised the close association of soil type to world climatic zones, such as the tropics, sub-tropics, temperate, arid, and cold regions. Most soils of the world can be linked to these climatic zones. In the tropical and sub-tropical zones which straddle the equator the main soil forming factors are the hot climate and the high rainfall, although the latter can be seasonal. The main effect of these factors is strong weathering of the parent rock and strong leaching of the soils. Many of the soils are deep, strongly weathered, and poor in nutrients apart from in the thin topsoil. In particular, the appearance of many of the soils in this zone is dominated by the forms of iron which give such soils their overall reddish colours. The fertility of these soils is largely maintained through the thin organic-rich top horizon and this supports much of the luxuriant vegetation that grows on these soils.

As we move north or south from the tropical and sub-tropical regions there is a decrease in temperature and rainfall and more temperate conditions prevail. The rainfall ranges from about 250 mm to 800 mm. Both the prairies of North America and the steppes of eastern Europe are part of this zone. Both have a dominantly grassland vegetation. The steppes of Russia are where some of the first soils were described and classified, including the famous chernozem. These soils are characterised by thick, blackish surface horizons rich in organic matter over a brownish B horizon. The soils have a natural high fertility. Where the rainfall is above 500mm brown earths with weathered brown B horizons (brown earths), soils with bleached horizons overlying B horizons enriched with clay, and podzols with B horizons enriched in iron, aluminium and organic carbon, on lighter textured materials, can all occur. These are found under deciduous woodland, except in the case of podzols which occur under coniferous forest. Apart from podzols the soils of this zone are among the most fertile soils in the world.

In the Antarctic, Arctic, sub Arctic and boreal climatic zones, soil formation is strongly affected by much of the soil depth being permanently frozen. The soils have a top horizon formed of organic matter in various stages of decomposition. There is weak development of a brownish B horizon which is permanently frozen or with just short periods free from ice. The soils are able to support some grasses and shrubs on which feed grazing animals, such as reindeer. In the hotter arid and semi-arid regions between 10° and 35° latitude, under less than 500mm of rainfall, and in the case of the desert regions with less than 50mm of rainfall, the combination of hot conditions and low rainfall has led to the development of saline and sodium-rich soils. The soils that occur include solonchaks which have high concentrations of soluble salts, solonetz soils with a high proportion of sodium ions, gypsisols with a high proportion of gypsum and calcisols with high levels of calcium carbonate. The B horizons of the soils generally contain such high amounts of these compounds that many plants cannot grow and they are thus characterised by a limited range of vegetation. The soils are also difficult to farm because of the high salt contents.