Farming the Rock: A biogeochemical perspective on intensive agriculture in Polynesia

Abstract

In pre-contact Hawai‘i, large and intensive rainfed agricultural systems were established only where ongoing weathering of basalt-derived minerals could provide a sustained source of nutrients to crop plants. We demonstrated that the high-elevation, high- rainfall boundary of the Leeward Kohala Field System corresponded to a well-defined threshold in soil fertility, above which soils were acidic and infertile and rock weathering was depleted as a source of biological nutrients. The single most reliable indicator of this boundary was the concentration of exchangeable Ca in soil; rainfed agricultural infrastructure was absent where exchangeable Ca was below ~10 meq/100 g of soil. However, irrigated taro pondfields frequently were developed in windward valleys in soils with <10 meq/100 g exchangeable Ca. In these areas, irrigation water brought a sustained supply of basalt-derived nutrients to pondfields; in effect, rainfed systems brought crops to where basalt was breaking down and supplying nutrients, while irrigation water brought the nutrients resulting from the breakdown of basalt to crops. In contrast to Hawai‘i, most of Rapa Nui has soils with <10 meq/100 g exchangeable Ca, but fine-scale erosion and deposition (probably reinforced in some cases by cultural practices) enhanced soil fertility to near this threshold within intensively cultivated rock gardens.