Soil Formation

Soil is the thin layer of material covering the earth’s surface and is formed from the weathering of rocks. It is made up mainly of mineral particles, organic materials, air, water and living organisms—all of which interact slowly yet constantly.

Soil minerals are produced from rocks (parent material) through the processes of weathering and natural erosion. Water, wind, temperature change, gravity, chemical interaction, living organisms and pressure differences all help break down parent material. The types of parent materials and the conditions under which they break down will influence the properties of the soil formed.

The pages here provide some basic information about the parent materials and climate setting within which the highly alkaline and coarse-textured, coral-derived soils of Kiribati have formed. These soils are among the poorest in the world. They are typically shallow with very low water-holding capacity, little organic matter, and low available macro- and micronutrients apart from calcium, sodium, and magnesium. Because the soils are alkaline, fertility is dependent on organic matter for the concentration and recycling of plant nutrients and for soil water retention in excessively well drained soils. Organic carbon values for subsoils are always low (< 0.5%) unless there has been considerable disturbance such as that associated with the digging of babai or swamp taro (Cyrtosperma chamissonis) pits. Phosphate soils, which were once extensive on Banaba, are found in scattered locations throughout Kiribati. These brown-red soils are also slightly more acidic than the surrounding areas and originated from guano deposits accumulated over long periods of time under groves of buka (Pisonia grandis), a favored seabird rookery tree.

Information about land use and major soil related projects is also provided.

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