Yield and Composition of Soybean Seed as Affected by N and S Fertilization

Soybeans (Glycine max (L.) Merr.) obtain nitrogen from soil and from symbiotic fixation when nodulated with effective strains of rhizobia. These two sources of N may need to be supplemented with fertilizer N for maximum seed yields. The objectives of the present study were to determine the effects of N and sulfur fertilization on soybean seed yield and other seed properties (protein, oil, amino acid, fatty acid, and S composition). Chippewa 64 and a nodulating-nonnodulating (nodnonnod) pair of isolines of Chippewa background were examined at three field locations over 2 years for yield response to different forms of fertilizer N (224 kg/ha) and to fertilizer S (224 kg/ha). Three readily available fertilizers (ammonium nitrate, urea, and urea + sulfur) and two slow release fertilizer (S-coated urea and ureaformaldehyde) were mixed in the top 20 cm of soil before planting. The soils were Nicollct silty clay loam (Aquic Hapludoll), McIntosh silt loam (Aeric Calciaquoll) Webster clay loam (Typic Haplaquoll). Response to N fertilizer was greater with the nonnod isoline than with the other two lines. Nitrogen fertilization increased seed yield, weight per seed, seed protein percentage, and kg protein/ha. Plant height and lodging were either increased or unaffected, depending upon locations and/or year. Seed oil percentage decreased following N fcrtilization. However, total oil production usually increased due to larger yields. All sources of fertilizer decreased N₂ fixation, plant nodule weight, nodule number, and weight per nodule. Amino acids increased uniformly with protein increases but fatty acid composition was unchanged. Sulfur fertilizer increased seed yield in one case and decreased seed yield in two cases depending on location and year. Total S percentage of the seed and S-containing amino acids of the seed were not increased with S additions. Increases in seed yield and/or seed protein percentage in the nodulating lines suggest N₂ fixation failed to supply amounts of N essential for maximum seed yield and/or protein percentage. With the recent concern for energy, N₂ fixation must be increased to provide the N needed for maximum seed yields of soybeans rather than relying on fertilizer N.