Widespread in semi-arid and sub humid tropical and subtropical regions of the world are soils of reddish brown surface colour which grade downward into a heavier dull-red or reddish-brown subsoil, then into a zone of lime accumulation. These are classified as reddish-chestnut and reddish-brown soils and grouped with the reddish prairie soils on the world soils map.
Comparison of the distribution of these reddish soils with the distribution of climate and natural vegetation suggest that these soils are found in regions of tropical wet-dry climates with a short wet season and with their bordering tropical steppes. They also occupy regions of Asiatic monsoon wet-dry climate and regions of Mediterranean climates in Australia, North Africa, and the Middle East. Vegetation associated with reddish-chestnut and reddish-brown soils is varied, being tall-grass prairie, short-grass steppe, raingreen savanna and scrub, or monsoon forest. All of the areas of reddish-chestnut and reddish-brown soils have in common a marked degree of aridity of climate, whether associated with an annual cycle or drought in the winter (low-sun season) or in the summer (high-sun season). Very high air temperatures and a severe water deficiency occur at one season of the year. Otherwise, in terms of latitude, these reddish soils occupy the same global positions as areas of latosols and lateritic soils, into which the reddish soils grade in Central and South America, Africa, southern Asia, and Australia.
Aridity in the seasonal climatic cycle explains the occurrence of calcium carbonate in lower layers of the reddish-brown and reddish-chestnut soils, for this is a characteristic of the pedocal soils. Red colour is evidence of art abundance of the sesquioxides of iron, which accumulate in warm climates where organic acids are not produced in quantity.
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