NITROGEN-FERTILIZER REQUIREMENTS IN AN ANNUAL DRYLAND CROPPING SYSTEM

Reduced tillage systems in the Central Great Plains have improved precipitation storage efficiency and increased the potential to crop more intensively than with the traditional crop-fallow system. More intensive cropping will require additional N input to maintain economical yields. Nitrogen fertility requirements for optimum crop yields in a dryland, annual cropping system were studied. Six N fertilizer rates (0, 22, 45, 67, 90, and 134 kg N ha-1) were applied to the same plots for 8 crops on a Weld silt loam (montmorillonitic, mesic Aridic Paleustoll). Spring barley (Hordeum vulgare L.), com (Zea mays L.), and winter wheat (Triticum aestivum L.) were grown in rotation from 1984 through 1991. Although grain yields varied with crop and year, average annual grain production was 2108, 2683, 3162, 3459, 3382, and 3411 kg ha-1 for the above N rates, respectively. This included 1 yr of zero yield, when a corn crop was lost due to hail, and 1 yr of low barley yields due to heat and water stress at anthesis. This compares with annualized winter wheat yields of 1381 kg ha-1 yr-1 produced in an adjacent crop-fallow system with adequate applied N. Application of 67 kg N ha-1 each crop year or, based on regression analysis, an average available N (soil plus fertilizer) supply of 170 kg N ha-1 was sufficient to optimize (95% of maximum) grain yields over the 8-yr period. These results indicate a high potential for adopting more intensive dryland cropping systems in the Central Great Plains to increase water use efficiency and better maintain soil quality.