Human Impact on Soils
Natural soil-forming processes produce soil horizons, for example, the A (topsoil), B (subsoil), and C or R (parent material) that are essentially homogeneous. The effects of human impact can be identified because anomalous soil characteristics are produced. As noted for cultivation (above), the A horizon may become mixed, and homogenized with the subsoil (B horizon) by plowing. This mixing of subsoil material is exacerbated if cultivation is accompanied by down-slope soil loss (erosion), and plowing of an eroded soil increasingly mixes subsoil into the Ap (plowed topsoil). Colluvium accumulated toward the base of the slope may well include topsoil mixed with subsoil and even eroded parent material and bedrock if erosion is severe. Another example of human impact on soils is when soils are manured, and once-impoverished soils like podzols become fertile and over-thickened. Not only dung may be added, but also turf from other localities (grassland and woodland), ashes, and other domestic waste. The resulting ‘cultosols’ (e. g., plag-gens) may thus include allochthonous (‘foreign’) inclusions relict of this manuring.
Although humans may have had no or negligible impact on soils, soil data are extremely relevant to the reconstruction of past environments. In the Quaternary, soils provide important information on the landscapes that hominins inhabited. At Boxgrove, Sussex, UK, soil studies showed that Early Palaeolithic hominins visited mudflats during low tide more than 480 000 years ago, where they seemed to have hunted and butchered animals, and manufactured flint tools. These investigations also showed that, through time, mudflat sediments weathered into an immature palaeosol, which was also occupied, and that hominins were present - at least during interstadials - after the climate had become cold and humid (glacial period).
Clearance Several forms of clearance can be carried out. Brush can be grubbed up and burned. This normally only disturbs the uppermost part of the soil profile, which when preserved (e. g., by rapid burial) can retain charcoal fragments. Sometimes, piles of brush are burned in one place and produce a reddened soil patch, which is also characterized by an enhanced magnetic susceptibility (w). In contrast, the heat from simple burning of scrub and shrubs in a landscape does not penetrate deeply into the soil, and there may well be no reddening or enhanced w.
Tree-throw holes are features that are often worth investigating. Due to many archaeological investigations taking place on eroded landscapes, negative features (ditches, pits, tree subsoil hollows) may contain the only soils that are contemporary with the archaeology. The fragments of soil that these negative features contain can therefore be vital clues to the earlier soil cover. Tree-throw holes are formed when a tree falls and the soil around the roots is disrupted and thrown up, creating a hummock and hollow landscape if many trees are involved through time. Under natural conditions, the infilling of the hollow is slow - perhaps taking 500 years or so. In plan, often two-thirds of the hollow is filled with subsoil and parent material falling off the root bole, while the other third fills more slowly under the fallen tree, and includes high amounts of organic matter, for example, leaf litter. The land snails present in the fill and the relict enhanced organic content reflect this humic infilling - the latter perhaps ‘preserved’ as iron and manganese mottle concentrations. At some sites, these tree-throw fills contain both charcoal (for instance mainly only from one species) and burned soil. This implies that people were involved, the fallen tree being burned in situ. Possibly, the tree was ring-barked and died before being pulled over. There are both Neolithic and Beaker examples of this in the Midlands of the UK.
The evidence of clearance of shallow rooting brush, and/or ‘slash and burn’, may be more difficult to recognize because the effects are less deep in the soil, and thus much less obvious. Subsequent soil homogenization by biological activity will destroy micropedological features of this clearance and once-coarse charcoal will become finely fragmented. Trampling can exacerbate this. Homogenization is less rapid on acid soils because levels of biological activity are lower. If sites are rapidly buried, for example, by an earth mound or rampart, evidence of clearance may consist of charcoal and soil mixed from several horizons; management by fire may also lead to concentrations of phytoliths and fine charcoal (from burned Gramineae). This burning can lead to a more rapid loss of bases (from ash) and eventually to soil impoverishment - as in the case of some European lowland podzols. As noted above, concentrated areas of burned brush can produce reddened soils.
A number of features produced by clearance can be confused with those caused by cultivation, and this is especially the case if clearance is followed
By short-lived cultivation. Prehistoric ‘middening’/ occupation can also produce concentrations of charcoal (see below).
Cultivation Experiments and case studies have shown that cultivation evidence can be highly variable and reflects soil type, slope unit position, technology and culture, and thus has to be investigated on a case-by-case basis. For example, cultivation on a slope may induce erosion and ‘plowing’ into the subsoil and parent material, whereas down-slope cultivation may be taking place in an accumulating soil. In the latter, features such as ard marks may become buried in colluvial layers and be difficult, but not impossible, to discern. Experiments involving plowing with ards show mixing of the plowed soil and the potential for soil to become destabilized, forming textural pedofeatures in situ and down-profile. This is consistent with findings from both modern agronomy and archaeological soil studies.
It has been found that some European soils when cultivated in prehistory at a time manuring was not commonly practiced (e. g., Neolithic) were strongly destabilized by plowing as their natural organic content - and its binding properties - was depleted (as found in modern arable soils). This led to erosion on slopes, and the exposure of less humic and, consequently, less stable subsoils to plow impact. This resulted in the translocation of much fine soil through voids down-profile.
Manuring regimes As societies became more complex, the need for sustainable agriculture became important. Household waste (e. g., Near and Middle East), nightsoil (e. g., Medieval Europe and Far East), animal dung from mixed farming (e. g., Bronze Age and Iron Age Europe), dung, household and plant ashes (e. g., c. 1500-year-old Peru) were all employed; in New Mexico, USA (AD 1000-1500), terraced sols were ‘manured’ through runoff entrapment. In the historic infield cultosols of the Scottish Isles, Medieval to modern plaggen soils (e. g., podzols thickened with additions of turf and stable manure), and terrace ‘garden’ soils worldwide, agricultural soils show high organic contents and fertility (e. g., bases and P), and have highly biologically worked microfabrics. Such soils may often demonstrate little evidence of soils being mixed from different sources because everything has been efficiently homogenized. Horticultural activity can be long - or short-lived. At St. Julien, Tours France, horticultural activity is recorded from the Late Roman (third century) to the eleventh century in an area between two remaining urban centers at Tours. In contrast, at Colchester and London, UK, building debris, apparently relict of the Boudiccan revolt and destructions (AD 59-60), was briefly cultivated - perhaps over 5-10 years - before urban reconstruction took place. Other types of cultivation can be cited from dry areas (irrigation) and the Far East where paddy soils from prehistory onward are typically characterized by waterlogging features and additions of organic fertilizers (and rice phytoliths); terra preta (Amazonian dark earths) are seemingly variable according to their origin as occupation or manured garden soils - and are still favored today for cultivation.
World History
