Hilo Soil Series

Hilo soil is an Andisol — a volcanic ash soil — found only on the windward slopes of Mauna Kea along the Hāmākua Coast of Hawaii County. It formed from repeated layers of volcanic ash falling and weathering over 100,000 to 300,000 years. The result is a very deep, dark soil unlike any found on the U.S. mainland.

The soil is dark reddish-brown from top to bottom. That color comes from ferrihydrite and other iron-rich volcanic minerals that formed as ash dissolved in centuries of rain. Despite receiving more than 140 inches of rain per year, Hilo soil drains well — the volcanic minerals that give it its color also give it a structure that passes water freely while holding nutrients for plant roots.

The Hilo series was established in 1949 in the Soil Survey of the Territory of Hawaii — one of the first systematic soil surveys of the islands. At the time, the Hāmākua Coast where Hilo soil is concentrated was the center of Hawaii's sugarcane industry, and understanding this soil was directly tied to understanding the state's agricultural economy.

Hilo soil was designated Hawaii's official state soil in recognition of its role as the foundation of Big Island agriculture and as a striking example of volcanic soil formation. No other state soil in the United States formed the same way — layer by layer from volcanic ash, on an island that is still growing.

The series takes its name from the city of Hilo, the largest city on the Big Island and the hub of the Hāmākua Coast agricultural region.

Unlike most soils, which show a clear color change from dark surface to lighter subsoil, Hilo soil stays dark reddish-brown from top to bottom. Every layer formed from volcanic ash and carries the same iron-rich minerals. The profile is more uniform than almost any other state soil.

The soil is silty clay loam throughout — finer than sandy soils but not heavy clay. The volcanic minerals give it a sponge-like quality: it holds enormous water per unit of weight but releases it easily to roots. This property, called high water retention at low suction, is the signature of a true Andisol and is what made these slopes so productive for sugarcane.

Hilo soil covers approximately 21,000 acres on the Hāmākua Coast of the Big Island — the wet, forested windward slopes running northeast from Hilo toward Waipiʻo Valley. It sits on the upper slopes of Mauna Kea, at elevations where trade winds push moisture-laden air upward and rainfall is constant.

Mean annual rainfall on Hilo soil reaches about 145 inches — more than twelve feet of rain per year. That makes the Hāmākua Coast one of the wettest agricultural regions in the United States. The soil is well suited to this climate because volcanic Andisols drain rapidly; standing water is rare even in the heaviest rain.

Slopes range from nearly flat to steep — up to 35 percent in places, with even steeper terrain in the deep gulches that cut through the coast. Hilo soil is found only on the Big Island, in Hawaii County. No other county and no other island in Hawaii has significant Hilo series acreage.

The Hāmākua Coast sugarcane industry ran for more than a century on Hilo soil. At its peak, sugarcane plantations occupied most of the coast's flat and gently sloping land. The deep, fertile volcanic soil and constant rainfall meant sugarcane could grow year-round without irrigation — a rare combination that made this region one of the most productive sugar-producing areas in the world. The last Hāmākua Coast sugar mill closed in 1994.

Since the end of sugar, Hilo soil supports orchards, agroforestry, and livestock grazing. Macadamia nuts, tropical fruit, and specialty crops have replaced cane on many former plantation acres. Some land has returned to native forest and is managed for watershed protection.

Natural vegetation on Hilo soil includes native Hawaiian forest — ʻōhiʻa lehua, hapu'u tree ferns, and other species of the wet montane forest. Where forest cover is intact, Hilo soil anchors some of the most biologically diverse native habitat remaining on the Big Island.