Response of spring wheat to nitrogen fertilizers of different nitrification rates

Hydroponic and greenhouse studies have shown that grain yields of wheat (Triticum aestivum L.) are maximized when the N supply contains a mixture of NH4 and NO3. Tillers per plant is the yield component most commonly increased by mixed N nutrition in greenhouse studies. The objective of this study was to evaluate, under field conditions, the response of 'Butte 86' spring wheat to N sources differing greatly in nitrification rate. Nitrogen was applied at 0 or 112 kg N ha(-1); the N sources were calcium nitrate (CN), urea, urea-dicyandiamide (DCD), 0.1-g urea granules, and 0.1-g urea-DCD granules. Six field trials were performed. Topsoil (0-15 cm) analysis of fertilized plots during tillering indicated a wide range of mineral N contents (19-101 mg N kg(-1)) and of NH4:NO3 mole ratios (0.04-2.06). Correlation analyses suggested that tillering was sensitive to the total mineral N supply, but quite insensitive to the NH4:NOB ratio in the topsoil. Vegetative growth and N uptake were generally lower for CN than for the other N sources. At some locations, more heads per square meter at harvest were observed with the slower-nitrifying N sources. Grain yields were increased by N fertilization, but significant differences between N sources were not generally observed. For both years of the study, the 0.1-g urea-DCD granules gave the highest N uptake efficiency. However, the dramatic responses in growth, tillering, or grain yield routinely observed with mixed N nutrition in hydroponic or greenhouse studies could not be duplicated in the field.