Pacific Technologies A publication of the Agroforestry Information Service April 1993, Number 4 Nitrogen Fixing Trees as Atoll Soil Builders Atoll soils form from coral reefs that grew on the tops of subsiding volcanoes during various periods throughout the Pacific. Most of these soils are quite young in geologic terms and thus still similar in property to this coral parent material. Coralline soils are extremely nutrient deficient and highly alkaline. They are especially lacking in iron, potassium, and nitrogen.
Organic matter is all that really does distinguish most atoll soil from crushed coral.
Atoll farmers have always practiced mulching and composting to some degree. Most consider regular organic additions integral to any agricultural activity. Organic matter holds nutrient ions, retains precious soil moisture, and buffers soil pH. In all soils, it builds and maintains good soil structure and provides essential plant nutrients. In atoll soil, it must also take the place of the missing clay component in providing nutrient cation exchange sites that are crucial to nutrient cycling processes. Most simply pile this material next to a target crop or tree as a mulch.