Another class of intrazonal soils consists of the calciotropic soils. These are soils whose characteristics are strongly related to the presence of lime-rich parent material. The process of calcification (introduction of calcium) is dominant in the formation of calciotropic soils. An important example of the calciotropic soils are the rendzina soils.
Rendzina soils have dark-gray or black surface layers over-lying soft, light-gray or white material which is highly calcareous (lime-rich), soft limestone, or chalk, all of which are forms of calcium-carbonate. The soil profile is regarded as immature. The natural vegetation is typically grassland, which results in a well-distributed humus in the upper layers and imparts the dark colour to the soils, just as in the chernozem.
Distribution of rendzina soils in the United States is fragmented into a number of geologically favorable areas. Substantial areas of these soils are found in undulating prairies of central and north-eastern Texas and central-southern Oklahoma (Blackland Prairie and Grand Prairie). Another geographically important area in a Black Belt of Alabama and Mississippi, lowland underlain by soft limestone. These areas are in the humid subtropical climate. Other rendzina areas occur on a high limestone plateau in northwestern Arizona, and in the Mediterranean climate regime of southern California.
Rendzina soils of the humid subtropical climate are productive agriculturally and yield cotton, corn, and alfalfa. In more arid grassland environments, uses are for grazing and some dry farming.
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