The Nature, Properties and Management of Volcanic Soils

Abstract Soils formed in volcanic ejecta have many distinctive physical, chemical, and mineralogical properties that are rarely found in soils derived from other parent materials. These distinctive properties are largely attributable to the formation of noncrystalline materials (e.g., allophane, imogolite, ferrihydrite) containing variable charge surfaces, and the accumulation of organic matter. Formation of noncrystalline materials is directly related to the properties of volcanic ejecta as a parent material, namely the rapid weathering of glassy particles. The composition of the colloidal fraction forms a continuum between pure Al–humus complexes and pure allophane/imogolite, depending on the pH and organic matter characteristics of the weathering environment. For soil management purposes, volcanic soils are often divided into two groups based on the colloidal composition of the surface horizons: allophanic soils dominated by allophane and imogolite, and nonallophanic soils dominated by Al–humus complexes and 2:1 layer silicates. Volcanic soils exhibit a wide range of agricultural productivity, depending on the degree or intensity of pedogenic development and the colloidal composition of the rooting zone. The different charge characteristics of allophanic and nonallophanic soils is the most important factor regulating chemical fertility attributes. Phosphorus fixation, strong acidity, and aluminum toxicity are the primary chemical limitations to agricultural productivity. Volcanic soils generally have high physical fertility (tilth) and mature soils are relatively resilient to erosion and compaction. To maximize the productivity of volcanic soils, proper management based on an understanding of the unique physical, chemical, and mineralogical properties of these soils must be practiced.