Fig. 3

a Illustration of experimental design in the third round of experiments. Black dots represent full factorial designs with respect to NaNO₃, CaCl₂, and trace elements. Blue dots indicate additionally added points. The gray dots represent the respective center points. Minus and plus signs symbolize minimum and maximum concentrations of the respective components. b Contour plots of the Kriging interpolation. The non-varied components were fixed at MgSO₄ = 1.7 × Ref, CaCl₂ = 0.82 × Ref, Trace = 1 × Ref, or NaNO₃ = 0.37 × Ref. Red dots indicate measurement data