Serpentine Soils as Media for Plant Growth

Plants and animals require water, energy sources, and nutrients to make tissues and perform vital functions. The primary source of energy is the sun. Green plants use solar energy to manufacture organic compounds that are later oxidized to produce energy for both plants and animals. Many microorganisms produce energy by inorganic chemical reactions, but that source of energy is minor compared to the very large amounts of solar energy used by green plants. The major source of water and nutrients (other than CO2) for green plants is soil. Barren rocks, including ultramafic rock outcrop and talus, are colonized by lichens, which are symbiotic alliances of fungi and either cyanobacteria or green algae. These and other small organisms promote weathering and contribute to soil formation. Once soils are deep enough to support vascular plants (plants with roots), plants are the primary users of soils and producers of ecosystem biomass. Vascular plants send roots into soils and exploit both a high soil particle surface area and soil solutions, neither of which are available to lichens growing on rock surfaces. The surface area of particles in a soil 10 cm deep is about a thousand times greater than a planar bedrock surface if the soil is coarse sand, or about a billion times greater if the soil is clayey. With these dramatic increases in surface area accompanying soil formation, and lack of water retained on rock surfaces, it is easy to understand that ecosystem productivity is relatively low on rock surfaces and increases greatly with soil depth in very shallow soils. Annual plants approach maximum productivity in moderately deep soils and trees in deep or very deep soils. Ecosystems with serpentine soils are generally less productive than ecosystems with other kinds of soils, and they have unique plant species distributions. Therefore, serpentine soils attract attention from botanists who are interested in the profound effects that serpentine soils have on plant distributions and growth. These effects include those that affect the supply of water (section 8.1) and those that affect the supply of nutrients (section 8.2) to plants. These in turn affect plant growth and productivity (section 8.3).