Physicochemical analyses of plant biostimulant formulations and characterisation of commercial products by instrumental techniques

The objective of this study was to develop instrumental protocols for evaluating physicochemical characteristics of plant biostimulant/biofertiliser formulations. Six formulations (Rygex, Algavyt, Ryzoset, Manek, Ecoryg and Algavyt Zn/Mn) containing algal/plant extracts, humic and amino acids, lipids and inorganic components were assessed for particle size distribution and zeta potential (ZP) by dynamic light scattering (DLS) and gross compositional differences by thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The six commercial formulations were assessed by mung bean root and wheat leaf senescence bioassays to determine auxin- and cytokinin-like activities. Among them, Rygex, Manek and Ecoryg showed significant auxin-like activities compared to the other formulations. Only Manek showed very low cytokinin-like activity. In combination, the four instrumental techniques highlighted gross differences in the particle sizes, ZP and compositions of the products, and provided key results on the physicochemical characteristics of the formulations including potential stability of the products. The three techniques (TGA, FTIR and Py-GC/MS) require no sample preparation beyond drying of the materials. The activities of the plant growth hormones have been compared to show advantages of the growth hormone bioassay methods for selecting better formulations.