Organic Soils on Basaltic Lava Flows in a Cool, Arid Environment

Abstract

Holocene‐aged basaltic lava flows offer a unique opportunity to study the initial stages of soil formation. In the cool, dry climate at Craters of the Moon National Monument and Preserve (CROM) in southern Idaho, organic soils (Histosols in Soil Taxonomy) are the dominant soil on recent lava surfaces. Unsaturated Histosols, or Folists (Folic Histosols in World Reference Base [WRB]), occupy crevices and depressions on the lava surface and support vegetation adapted for life in this harsh environment. These soils are comprised of either a thin fibric (Oi horizon) over a hemic (Oe horizon) or a single, relatively homogenous Oe horizon over relatively unweathered basalt bedrock. The parent material for organic soils is biologic rather than geologic in origin with parent vegetation growing in crevices supplying the necessary organic debris. Soil acidity is directly influenced by the type of vegetation growing in the soil, with more acidic leaf litter creating lower pH soils while more alkaline plant debris generates soil with more neutral pH. Within these shallow soils (12–35 cm), organic C content (mean 333 g kg−1) is proportional to fiber content and cation‐exchange capacity (CEC) and inversely proportional to bulk density. Compared with Folists of wetter climates, those forming at CROM have substantially higher bulk densities with a mean of 0.41 g cm−3 The influence of loessial mineral material likely accounts for the increased mineral content and subsequent increase in bulk density. Knowledge of the morphological, physical, and chemical properties of Folists at CROM is essential to improve our understanding of the ecological significance of Folists and co‐evolution of soils and plant communities in a cool, arid environment.