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The leeward Kohala Field System on the island of Hawai'i was one of the most intensive pre–European contact dryland agricultural systems. Archaeological and soil analysis has documented changes in soil nutrients over time. Soils were collected under agricultural field walls of different relative ages within the Kohala Field System. These field walls preserved soil from the time of their construction ( between ca. A.D. 1400 and 1800), so soil samples from underneath older field walls have been exposed to a shorter period of cultivation than the soils under more recent field walls. Total P and P:Nb ratios of these buried soils were greater under walls than in once-cultivated surface soils, and greater under older walls than under younger walls. These results suggest that precontact cultivation decreased soil P reserves in this intensive agricultural landscape.