Changing nitrogen source on the yield and nitrogen content of soybeans

Summary Soybean plants were grown in nutrient culture solutions containing 150 ppm of N either as an equal concentration of NH₄⁺ or NO₃^–, or all NO₃^–. At the R2 stage of growth for some plants, the N form was changed to either all NO₃^– or all NH₄⁺, but at the same total N concentration as before. Highest seed yield was obtained with all NO₃^– over the entire growth period, the poorest when the N form was switched from an equal ratio of NH₄⁺ and NO₃^– to all NH₄⁺ at the R2 stage. Kjeldahl N concentrations in the plant leaves and seed were highest when NH₄⁺ was part or all of the N source in the nutrient solution. These results may partially explain why the literature is inconsistent on the effect of added fertilizer N on soybean seed yield, and may pose a problem in using leaf Kjeldahl N concentration to determine plant N sufficiency.