Most of the ultramafic rocks from Newfoundland to Alabama, inland from the Atlantic Ocean, and from Arkansas to Texas, inland from the Gulf of Mexico, are peridotites and serpentinites derived from the mantle in oceanic or magmatic-arc settings. They were accreted to a precursor of the North American continent more than 0.25 Ga ago. The serpentine soils range from very cold Entisols and Histosols in Newfoundland and Quebec to cold or cool Inceptisols southward to the limit of late Pleistocene glaciation about latitude 41°N. They are warm to hot Alfisols in the unglaciated areas from New Jersey south to Alabama, with some Mollisols in the Blue Ridge Mountains. Mollisols are the dominant serpentine soils in the drier Llano uplift of Texas. The woody vegetation on the serpentine soils is relatively sparse or stunted, or both. Many of the plant species grow mainly or only on serpentine soils, and some that are common on other soils do not grow on serpentine soils. Some of the species are circumpolar and are common on serpentine soils in both eastern and western North America, and some have distributions that are disjunct from populations on nonserpentine soils of midcontinental prairies. The most distinctive features of serpentine soils are low exchangeable Ca/Mg ratios and high first-transition element concentrations from Cr through Mn, Fe, and Co to Ni. Although some of the serpentine plants have relatively high Ni contents that are toxic to some plants, it is mostly the low Ca/Mg ratios that are responsible for the unique plant assemblages on serpentine soils. The serpentine soils have soil organic matter contents comparable to those of nonserpentine soils.
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