The intensive agricultural systems that existed in the Hawaiian Islands prior to European contact have been separated into two major groups: irrigated pondfield and rain-fed upland systems. Pondfields were concentrated in well-watered alluvial and colluvial soils, which occur primarily on older islands, while rainfed systems were confined to fertile soils with moderate rainfall. Because soil fertility decreases with increasing substrate age, rain-fed systems occur primarily on younger islands. We evaluated soil properties related to agricultural potential within and surrounding major valleys in two landscapes that differ in substrate age – Pololu on the Island of Hawai`i (<0.5 million y), and Halawa on Moloka`i (>1.5 million y). The Pololu landscape is subsiding rapidly (~2 mm/y) as active volcanism increases the mass of Hawai`i Island and deforms the underlying crust, while subsidence of the Halawa landscape is orders of magnitude slower.

Results/Conclusions

Soils of the uplands surrounding both valleys are nutrient depleted (base saturation <15%, available P < 2 ppm), and neither supported intensive rain-fed agriculture prior to European contact. The alluvial soils of both valley floors are enriched (base saturation > 40%, available P > 10 ppm), and both supported systems of irrigated pondfields. The landscapes differ in that Halawa Valley contains a band of colluvial soils between its steep cliffs and alluvial floor; these soils are younger than those in the uplands, and far richer in nutrients (base saturation ~60%, available P ~20 ppm). These colluvial areas support the remains of field walls and small cleared terraces that mark rain-fed cultivation prior to European contact. Subsidence of the younger Pololu landscape drowns these colluvial areas, and alluvial soils border the steep cliffs. The lack of subsidence on older islands allows the development of agricultural landscapes that integrated both pondfields and intensive rain-fed systems within a single valley, while these systems are spatially segregated on the younger islands of the Hawaiian Archipelago.