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Response of Spring Wheat to Nitrogen Fertilizers of Different Nitrification Rates

Dep. of Soil Science, North Dakota State Univ., Fargo, ND 58105

Bert R. Bock and Brian E. Johnson

Natl. Fertilizer and Environmental Res. Ctr., Tennessee Valley Authority, Muscle Shoals, AL
Dep. of Soil Science, North Dakota State Univ., Fargo, ND 58105

^* Corresponding author (goos{at}badlands.nodak.edu).

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, ureadicyandiamide (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:NO₃ 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.

Received for publication January 21, 1998.

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