Hawaiian agriculture prior to Cook: Sources of nutrients to windward agricultural systems

We evaluated sources of the nutrients that sustained intensive agriculture prior to European contact in windward Kohala, Hawai`i, focusing on three potential sources – weathering of primary and secondary minerals in upland soils, enhancement of  weathering supply on eroded colluvial and alluvial landforms, and transport of rock-derived nutrients to crops in irrigation water. Extensive sampling across windward Kohala demonstrated that weathering was insufficient to sustain intensive agriculture in upland soils; all areas were more depleted in cations and phosphorus than were soils at a fertility threshold that bounds nearby rain-fed leeward agricultural systems. In contrast, we found enhanced supplies of rock-derived nutrients on the slopes and alluvial/colluvial areas of a large valley (>200 m deep); these areas retained 40-60% of Ca from parent material, versus ~ 3% for little-eroded upland surfaces, and they supported pre-contact agriculture. However, alluvial/colluvial soils of smaller valleys are less enriched (8-13% of parent material Ca retained), and many of these smaller valleys contained pre-contact irrigated agricultural systems. Analyses of streamwater in these valleys demonstrated that most dissolved cations derive from rock rather than atmospheric sources; for example, analyses of ⁸⁷Sr/⁸⁶Sr show that >90% of dissolved strontium comes from basalt sources at low and moderate streamflows.  We calculate that irrigation water could supply >50 times more Ca to taro crops than could in situ weathering on upland soils.