Soils form through a combination of influences, the most important of which are parent rock, climate, relief, vegetation and organisms, time and the influence of human beings over the centuries. This combination of influences varies greatly throughout the world so it should not be a surprise that there are many thousands of different soils in the world. Just as the members of the plant and animal kingdom have been classified, there have been several attempts in the past 150 years to classify soils and give names to them. Unlike plants and animals, which are discrete entities and have recognisable features on which to base their classification, the classification of soils is much more complex because soils are a continuum across the landscape and grade into one another. Furthermore the features and characteristics of the soil which need to be used in classification are mainly below ground and largely out of sight unless you dig a hole. Soil classification has thus been difficult and remains so. There have been three major classifications of world soils: the US Soil Taxonomy, the Food and Agriculture Organisation (FAO) classification, and more recently the FAO-UNESCO World Reference Base (WRB) for Soil Resources. Rather than make things complicated for you with the difficult terminology of these soil classifications, we shall take a simpler look at how soils are distributed around the world. We shall also give some examples of where factors such as relief, geology and human activity have been a stronger influence on world soils than that of climate. Some of the early classifications, even 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 the 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 800mm. 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. Some world soils are strongly influenced by parent material. These include organic soils formed in fen and peat deposits. Most of the soil profile of these soils is composed of organic matter. Some of these soils are among the most fertile in the world. Other soils in which parent material has a strong influence include those on sandy deposits (Arenosols), on volcanic ash deposits (Andosols) and on clay deposits with high shrink-swell capacity (Vertisols). In some other world soils, the dominant soil forming influence is topography. In many of the mountainous regions of the world, the steep slopes are regularly being eroded and it is difficult for deep soils to form. Commonly, the soils here will just have a thin topsoil directly overlying the parent rock as there has been no period of stability for a B horizon to form. There can be other weakly developed soils which are much deeper. These are generally found in recent alluvial deposits such as in river floodplains subject to regular immersion and new deposits added to them. Finally there are soils which have been greatly affected by human activities, including additions of organic materials or household wastes, irrigation or cultivation. These have altered A and B horizons and are now known as anthrosols.
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