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This article in AJ

  1. Vol. 77 No. 1, p. 57-62
    Received: Mar 19, 1984

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Nitrogen Nutrition and Yield of Seedling Alfalfa as Affected by Ammonium Nitrate Fertilization1

  1. B. D. Eardly,
  2. David B. Hannaway and
  3. P. J. Bottomley2



Nitrogen fertilization of seedling legumes is a controversial management practice intended to provide an N source to the plant prior to the development of N2-fixation activity. Field experiments were conducted in 3 consecutive years (1980,1981, and 1982) to evaluate the effect of ammonium nitrate (NH4NO3) at establishment on nodulation, acetylene reduction (AR), percentage herbage N, and dry matter yield of 10-week-old seedling alfalfa (Medicago sativa L. cv. Apollo). The soil at the field site was a Woodburn silt loam, a member of the fine-silty, mixed, mesic Aquultic Argixeroll. The soil contained 8 to 12 mg kg−1 of available soil N. Six rates of NH4NO3-N were applied preplant in 1980 and nine rates were applied preplant in 1981 and 1982 (0-224 kg N ha−1) in a randomized complete block design. Data from the 1980 experiment suggested that the native population of Rhizobium meliloti Dang. was ineffective in N2 fixation. This was confirmed by effectiveness tests of native strains of R. meliloti in 1981. In 1982 an inoculation variable was added in a factorial treatment combination to examine the effects of NH4NO3 fertilization on the N nutrition and yield of both effectively and ineffectively nodulated alfalfa. Results from the three field experiments indicated that only plants which were ineffectively nodulated exhibited substantial increases in herbage yield and Kjeldahl N concentration in response to the "NH4NO3 applications. All rates of NH4NO3 fertilization decreased nodulation in a curvilinear response. For example, application of 45 kg N ha−1 reduced both nodulation and AR by 45% at 10 weeks (early bloom) regardless of rhizobia1 effectiveness. Nitrogen fertilization also increased weed competition in 1981 and herbage nitrate levels in 1982.

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