On a broad, general scale, virtually the whole of France can be classified in the zone of brown forest soils, or brown earths. These soils, which develop under deciduous forest cover in temperate climatic conditions, are of excellent agricultural value. Some climate-related variation can be detected within the French brown earth group; in the high-rainfall and somewhat cool conditions of northwestern France, carbonates and other minerals tend to be leached downward, producing a degraded brown earth soil of higher acidity and lesser fertility; locally this may approach the nature of the north European podzol. The brown earth zone gives way southward to the zone of Mediterranean soils, which in France cover only a limited area. They are developed from decalcified clays with a coarse sand admixture and are typically red in color because of the upward migration of iron oxides during the warm, dry summers. These soils can be quite fertile.
Over large areas of France, soils have developed not directly from the disintegrated bedrock but from the waste sheets created by periglacial action. These may provide a particularly favorable soil material; most notable is the windblown limon that mantles the Paleogene and Neogene limestone plateaus of the central Paris Basin and the chalk beds to the northwest, the basis of the finest arable soils of France. The quality of the soils depends heavily upon the origin of their waste sheets; sand spreads derived from the granites of the Hercynian massifs, for example, provide only poor soils. The bedrock, however, is not without influence. Soils developed over clays are likely to be heavy and wet, although not necessarily infertile, as in the Jurassic clay and chalk vales of the eastern Paris Basin. Limestone and chalk enrich soils with lime, which is generally favorable, but there is a marked north-south contrast. The limestone areas of southern France tend to be swept almost bare of soil by erosion; the soil then collects in valleys and hollows. The soils of the higher mountains are naturally stony and unfavorable.
Finally, human action is an extremely important factor in soil quality. As soon as the original forest was cleared, some modification of the soil was inevitable. Generally, farmers through the ages have maintained or improved soil quality by draining and manuring; especially noteworthy were the activities of Flemish peasants who virtually created their soil out of a marshy wilderness. Not all human intervention has been as successful, however. For example, the degradation of brown earths under heath in western France is not a natural feature but the product of human clearance and grazing practices. Large-scale arable cultivation with no use of animal manure is leading in places to soil degradation and soil erosion.
Source: https://www.britannica.com/place/France/Soils
Don't ask me why I found the topic of "brown earths" of interest.
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