Sources of nutrients to windward agricultural systems in pre-contact Hawai'i

Prior to European contact in 1778, Hawaiians developed intensive irrigated pondfield agricultural systems in windward Kohala, Hawai'i. We evaluated three potential sources of nutrients to windward systems that could have sustained intensive agriculture: (1) in situ weathering of primary and secondary minerals in upland soils; (2) rejuvenation of the supply of rock-derived nutrients on eroded slopes and in alluvium; and (3) transport of rock-derived nutrients to crops via irrigation water. Our results show that most windward soils are infertile and suggest that weathering of minerals within upland soils was insufficient to sustain intensive agriculture without substantial cultural inputs. Erosion enhances weathering and so increases nutrient supply, with soils of the largest alluvial valleys (>200 m deep) retaining 37% of calcium from parent material (compared to 2% in upland sites). However, soils of smaller valleys that also supported pre-contact agricultural systems are substantially less enriched. Isotopic 87Sr/86Sr analyses of stream water demonstrate that at low to moderate stream flow over 90% of dissolved strontium derives from weathering of basalt rather than deposition of atmospheric sources; most other dissolved cations also derive from basalt weathering. We calculate that irrigation water could have supplied approximately 200 kg ha(-1) yr(-1) of calcium to pondfield systems, nearly 100 times more than was supplied by weathering in soils on stable geomorphic surfaces. In effect, irrigation waters brought nutrients from rocks to the windward crops.