Differences in substrate chemistry, including pH, govern the availability of essential plant nutrients. Soils in the circumneutral range (pH 5.5-7.2) are generally mineral rich, whereas the full suite of essential nutrients is often unavailable in acidic soils (pH 7.2). The latter often have unique assemblages of plant species. The substrate chemistry map is derived from a wide variety of sources including soil, surficial geology and bedrock geology maps, and from spectral patterns that could be recognized on the AVHRR base image. Carbonate substrates occur mostly in mountainous regions, while circumneutral substrates are mostly fine-grained loess occurring at lower elevations in foothills or plains. The Alaska Arctic Substrate Chemistry Map is a more detailed map of the Alaska portion of the Circumpolar Arctic Vegetation Map (CAVM). The substrate chemistry mapping is the same as the Circumpolar Arctic Vegetation Map.