ANCIENT AGRICULTURAL SOILS IN THE ANDES OF SOUTHERN PERU

Knowledge about soil changes resulting from long-term agriculture is important because agricultural influences on soil continue to increase in extent, intensity, and duration. Agricultural terraces in the Colca Valley, Peru, present an opportunity to study soils that have been cultivated for at least 15 centuries. Physical and chemical properties of soils were determined to explore possible effects of traditional agricultural practices on soils. Relative to nearby uncultivated Mollisols, agricultural A horizons are commonly 0.3 to 1.3 m thicker, contain buried organic-matter-enriched horizons, and are lower in bulk density. They also have more organic C and N and lower pH. Cultivated A horizons, particularly in agricultural terraces that have been abandoned for about four centuries, are enriched in P. Data suggest that P applied as fertilizer has been translocated deeply into soils during centuries of agriculture and accumulated in physically undisturbed B horizons. Agricultural practices inferred to be causal factors in changing these soils include terracing, tillage, fertilization, and irrigation. These ancient agricultural soils contain features characteristic of anthropic and plaggen epipedons and agric horizons.