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.
Mollisols, found beneath temperate grassland, also have a thin A0 horizon, because grass produces a
dense but shallow mat of roots. Organic matter decomposes rapidly. The humus-rich A1 horizon is
deep and the leached A2 horizon correspondingly thin, with a deep B horizon, where nutrients accumulate.
It is the Aridisols that provide dramatic contrast. Developed under desert conditions, they support
almost no vegetation and consequently have no surface litter at all. Because there is no A0 horizon,
there can be no A1 horizon either, because no humus is being produced. Occasional rains produce
weathering, and soluble compounds are leached into a deep B horizon, beneath which there is a
further horizon where calcium carbonate accumulates.
The combination of climate and vegetation has further effects on soils. Spodosols are prone to
‘podzolization’. The slow decay of organic matter releases acids that drain downward, removing
carbonates as they do so, and making the whole of the A horizon acid. In extreme cases it may
be so acid as to cause the leaching of clays and their accumulation as a hard, impermeable layer
(a ‘hardpan’) in the B horizon. In permafrost regions, the winter freezing of the surface layer
causes it to expand, compressing the soil beneath against the underlying permafrost. This inter-
Figure 3.13 Profiles of four soils, with the vegetation associated with them