The soils of Hawaii have developed from volcanic rocks under a warm tropical climate. The chief parent soil materials are basaltic lava, products of explosive eruptions like volcanic ash and cinders, alluvial deposits made by running water, and the coralline limestone on raised beaches and lowlands that were once the ocean floor. The characteristics of the soils depend upon various factors, among which are nature of the original materials, time available for the development of the soil, rainfall that varies from arid to humid, temperature, surface relief features, drainage, natural vegetation, and the organisms present in the soil. Only about 10 per cent of the area of the islands has deep, well-developed soils because of the youthfulness of some of the volcanic rock and other parent materials and the steep slopes and excessive rainfall of the mountains that permit the rapid removal of weathered materials. The deep, more mature soils are mostly on the surface of ancient lava flows that slope at a moderate degree. On the valley floors and along some coasts there are small areas of alluvial soil, which are too youthful to show much effect of climate and other influences. On recent lava flows the soil is thin but generally fertile; such pasture soils are successfully cropped in several areas, for example, in the Kona district on the island of Hawaii.
The rate of rock decomposition in Hawaii is very rapid largely because of the hot, humid climate but partly because the heavy rainfall and good drainage of the porous soil promote such rapid leaching that minerals, characteristic of mature, zonal soils elsewhere in the world, rarely accumulate. Consequently Hawaiian soils have, the A zone (topsoil) and C zone (weathered original material) but not B zone (subsoil) of accumulation. In areas with humid climate the chief soil-forming process is laterization, in which silica is leached out, and the oxides of aluminum and iron accumulate as the residual components. These lateritic soils have been named latosols by soil scientists. The latosols, developed under very humid conditions, have a high organic content (10 to 35 per cent by volume), possibly because plant materials in the ground are soaked with water that would delay their decay. All the latosols have a high iron content, and those with a low amount of humus are likely to have a large content of manganese. Several hundred soil types have been recognized in Hawaii, but a simplified general classification, would include low humic latosols, humic latosols, hydrol humic latosols, and humic ferruginous latosols, all developed from basalt flows under a rainfall in excess of 30 inches annually; red desert soils; reddish prairie soils and brown forest soils, but developed from volcanic ash; alluvial soils; bog soils; and lithosols from very rocky materials.
The low humic latosols have under 5 per cent organic matter, were developed under an annual rainfall of 30 to 60 inches, and originally had a cover of moderately open forest. Huge areas of these soils have been cleared for cultivation and are the most widespread Hawaiian soils, comprising much of the land planted to sugar and pineapples, for example, the Wahiawa Plain of central Oahu, west Molokai, Lanai, and portions of the drier slopes on Maui and Kauai. These soils are friable clays and clay loams with a granular structure, rather uniform texture, high porosity, and an acid reaction. Their natural fertility is moderately high, and good yields of crops are maintained by the heavy application of fertilizer, which helps to replace the nitrogen that disappears under steady cropping.
The low humic latosols may develop into two end products.
(1) Corresponding to increasing rainfall of 60 to over 150 inches annually, they pass through a transition type of humic latosols (6 to 9 per cent organic matter) into hydrol humic latosols (10 to 20 per cent organic matter), which are high in alumina (Al 2 O 3 ).
(2) In places with a wet-dry rainfall regimen they develop into humic ferruginous latosols, which are high in iron and titanium accumulation. The humic latosols have developed on the lower slopes of the rainy Hamakua coast of Hawaii and under similar conditions on Maui and other islands. Much of these soils on Hawaii and Maui has been cleared of dense forests and planted to sugar cane. The hydrol humic latosols coincide with the rainforest that grows in regions of very heavy rainfall, especially on Hawaii, Maui, and Molokai, but except for some sugar fields near Hilo, comparatively little of this soil type is in crops. Much of the humic ferruginous latosols is tilled, for example, on Maui, Kauai, and Oahu.
The soils formed from volcanic ash are reddish prairie soil developed under a rainfall of about 35 inches or less annually and that is mainly for grazing rather than crops, and brown forest soils developed under a rainfall in excess of 35 inches yearly and covered by dense forests, very little of which has been cleared for tillage. These soils are mostly found on Hawaii and Maui. Both types are fertile and low in silica, but because of their location in rather high altitudes they are seldom planted in crops. Another soil type is of alluvial origin. Soils of this type are generally very fertile and much used for taro, rice, and vegetables as well as sugar.
Hawaiian soils are subject to erosion on the uplands and slopes, and farming practices have had to be adapted to reducing the loss of soil in this way. Some grazing lands also show the effects of erosion both by water and by wind. This condition usually is the result of overgrazing or other carelessness.