Changes in Physical and Chemical Properties of Surface Soil in a Swidden and Subsequent Fallow in a Northwestern Region of Malaita Island, Solomon Islands

Changes in the physical and chemical properties of the surface layers of soil of a slash-and-burn swidden(garden) in a tropical and ever-humid climate region in northwest Malaita Island, Solomon Islands, were investigated.Soil samples were obtained from the same site nearly spanning the entire swidden cycle and from an almostadjacent long fallow forest. The fallow period in the swidden cycle was slightly more than ten years. The soil therewas found to be acidic. The changes in physical properties suggest that slight soil compaction of the surface layersoccurred during the fallow, and that the soil became softer, and more permeable with increasing fallow length. Thechanges in the content of organic carbon seem to parallel this change. Both organic carbon and total nitrogen inthe topsoil had the lowest value in the middle of the fallow period. The surface layers accumulated much organicmatter during the long fallow period. The levels of available phosphorus, exchangeable calcium, magnesium,potassium, and sodium, base saturation percentage and pH of the topsoil were lowest during the fallow period. Onthe other hand, effective cation exchange capacity was high in the middle of the fallow period owing to a highvalue of exchange acidity. Exchangeable calcium was noticeably abundant in both the topsoil and the 15 20cm soillayer in the long fallow forest. After burning the plant debris, no rise of pH value measured in de-ionized waterwas recognized.From the preceding results, it can be concluded that some properties of the topsoil in fallows far less than tenyears after garden abandonment are less favorable for swiddening even when the beneficial effects of heating andadding ash are taken into account. Furthermore, the present investigation suggests that the ecological characteristicsof swiddening in an ever-humid region are substantially different from those in a region with humid anddistinctly dry seasons