Topographic relief as a factor

Abstract

Variation in the nature of soils with topography can readily be appreciated by comparing thin, rocky soils of mountain tops with thick, fertile soils of lowland plains. However, even in relatively featureless lowlands, the nature of soil varies profoundly, depending on whether it is well or poorly drained. This aspect of soil formation is not completely independent of the others discussed here because vegetation, microclimate, and age of land surfaces vary in different parts of the landscape. In small areas, however, other factors may be limited to such an extent that variation across the landscape constitutes a well constrained set of soils (toposequence) for obtaining mathematical expressions for variations in topographically related soil features (topofunctions). The undulating till ridges abandoned from the last glacial advance (Cary Till, ca. 14 000 years old) in north central Iowa provide examples of toposequences that are all the more striking because of the subtle topography involved, i.e., a relief of only 5 m from well drained summit to boggy bottoms. Soils from the top of the ridge to the bog show marked decreases in gravel and mean grain size and increases in profile thickness and amounts of clay, organic matter, and carbonate (Fig. 11.1). As an example of a topofunction, the increase in thickness (y) of the soil with distance from the summit down the slope (x) can be expressed by a polynomial equation: y = 1.41 − 0.91x + 0.49x² − 0.034x³ (Ruhe 1969).