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Influenced of N Nutrition on Total N, Nitrate, and Carbohydrate Levels in Soybeans¹

A better understanding of the effects of varying the N supply to the soybean (Glycine max (L.) Merrill) be obtained by studying, after treatment, the resultant changes in N and carbohydrate levels in the various plant parts. Greenhouse and field experiments were conducted to determine the effect of varying the N supply to the plant during flowering and pod-filling on the dry weight, total N, NO₃^-, and the total available carbohydrates. The greenhouse experiment was conducted in a gravel culture hydroponics system with 5 meq NO₃^-/liter m the nutrient solution. Some plants were shifted to 12.5 and 18.0 meq NO₃^-/liter at initial bloom and maintained at these levels until maturity, while others were exposed to the high NO₃^- levels only until the end of bloom. Treatments in the field consisted of 168 kg N/ha as NH₄NO₃ applied at initial bloom, at the end of bloom, or at both times (total of 336 kg N/ha). Increasing the N supply to the plant during the period from initial bloom increased the dry weight of the vegetative plant parts at the end of bloom and at mid-podfill. The effect was much greater in the greenhouse than in the field. Increasing the N supply to the plant also increased the concentration of N and NO₃^- in the vegetative tissue at the end of bloom; differences in N and NO₃^- were also apparent at midpodfill although the magnitude was not so great. The concentration of total available carbohydrates in the vegetative tissue decreased when the N supply to the plant increased. Treatment effects were similar in the greenhouse (non-nodulated) and in the field (nodulated) experiment. A previous paper reported that yield was increased by increasing the N supply to the plant during flowering and pod set, primarily as a result of an increase in seed number. The increase in yield was associated with increased tissue levels of N and NO₃^- and decreased levels of total available carbohydrates. The data demonstrate the importance of high tissue levels of N during the period of flowering and podset.

Key Words: Glycine max (L.) • Merill Vegetative growth • Total available carbohydrates

² Graduate research assistant, associate professor aud professor, Dep. of Agronomy, Uuiv. of Kentucky, Lexington, KY 40506. Present address of the senior author is Departamento de Agronomia, Universidad Nacional del Sur, Rondeau 29, Bahia Blanca, Argentina.

Received for publication January 23, 1977.