Soil discolorations in aerial photography are helpful aids for archaeology as well as soil science. From the small-scale distribution of soil units, conclusions about soil thickness and degradation state of the soil may be drawn. Not only old postholes, pits, or walls can be detected from soil color changes, but also individual soil horizons distinguished by their colors (Fig. 10-40). Topsoils (Ah horizon) rich in humus are usually darker than the parent material (C horizon) in which the soils have developed. Subsoil horizons that are situated in between usually show their own distinctive colors, mainly brown or reddish-brown (B horizon). This is typical at least for the loess landscapes in Europe, Asia, and northern North America with their Luvisols, Chernozems and Kastanozems. When the soils are eroded, truncated profiles result, exposing the underlying soil horizons or even the parent material (Fig. 10-41). Such erosion areas may be detected easily from the air by their lighter colors.

Truncated horizons are particularly frequent in the landscapes of central and southern Europe and of the Near East that have been cultivated for millennia. In the United States and southern Canada, soils that were still largely intact until two centuries ago have been affected severely also by mechanized agriculture. The intensity and small-scale distribution of soil impairment is today particularly important for precision farming techniques. Soil mapping from remotely sensed images requires a soil surface largely devoid of vegetation and a homogeneous, preferably fine-aggregated condition of a dry soil surface. In central Europe, such conditions may be found for only a few days between the end of February and mid-May. Thus, flexible timing of aerial surveys, such as offered by SFAP, is needed for soil mapping applications.

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