Wet air oxidation (WAO) of grape pomace, an organic waste generated in massive amounts worldwide and with low biodegradability due to phenol-acid content, was performed to evaluate the potential production of an organomineral fertilizer with biostimulant properties. The synergistic effect of co-processing kimberlite rock powder, a diamond mine tailing, was tested as a potential waste-derived catalyst due to its ferrous iron content and as a potential carbon sink due to its alkaline earth-silicate content. The gaseous, liquid, and solid phases were characterized by gas chromatography, wavelength-dispersive X-ray fluorescence, pH, loss on ignition, thermogravimetric analysis, attenuated total reflectance Fourier-transform infrared spectroscopy, calcimetry, and turbidity, and the results were also subjected to bivariate and multivariate analyses (Pearson correlation coefficient and principal component analysis) to uncover data patterns. The products obtained from WAO were evaluated in bioassays for their soil amendment properties: one assay as a nutrient source to alfalfa (Medicago sativa L) and another assay with soybean (Glycine max L. Merrill) under abiotic stress. The results revealed that temperature and kimberlite play an important role in the oxidation performance of steadfast humic-like substances, and kimberlite is a sink for part of the CO2 that is produced during WAO via mineral carbonation. The WAO products were found to possess beneficial nutrients and properties as soil amendments in terms of sustaining early plant development and alleviating salinity stress.