The effect of N fertilizer strategy on N2 fixation, growth and yield of vegetable soybean

Vegetable soybean (Glycine max) is a lucrative cash crop predominantly grown in Japan, Taiwan, China, Thailand, and Vietnam. Although soybean has the capacity to satisfy a large proportion of its own nitrogen (N) requirements via N₂ fixation, farmers are recommended to apply several side-dressings of fertilizer N during growth in the belief that this will maximize pod yield. A field experiment was conducted in northern Thailand to evaluate the effect of six different N fertilizer management strategies on crop growth and marketable pod yield of vegetable soybean and to assess the impact of fertilization on the potential carry-over of residual fixed N remaining after harvest. A vegetable soybean crop was supplied with 25 kg N ha⁻¹ ammonium sulphate as starter N at sowing, then areas of the crop either received no further fertilizer N, or received one (50 kg N ha⁻¹), or two (50 + 25 kg N ha⁻¹) urea top-dressings at different stages of vegetative and/or reproductive growth.

The field trial demonstrated that the appropriate timing of N fertilization could improve both crop growth and pod yield. Crop biomass was increased by 11 to 16% and pod yield improved by up to 44% if starter N was followed by a single top-dressing at either early vegetative growth or at flowering. However, there was no additional benefit from the standard farmer practice of supplying two top-dressings prior to flowering. However, if an early vegetative application of fertilizer N was followed by a second dressing either during flowering or pod-fill a further 20% increase in marketable pod yield was achieved.

The effect of fertilizer N on soybean's capacity to fix N was complex. The proportion of plant N derived from N₂ fixation (Pfix) was highest when only starter N₂ was applied (seasonal average of 84% of crop N₂ coming from N₂ fixation). Any further top-dressing with N-fertilizer depressed nodulation and Pfix, but also increased crop growth so that amounts of N, fixed were similar for most fertilizer treatments (adjusted to account for below-ground N: 149 to 153 kg N ha⁻¹ cf 147 kg N ha⁻¹ fixed in the presence of starter N alone). However, N₂ fixation was significantly depressed by the farmer practice of top-dressing N twice before flowering (117 kg fixed N ha⁻¹, and was highest when fertilizer N was supplied during a period of peak N demand at pod filling (180 kg N ha⁻¹).

Between 53 and 92 kg N ha⁻¹ was removed from the crop in marketable pods. But since N₂ fixation remained the dominant N source for crop growth in all treatments, substantial amounts of fixed N (64 to 91 kg of fixed N ha⁻¹ were estimated to remain in leaf, stem and root residues after harvest. However, the potential N benefit from including vegetable soybean in a cropping sequence disappeared if the above-ground residues were removed from the field as is the common practice in many Asian farming systems.