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Long-term tillage and nitrogen fertilization in a west central Great Plains wheat-sorghum-fallow rotation

Tillage and N management are important in dryland crop production of the west central Great Plains (area between the 99 th meridian and the eastern edge of the Rocky Mountains) because of frequent periods of limited soil moisture. Therefore, judicious use of N fertilizer is a management priority in wheat (Triticum aestivum L.)-sorghum [Sorghum biocolor (L.) Moenchl- fallow (W-S-F) rotations. The objectives of this study were to: (i) determine the long-term effects of N fertilization (0, 20, 40, and 60 Ib N/acre) on grain yields of winter wheat and grain sorghum under three tillage systems, (ii) investigate the effect of soil moisture at or near planting on grain yields, and (iii) evaluate the residual profile soil inorganic N after 20 yr of N fertilization in the three tillage systems. The study involved a W-S-F rotation under three tillage systems on a nearly level Harney silt loam soil (fine, montmorillonite, mesic Typic Argiustoll). The three tillage systems were clean-till (CT), reduced-till (RT), and no-till (NT). Nitrogen was broadcast preplant as ammonium nitrate on each crop at rates of 0, 20, 40, and 60 lb N/acre. As the level of soil moisture increased in each tillage system, there was a corresponding larger yield increase of wheat and sorghum to applied N. The correlation of grain yields of wheat and sorghum with soil profile N at all depths was highest for nitrate N and lowest for ammonium and total inorganic N. For all three tillage systems, sampling deeper than 6 in. resulted in little improvement in the coefficient of determination (R 2 ) for grain yields regressed on soil nitrate N. Residual soil nitrate N was highest in the top 6 in., dropped significantly in the 6- to 12-in. depth, and remained relatively low thereafter throughout the 72-in. sampling depth. Data from this long-term study showed the optimum broadcast N rate was approximately 60 lb N/acre applied on each crop grown in a W-S-F rotation with the exact rate depending on soil moisture, fertilizer, and crop prices. Yields from CT were comparable with RT on this nearly level upland soil but failed to meet the residue requirements mandated in conservation compliance plans. Poorer stands, increased weed competition, and drier soils resulted in generally lower yields from NT plots. Considering all factors, RT systems for dryland wheat and sorghum production are recommended on upland fertile soils in the west central Great Plains.