A dynamic optimization model which introduces an intertemporal nitrate-nitrogen residual function is used to derive and evaluate nitrogen fertilizer and irrigation water optimal decision rules for cotton production in the Southern High Plains of Texas. The results indicate that optimal nitrogen applications critically depend on initial nitrate-nitrogen levels and nitrogen prices. Also, the results indicate that the optimal levels of irrigation water are critically dependent on its price. Duality theory is used to derive irrigation water and nitrogen demand functions and a cotton supply function.