Dynamic critical potassium concentrations in soybean leaves and petioles for monitoring potassium nutrition

Soybean [ Glycine max (L.) Merr.] critical tissue‐K concentrations associated with the bloom to early pod set growth stages have been used to diagnose K deficiency for decades. The ability to interpret tissue concentrations beyond the R2 stage would strengthen soybean tissue monitoring programs. Our objective was to develop continuous critical‐K concentrations for the trifoliolate leaf and petiole of irrigated soybean. Trifoliolate leaf‐ and petiole‐K concentration data were collected during soybean reproductive growth from 10 research trials. Multiple regression was performed to predict relative soybean yield as a function of tissue‐K concentrations and days after R1 development (DAR1). The 10 research trials provided >1,400 leaf‐ and petiole‐K concentration observations from the R1–R6 stages of soybean cultivars from the 4.4–5.5 relative maturity groups. Among the 10 trials, soybean receiving no fertilizer‐K produced 59.2–83.2% of the maximum yield produced by soybean receiving fertilizer‐K. The sufficient leaf‐K concentrations associated with 95% relative yield were 20.2 g K kg –1 at 1 DAR1, 18.5 g K kg –1 at 20 DAR1, 15.5 g K kg –1 at 40 DAR1, and 11.2 g K kg –1 at 60 DAR1, which occur at the R1, R2, and R3‐R4, and R5‐R6 growth stages, respectively. Compared with leaf‐K concentration, the sufficient petiole‐K concentrations associated with 95% relative yield were 2.5 times greater at R1 but similar at 70 DAR1 (R6). Agricultural analytical laboratories can use these sufficiency equations for soybean leaf‐ or petiole‐K concentrations, with planting date and maturity group information, to interpret soybean K sufficiency.