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Critical Levels of Manganese in Soybean Leaves at Various Growth Stages¹

Manganese deficiency in soybeans [Glycine max (L.) Merr.] is a common problem in the Atlantic Coastal Plain. Leaf Mn concentration may be used to diagnose this disorder. For plant analyses to be effective, critical levels need to be established at different growth stages. Data from 38 experiment-year field rate studies were used to relate leaf Mn concentration at the R2 growth stage to yield response to fertilization. In three of the experiments, leaf samples were also taken at the V4 and V10 growth stages. The critical Mn concentration varied from 16 to 22 mg kg^–1 over the three sampling dates. With a severe Mn deficiency, the critical Mn concentration remained relatively constant at 17 mg kg^–1 at the V4, V10, and R2 growth stages. With a less severe deficiency, however, the leaf Mn became greater over the season, and the critical level increased to 22 mg kg^–1 at the R2 growth stage. It appears that the R2 growth stage is the most reliable time for plant sampling in order to predict a yield response. The critical Mn concentration may range from 17 to 22 mg kg^–1 at that time. Sampling prior to the R2 stage is less reliable. The critical Mn concentration at the V4 growth stage was estimated to be 16 to 17 mg kg^–1, but plants with even lower values did not always remain deficient.

Key Words: Calibration • Linear plateau model • Mn fertilization • Yield response • Critical deficiency levels • V4 • V10 • R2 growth stages • Deficiency symptoms • Glycine max L.

² Former graduate research assistant and professor of soil science, respectively, North Carolina State Univ., Raleigh, NC 27695.

Received for publication July 16, 1984.