andisols

Andisols are a soil order defined by the United States Department of Agriculture in Soil Taxonomy and, in the World Reference Base for Soil Resources, correspond to Andosols. They form from volcanic ash (tephra) that has been deposited, weathered, and reorganized in place. The defining feature is the presence of highly reactive amorphous materials—especially allophane and imogolite—and glassy volcanic fragments in the surface horizons. These materials give Andisols a high specific surface area, low bulk density, and typically good drainage with substantial water-holding capacity.

Key properties commonly associated with Andisols include a high nutrient-holding capacity and strong phosphorus fixation caused by the amorphous aluminum and iron minerals. As a result, phosphorus availability can be highly variable and may require careful management. Many Andisols are fertile under appropriate management, but their suitability depends on pH and the ability to supply phosphorus and other bases. Organic matter content can be substantial in forested or cultivated soils, contributing to soil structure and fertility.

Distribution and occurrence: Andisols occur in volcanic regions around the world, particularly in areas associated with the Pacific Ring of Fire. Notable regions include parts of the Pacific Northwest, the Andes, Japan, New Zealand, the Philippines, Indonesia, and East Africa. They can range from young ash deposits to older, developed soils and may retain distinct Andic horizons within a soil profile.

Uses and management: Their favorable structure and moisture properties support agriculture and forestry in many regions. Management commonly emphasizes maintaining soil pH, supplying phosphorus due to fixation, preventing erosion of exposed ash surfaces, and incorporating organic matter to sustain fertility.