Serpentine soil is an uncommon soil type produced by weathered ultramafic rock such as peridotite and its metamorphic derivatives such as serpentinite. More precisely, serpentine soil contains minerals of the serpentine subgroup, especially antigorite, lizardite, and chrysotile or white asbestos, all of which are commonly found in ultramafic rocks. The term "serpentine" is commonly used to refer to both the soil type and the mineral group which forms its parent materials.
Serpentinite is a meta-igneous rock formed by the metamorphic reaction of olivine-rich rock, peridotite, with water. Serpentinite has a mottled, greenish-gray or bluish-gray color and is often waxy to the touch. The rock often contains white streaks of chrysotile running through it, which are a type of naturally occurring asbestos. Asbestos is linked to an array of human health conditions such as mesothelioma from long time exposure of breathing in the dust particles. Caution should be taken when working in serpentine soils or when working with crushed serpentine rocks.
Serpentinite most often forms in oceanic crust near the surface of the earth, particularly where water circulates in cooling rock near mid-ocean ridges: masses of the resulting ultramafic rock are found in ophiolites incorporated in continental crust near present and past tectonic plate boundaries.
Serpentine soils are widely distributed on Earth, in part mirroring the distribution of ophiolites. There are outcroppings of serpentine soils in the Balkan Peninsula, Turkey, Alps, Cuba, and New Caledonia. In North America, serpentine soils also are present in small but widely distributed areas on the eastern slope of the Appalachian mountains in the eastern United States. However, California has the majority of the continent's serpentine soils.
In order to overcome the chemical and physical challenges presented by serpentine soils, plants have developed tolerances to drought, heavy metals, and limited nutrients. Low calcium:magnesium ratios cause limited root growth and root activity, weak cell membranes, and reduced uptake of essential nutrients. An adaptive mechanism to high magnesium soils allocates more resources to deep-growing roots. Heavy metals stunt growth, induce iron deficiency, cause chlorosis, and restrict root development. Multiple adaptive mechanisms to heavy metals include the exclusion of metals by restricting the uptake by the roots, compartmentalization of metals in various organs, or the development of toxicity tolerance. In nitrogen-poor sites, physiological effects on plants include impaired protein synthesis, chlorosis, reduced leaf turgor, reduced leaf and tiller number, reduced growth rate, and low seed yield. Low phosphorus levels cause similar effects of low nitrogen but also cause reduced seed size, lower root to shoot ratios, and increased water stress. Low soil moisture causes reduced nutrient uptake and transport, decreased stomatal opening and reduced photosynthetic capacity, and also reduces plant growth and productivity. Serpentine plants have strongly developed root systems to facilitate uptake of water and nutrients. For example, Noccaea fendleri (aka Fendler's penny grass) is a hyper-accumulator of nickel and Sedum laxum ssp. expresses succulence. In some cases, symbioses with serpentine tolerant ectomycorrhizal help facilitate plants’ adaptation to edaphic stressors on serpentine.
The unique plants that survive in serpentine soils have been used in the process of phytoremediation, a type of bioremediation. Since these plants developed specialized adaptations to high concentrations of heavy metals, they have been used to remove heavy metals from polluted soil.
They are named for minerals of the serpentine group, resulting in serpentine soils, with unusually high concentrations of iron, chromium, nickel and cobalt. Serpentine barrens often consist of grassland or savannas in areas where the climate would normally lead to the growth of forests.
In Chester County, Pennsylvania, the Nottingham Park, aka Serpentine Barrens, was recommended by UMCES as deserving of National Natural Landmark designation, on numerous grounds. They included supporting a number of rare and endemic species, an intact population of pitch pine, and also the site having historic significance.