Region 7: Hawai'i

Hawai’i’s climate, topography, and substrate age are all characterized by large gradients, and there is a similarly large variation in the types of soils found here, despite the somewhat uniform lithology of the underlying basaltic lavas. Of the 12 soil orders classified by the USDA, 10 are found in this state—Alfisols and Gelisols being the only orders not found on the island chain. As might be expected, fewer soil orders are found on the younger islands.

Unlike a continental system, each Hawaiian volcano begins as a blank slate—a barren basalt flow on which soil begins to develop only a few years after the eruption of the substrate. Soil development follows the arrival of early colonizing organisms, both plants and soil microbes, that begin to transform rock into soil. These early soils are relatively rich in nutrient elements derived from the parent material, but are relatively poor in nitrogen, a product of biological activity. Nitrogen-fixing lichens are among the first colonizers of lava flows, and they are critical to the creation of soils capable of supporting a young ecosystem (Figure 8.18).

Figure 8.18: Sword ferns and small gray dots of lichen colonize a lava flow on Kīlauea volcano. The lichen is a nitrogen fixer, and the fern contributes organic matter to the soil.

Soil development is rapid in the tropics, where high temperatures and locally abundant rainfall accelerate the process. Once soils are established, rainfall also dissolves and mobilizes compounds in the soil, mediating chemical reactions that alter pH and the availability of nutrients. The rate of soil development in Hawai’i depends strongly on rainfall. Dry areas evolve in the same direction as wet areas do, but much more slowly. Soluble compounds such as sodium, magnesium, and calcium are leached from the soil, while insoluble compounds such as oxides of iron and aluminum are left behind.

As Hawaiian soils develop, nitrogen also becomes more abundant due to its addition by nitrogen-fixing bacteria. Phosphorous is gradually leached from the soil at a rate faster than it is replaced. In very old soils, where nearly all the phosphorous in the soil profile has been lost, it can only be replaced by deposition from the atmosphere. Interestingly, in Hawai’i this input is principally via long-distance atmospheric transport of dust from Asia (Figure 8.19).

Figure 8.19: Satellite image of China, where continental dust (tan patch at center) blows off of the Gobi desert in a spring storm. Dust storms such as these are important for transporting nutrients to Hawaiian ecosystems.

The youngest soils and the oldest soils in Hawai’i are not well suited for agriculture. The youngest soils are thin, do not have much water-holding capacity, and are poor in nitrogen. In contrast, the oldest soils are acidic, have high levels of aluminum that can be toxic to plant roots, and are poor in phosphorous. But many soils in Hawai’i are quite fertile, making the islands highly productive for both traditional and modern methods of cultivation. Harvesting crops tends to remove the most nutrient-rich parts of plants from the system, and this can lead to nutrient depletion over time. Crop rotation and fallow periods can help limit nutrient losses, as can fertilizer use. In traditional wet taro cultivation, where taro is grown in flooded paddies, new sediment brought in by streams provides nutrient-rich material to the taro patch each year, supporting high yields over time. Traditional dry land taro cultivation tended to focus on the “sweet spots” where there was enough rain to support the crops but not so much that the soil had become too weathered, acidic, and nutrient poor.

In many parts of the world, certain types of soils were valued for the clays they contained that could be made into pots and vessels. Weathering processes in Hawai’i tend to produce clays that are not well suited for pottery, and thus pottery was not an important traditional craft, unlike in some other Polynesian areas.