مقاله اقلیم خاک و آمایش زمین

Soil, climate, and land use

Climate is by far the most important factor controlling the development of soils (pedogenesis) from their parent materials, but it is not the only one. Pedogenesis is also affected by the type of vegetation (which is also climatically governed), the activities of animals including humans, the parent material,

and topography. Most of the chemical reactions by which the mineral constituents of rock are modified do not commence until the temperature rises above 10°C, and their reaction rates double for every further 10°C rise above that. It follows that, provided water is available, soils develop and age faster in warm than in cold climates and it is in warm climates that biological organisms are more active.

Some of the early schemes for classifying soils were based on climate, grouping soils into cold, cool-temperate, subtropical, and tropical zones, within which many of the names were descrip-tive. In the cold zones, for example, are tundra and mountain meadow soils, and in the cool-temperate zones prairie soils (CRUICKSHANK, 1972, pp. 155 166). This led to the concept of ‘zonal soils’,

typical of the zones in which they occur. The zones might also contain soils formed under the influence of some local factor and therefore atypical. These were called ‘intrazonal soils’. Soils that had not developed at all might be found in any climatic regime, and were called ‘azonal’. Such classifications were based on examinations of only the A and C soil horizons, the B horizon being considered merely transitional between those above and below it. In time, soil scientists came to realize that zonal schemes were classifying not so much the soils as the environments in which they form. Modern soil taxonomy is based on the soils themselves, defined in terms of more than 20 surface and subsurface ‘diagnostic horizons’, called ‘epipedons’ and ‘endopedons’ respectively. An anthropic epipedon, for example, is a surface horizon formed where people have lived for a long time or have grown irrigated crops; cultivation over many years might lead to the formation of an agric endopedon just beneath the ploughing depth, where clay and organic

matter have collected.

116 / Basics of Environmental Science

Although soils are now classified according to their composition, their surface horizons are formed biologically, by the mixing of organic and mineral material. Since natural vegetation is usually typical of the climate in which it grows, it is impossible to dissociate pedogenesis and climate.

As Figure 3.13 shows, the relationship between plants and soil is intimate. Beneath a conifer forest, there is usually a deep layer (A0 horizon) of organic material, mainly needles. This decomposes only slowly, partly because conifer needles have a thick, waxy, outer layer that is not easily broken. This is an adaptation to climate: conifers grow in climates with a pronounced dry season or a long winter when water is frozen and so unavailable. The dry or cold season also reduces the rate of decomposition. The A1 horizon, dark and rich in humus (decayed organic material), is thin and the somewhat thicker A2 horizon is very pale, because its humus has leached into the B horizon. This soil is classifed in the order Spodosols.

Broad-leaved forests produce a much thinner A0 horizon, because the more delicate leaves, shed in the autumn, decompose fairly quickly during the mild, wet winter. The resulting humus forms a deep, dark, A1 horizon, a thinner, leached, A2 horizon, and a deep B horizon, where plant nutrients accumulate well within the reach of tree roots. This is an Alfisol.