Origins of Hawaiian Soil

What is soil? Basically it is made up of parent material--chemicals and minerals—and includes decomposing organic material and water. It’s easy to take it for granted. Dirt is dirt, and it’s everywhere, right?

Not exactly. Recently I was reminded of this fact on a hike I took with the Moku Loa chapter of the Sierra Club on the Mauna Iki trail at Hawai'i Volcanoes National Park. The trail led through flows of weathered pahoehoe and shallow pockets of volcanic mud. In this older flow, there were diverse native Hawaiian plants.

On that part of the trail we saw mostly pukiawe, 'ohi'a lehua, 'ulei, ko'oko’olau, and a'ali'i. I was amazed to see how well-adapted these plants are in this usually dry, rocky, harsh environment, and how they are able to grow tenaciously in the cracks and crannies where the tiniest amount of moisture could be found. These thriving pioneers reminded me that gardeners who want to xeriscape with less-thirsty plants would do well with these natives, with perhaps the exception of the 'ohi'a lehua. Once in awhile this area, which is part of Hilina Pali to the Ka'u Desert, also gets misty, which no doubt helps the survival of the few stunted 'ohi'a trees we saw along the way.

Further along the trail we crossed soft, black sand dunes to a newer flow where we were treated to a view of two very deep, scary pit craters, and a small jagged cinder cone, also very deep. These were so vertigo-inducing that a few members of our party refused to get any closer than 30 feet from the edge! The bright colors of the rock inside the pit craters were quite stunning, like a painted desert, indicating a different composition from the surrounding black pahoehoe. At the very bottom of the pits water collects, and assorted native ferns have been able to take hold. A few pioneer 'ohi'a trees flourish at the rims.

It’s amazing to think that all Hawaiian soils started out this way. While continental crusts consist of granite, schist, and gneiss, Hawai'i rests on oceanic crust, which is almost purely basalt. Unlike continents that have uniform climates over large areas, Hawai'i has microclimates that cause lava to weather differently from location to location. Also, since the archipelago drifts along on a hot spot, there are very young soils on Hawai'i and Maui, and very weathered soils on Kaua'i and 'O'ahu. This is why gardeners and farmers throughout Hawai'i have unique challenges – many different solutions have to be found for many variables when it comes to soil analysis and plant nutrition.

It’s a good idea to have your soil analyzed to determine what you can do to improve structure and nutrients. You might find that adding organic material, liming, and/or incorporating other amendments greatly improve the health of your garden. The UH CTAHR Cooperative extension service has a lab that can do a soil analysis for a fee; call your nearest office for directions on how to collect a sample from your yard. Or you can check online for other labs on the mainland.