Nitrogen management in corn systems is imprecise and inefficient in the humid regions due to dynamic, complex and locally-specific interactions between weather, soil and management variables. Large N losses via leaching and transport to the Gulf of Mexico and Chesapeake Bay cause hypoxia, while nitrous oxide emissions from denitrification make up the largest fraction of agricultural global warming contributions. Such losses occur primarily during a wet spring/early summer, but also in fall after a dry season when excessive N was applied. Adaptive N management should account for the temporal and spatial variability in N dynamics to allow for more precise and better-timed N fertilizer applications and improved adaptation to the effects of climate change. We developed the web-based Adapt-N tool for adaptive N management, and are validating its use for on-farm applications during the 2011 and 2012 growing seasons. Adapt-N uses farmer-specified management and soil test information and newly-developed high resolution, near-real-time climate data as inputs for the Precision Nitrogen Management (PNM) model. The PNM model simulates daily soil C and N transformations, water transport and N uptake during corn growth, to calculate in-season N fertilizer needs. A sidedress nitrogen recommendation and graphs visualizing seasonal dynamics are provided to the user via the Adapt-N web-interface. Access to this tool should incentivize lower N applications in average years when less is needed, but provide justification for higher in-season applications after wet springs. New York and Iowa growers, in collaboration with consultants and extension staff, are testing the Adapt-N tool in replicated on-farm strip trials in grain, silage and sweet corn in 2011. Preliminary data on agronomic, environmental and economic impact of using the tool will be presented.