Dry Matter and Potassium Accumulation and Partitioning in Three Soybean Genotypes.

Understanding the K uptake pattern during the growing season is required to develop sound fertilization programs. We measured season-long aboveground dry matter and K accumulation and allocation in representative determinate [maturity group (MG) 5.3 (2012) or 5.5 (2013)] and indeterminate (MG 3.9 and 4.7) soybean [Glycine max (L.) Merr.] cultivars in 2012 and 2013. Whole-plants and recently matured trifoliolate-leaves were collected 8-10 times between V5 and maturity. Each parameter was statistically analyzed by year due to differences in planting date. In 2012, the maximum dry matter accumulation among cultivars was similar (10.3-11.6 Mg ha^-1). In 2013, aboveground dry matter was similar for both indeterminate cultivars (7.5-8.0 Mg ha^-1) but less than the determinate cultivar (9.7 Mg ha^-1). Dry matter accumulation peaked at R6.0-6.5, 96-102 (2012) and 82-96 (2013) days after emergence (DAE). The predicted maximum crop growth rate occurred at R4-5 stage [180-197 (2012) and 144-167 (2013) kg dry matter ha^-1 d^-1]. Each year, the maximum aboveground K uptake was similar among cultivars totaling 118-148 (2012) and 125-132 (2013) kg K ha^-1 and peaking at R5.5-6.0, 93-111 (2012) and 74-94 (2013) DAE. The predicted maximum K uptake rate occurred at R3-4 stage [2.01-2.27 (2012) and 1.77-2.39 (2013) kg K ha^-1 d^-1]. Before leaf senescence (R7), seed comprised 40-51% of the aboveground dry matter (7.2-10.7 Mg ha^-1) and 49-69% of the aboveground K content (114-138 kg K ha^-1). In 2012, trifoliolate-leaf K concentration for all cultivars peaked at R2 (14.2-16.3 g K kg^-1) and declined linearly at a rate of 0.16 g K kg^-1 d^-1 until R7. In 2013, trifoliolate-leaf K concentration peaked at R1 (19.8-20.6 g K kg^-1) and declined linearly, but differently among cultivars (0.06-0.21 g K kg^-1 d^-1). This information may aid in developing critical trifoliolate-leaf K concentrations beyond the R1-2 stage for diagnosis of K deficiency.