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Molybdenum Limitations to Alfalfa Growth and Nitrogen Content on a Moderately Acid, High-Phosphorus Soil¹

Alfalfa (Medicago sativa L.) grown in an Aquultic Argixeroll at pH 5.5 had a low N content and produced very low yields even though soil and/or plant tissue analysis for Al, Mn, Ca, P, Mo, and Mg did not indicate deficiency or toxicity conditions. Field and greenhouse experiments with pH, Ca, N, and Mo variables were undertaken to characterize the factor(s) limiting production of alfalfa on this soil. Plants at all pH levels (5.5 to 6.4 in the field and 5.3 to 6.5 in the greenhouse) were well nodulated. Under greenhouse conditions the application of 1 mg Mo kg^–1 soil resulted in a total dry matter yield, total N uptake, and shoot N concentration of 9.87 g pot^–1, 0.216 g pot^–1, and 26.5 g kg^–1, respectively, compared with corresponding values of 2.96, 0.040, and 20.6 for plants grown without Mo. The application of 714 mg N kg^–1 (greenhouse) and 672 kg N ha^–1 (field) as NH₄NO₃ to alfalfa grown in unamended soil increased shoot dry matter production from 1.46 to 7.01 g pot^–1 in the greenhouse and 3.7 to 12.0 Mg ha^–1 under field conditions. No response in yield or nodulation to the application of CaSO₄ was measured. Responses to N and Mo, especially at the lower pH levels, suggested that the growth response to lime is due primarily to increased nodule efficiency, resulting from greater Mo availability as soil pH is raised. On moderately acidic fertile soils, the application of Mo to well-nodulated but N-deficient alfalfa may provide an economical, effective means for increasing production where low Mo availability is the major growth-limiting factor.

Key Words: Soil acidity • Nodulation • Nodule efficiency • pH • Medicago sativa L. • Gypsum • Lime

² Research assistant, Dep. of Soil Science; associate professor, Dep. of Microbiology and Dep. of Soil Science; and professor, Dep. of Soil Science. Senior author is now extension soils specialist, Dep. of Soil and Water Science, The Univ. of Arizona, Tucson, AZ 85721.

Received for publication November 26, 1984.

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