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

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.