Introduction

Compiling soil maps is an essential objective of classic soil science. Together with relief maps or digital elevation models and land-use maps, they form the basis for many questions and problems in physical geography, e.g., concerning water balance or soil erosion, and have many other practical and scientific applications.

In central Europe, conventional aerial photographs are of limited use only for delineating soil units, because the fields are densely covered with vegetation or crop residue from May to September, the months in which most of the aerial photography surveys are carried out (Fig. 17-1). The soil surface is not visible during this period. Accordingly, soil mapping is done by fieldwork using soil profiles and hand augers, usually in autumn, winter (if the soil is not frozen), and early spring.

Such soil inventories with sampling by drilling and digging are quite time-consuming, so the distance between two sample points is rarely less than 20 m. Instead, the delineation of the soil units is usually aided by the interpretation of local topography, such as changes in slope, escarpments, and breaks of profile that may occur, for example, between hillslope and floodplain, between convex slope shoulder and concave slope foot, or associated with vegetation and land-use changes. Soil samples are taken in the center of homogeneous-looking areas as well as along the assumed soil map-unit boundaries. This method usually brings coherent results but may lead in some cases to circular reasoning in subsequent geomorphologic/soil-scientific studies, when the actual distribution and explanation of soil units in maps are analyzed on the basis of the topography and the geomorphologic process knowledge.

This might result in causal connections such as ''summit position on the topographic map equates to intense erosion that results in a truncated soil profile.'' Soil-unit boundaries on a soil map would in most cases be verified by a digital

FIGURE 17-1 Agricultural landscape in the Wetterau (Hesse), Germany, in May. Nearly all fields are covered with crops and the soil surface is visible on solitary fallow fields only. As the automatically controlled image exposure is balanced for the complete scene (A), fine differences in soil colors are even less discernible; strong histogram adjustments (B) may help to enhance them. Hot-air blimp photography taken by JBR and A. Fengler, May 2000.

FIGURE 17-1 Agricultural landscape in the Wetterau (Hesse), Germany, in May. Nearly all fields are covered with crops and the soil surface is visible on solitary fallow fields only. As the automatically controlled image exposure is balanced for the complete scene (A), fine differences in soil colors are even less discernible; strong histogram adjustments (B) may help to enhance them. Hot-air blimp photography taken by JBR and A. Fengler, May 2000.

terrain model or contour map simply because of the intrinsical geomorphology background that the soil scientist relied on while compiling the map. Thus, two basic aspects have to be considered.

• Conventional soil mapping is usually based on expert knowledge from closely related disciplines. The map-unit boundaries are strongly correlated with changes in topography, vegetation, or land use.

• On areas that have been subject to land consolidation, which is particularly widespread in central and eastern Europe, this soil mapping technique is of limited use nowadays because such auxiliary topographic or structural features have been levelled out or cleared.

Recent studies on the current intensity and historic dimension of on-site soil erosion show highly differentiated results and sometimes surprising distributions of soil units.

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