Potassium and Phosphorus

Abstract

Critical K concentrations for diagnosis of K deficiency were determined in various leaf, petiole and stem tissues at growth stages R3–4 (37 days after sowing‐D37) and R5–6 (D55) of soybean (Glycine max (L.) Merr.) grown at eight levels of K supply on a K deficient soil. Critical concentrations declined substantially between D37 and D55 in all tissues examined except the basal stem internode and the petiole of the leaf below the youngest fully expanded leaf (YFEL). Calculation of K concentrations on the basis of tissue water content did not arrest the decline in critical concentrations. It is suggested that the main factors causing the decline in critical K concentrations were decreasing K supply from the soil with time and the remobilization of K out of leaves, petioles and stems into developing seeds. Until reasons for the decline in critical concentrations for deficiency diagnosis in soybean are clarified, critical values for YFEL must be related to growth stages of the plant. In the YFEL, critical concentrations were 0.88 and 0.39% at growth stages R3–4 and R5–6 (pod set and pod fill), respectively.

Key words:

• Soybean
• potassium
• critical concentration
• plant parts
• plant age and stage of development
• potassium supply