Unearthing the diverse soils of Hawai‘i: Can you dig it

When I ask people “What is soil,” many responses include “dirt” or “the brown stuff on the ground,” but soils are much more dynamic. Composed of an assortment of minerals, organic compounds and living organisms, soils come in all different colors and “flavors,” and literally serve as the foundation upon which society stands.

Although soils cover only a thin layer on the surface of our planet, the function they provide humans is critical for our survival. The most familiar function is that soils provide society with food crops. But what about the clothes you wear, the wooden frames for your home or part of the oxygen that you breathe? These necessities, in some way, were produced due to the ability of the soil to support plant growth. In addition to farming, humans also build upon soil. Soils filter impurities in water, cleansing the water that reaches our streams or aquifers.

Not all soils are equal. Climatic, geographical and mineralogical differences can produce soils ranging in color, texture, nutrient retention and drainage levels.

For example, in wetter areas of Hawaii, rain leaches nutrients from the soil profile, resulting in depleted soils that are generally dominant in aluminum and iron oxides, which have low nutrient retention. Iron oxides also impart a reddish hue.

Soils in drier spots have a higher nutrient supply and are characterized by younger clay minerals that are more gray or brown in color. Soil weathering is more pronounced on flat plains rather than valleys, where minerals and nutrients wash down with the slope and accumulate at lower levels.

Time is another important factor in soil development. Older islands, such as Kauai and Oahu, have more developed and clay-ey soils, while younger islands like Hawaii have volcanic ash, cinder or rock still present in the soil profile.

Due to the diverse climates and landscapes found across the islands, Hawaii exhibits enormous soil diversity.

Oxisols are one large group of soils that are typically found in rainy areas on Kauai and Oahu. They are rich in oxide minerals and have acidic pH and low nutrient levels due to high rainfall. Slightly more fertile oxisols with near-neutral pH can be found in dry areas, such as on Molokai, Maui and Lanai.

Ultisols are also acidic and depleted of nutrients and are usually found in mountainous areas. Despite their infertile nature, these two soil orders have great physical properties like drainage and stability, and with proper nutrient management, they can be very productive soils.

Mollisols are another large and agriculturally important soil type that is typically found on dry, coastal plains. They are rich in nutrients and have near neutral pH, making them very productive. However, they can be sticky and difficult to work when wet.

Andisols are the largest group of soils found in Hawaii. These soils recently formed from volcanic ash and cinder, and mostly occur on Maui and Hawaii island. In areas with high rainfall, andisols are depleted of nutrients like oxisols, but they have high water retention, are rich in organic matter and have good physical properties.

Andisols in slightly drier climates also have high organic matter levels and good physical properties, and due to less rainfall, they are rich in nutrients. Some scientists consider andisols in these areas with moderate rainfall to be some of the most productive soils in Hawaii.

If you want to learn more about the soils in your area, there are many online resources available.

The Hawaii Soil Atlas is an interactive map developed by CTAHR researchers (gis.ctahr.hawaii.edu/SoilAtlas). The map generates descriptions for soil fertility, pH, physical properties and much more. For those that prefer numerical data, the SoilWeb app (casoilresource.lawr.ucdavis.edu/gmap) is another useful map that displays actual soil data. Lastly, there are numerous CTAHR publications available online (www.ctahr.hawaii.edu/site/Info.aspx) that describe the intricacies of Hawaii’s soils.

Joshua Silva is a junior extension agent and Master Gardener coordinator with the Kauai Cooperative Extension Service in Lihue. Email him at jhsilva@hawaii.edu.