Extensive use of inorganic nitrogen (N) fertilizers in Nebraska began in the early 1950's, driven at least partially by the availability of ammonium production facilities following World War II. In Nebraska, the availability of inorganic N fertilizer coincided with the expansion of irrigation, and the transition from primarily dryland wheat production to irrigated corn production. From 1955 through the mid-1970's, the land area dedicated to irrigated corn production increased dramatically in Nebraska, as did the use of N fertilizer. Beginning in the mid-1970's, concern developed about increasing nitrate-N concentrations in groundwater, especially in areas with intensive irrigated corn production. Over the past twenty years, extensive research and educational efforts have increased the efficiency with which N fertilizers are used. Between 1965 and 2005, the amount of N required to produce a bushel of corn in Nebraska declined from 1.6 lb/bushel to less than 1 lb/bushel in 2004. While total irrigated land in Nebraska has remained relatively constant from 1995 – 2004, the proportion which is sprinkler irrigated has steadily increased, and furrow irrigation has declined. This trend has allowed fertigation of urea ammonium nitrate (UAN) solution through irrigation systems, resulting in a steady shift away from anhydrous ammonia to N solution as a primary N fertilizer material.