The soils of the islands of Micronesia have been derived from volcanic
materials or from depositions of coralline limestone. Volcanic soils
occur on the "high" islands of Micronesia. In many places, especially on
the islands of the northern Marianas there is little soil; there are
large areas of bare igneous rock, because the islands are geologically
of relatively recent origin and little erosion has occurred. On islands
where volcanic rocks have decomposed, the resulting soil may have a top
layer of humus. The richest soils of the islands are along drainage
areas and in alluvial deposits.
Coralline soils result from the decomposition of limestone, coral
fragments, shells, and sand, and are overlain by some humus. Where the
layer of humus is deep, the fertility is greatest. Coralline-volcanic
soils occur on some "high" islands where coral rock and volcanic rock
have become mixed in the decomposition process which forms soil. In
parts of the Marianas and elsewhere, unwise practices of burning and
overgrazing have allowed extensive erosion to occur, resulting in
reduced fertility of the soil. On the island of Yap certain sedimentary
rocks are exposed which are thought to have been elevated from the ocean
bottom. Soils at Yap which have developed from this rock are considered
more fertile than soils of coralline origin, although the fertility
there also is dependent on the depth of the layer of humus.