The soils developed in the Pacific islands depend on many factors, including the original soil materials, temperature, rainfall, drainage, slope, natural vegetation, and the kinds of bacteria and other organisms present. For example, soils recently formed from volcanic ash and those developed on the floodplains of streams are generally very fertile, but those developed on low-lying coral rock are likely to be very poor.
The volcanic islands have complex soil conditions because of differences in rainfall, temperature, plant life, rate of weathering, etc., with altitude and exposure to the winds. Under conditions of high temperatures and rainfall the parent lava rock weathers quite rapidly to depths of many feet, and the soil materials undergo changes, chief of which is laterization. In this process the soluble bases and silica in the soil are leached out, and the iron and aluminum compounds remain until by this concentration the soil chemicals become chiefly these. Lateritic soils are usually red in color from the increase and oxidation of the iron and are high in content of clay. However, the presence of much humus and the character of the chemical changes make the soil more porous to water than its content of clay would ordinarily indicate. On the well-drained lowlands and lower gentle slopes the products of weathering are removed in the order of their solubility; hence these soils may become in maturity generally deficient in nutrients and unbalanced in the elements present. The soils on the islands have seldom developed into true laterites, which may have low fertility, but they are called lateritic and possess moderate to high fertility. They are formed from rather recent lavas, and elements are still being liberated by the weathering of these rocks. On steep slopes erosion keeps pace with soil formation, and the soils are thin. Under tillage the original humus and soluble plant food are quickly exhausted by crops. Some foods grown on depleted soils are deficient in minerals needed by man, and those who consume the plant foods may suffer from malnutrition.
All the soils of coral islands have about the same character. Upon the sterile rock or sand are small amounts of humus and plant debris from decaying vegetation. But this layer is rarely over a few inches thick, its water-holding capacity is low, and frequent rains are necessary for sustained plant growth. Soil fertility is low, and there may be deficiencies in soil elements. The thinness of these soils and their excessive porosity to water are unfavorable to plants. Gardens are possible only when the soil has been improved with plant trash and waste of all sorts.