Influence of organic nutrients on soil quality in variable-charge soils, Pohnpei, Micronesia

Variable-charge soils of the humid tropics are frequently infertile in the extreme. In addition to low nutrient status, highly weathered subsoils may present physical and chemical barriers to roots. Limited resource farmers do not have the means to apply synthetic fertilizers but must maintain soil fertility through management of organic inputs. This work investigates differences in soil quality properties in highly weathered variable-charge soils as a result of additions of organic nutrients. Study plots are located on the Island of Pohnpei, Federated States of Micronesia, which is about 7N latitude and 159E longitude. The climate is tropical with about 5,000 mm annual rainfall and an average annual air temperature of 27C. Comparisons from six study sites on soils classified as Typic Acrorthox are included in this study. Organic nutrients (liquids and solids) from subsistence scale piggeries are applied to bananas and other crops on one agroforestry plot on each soil type. Comparison plots are in degraded savanna, forest and in conventional tillage monocropped plots. The agroforestry demonstration plots are uncontrolled experiments; the farmers control the amount and timing of manure application. Analyses from soil quality parameters such as pH, extractable nutrients, soil respiration, cation exchange capacity, total carbon, infiltration and bulk density are presented. Comparisons in soil quality parameters are made between the agroforestry plots and the savanna, forest and monocrop plots. In highly weathered, nutrient-poor soils, nominal additions of manure can make substantial improvements in crop yields. The low-tech approach to manure management employed in this study will presumably translate to higher yields. Initial visual results show dramatic differences in banana growth between the agroforestry and monocrop plots.