Optimizing Soil Water Retention through Compost and Vermicompost from Vert d'Iris Cooperative (Belgium): Chemical and Spectroscopic Characterization of Humic Acids (UV–Vis and FTIR Analysis)

The world faces major environmental challenges, including climate change, soil degradation, water scarcity, and pollution from unmanaged organic waste. Sustainable and scientifically validated strategies are therefore essential. This study evaluated the effect of a compost-vermicompost mixture (70:30, w/w), derived from food waste produced by the Vert d'Iris Cooperative (Belgium), on soil field capacity and humic acid quality. The amendment was produced and characterized according to international standards before application to the Betteraves Enz garden (Anderlecht). Soil samples collected at 0–30 cm depth from amended plots (n = 15, n = number of samples) and control plots (n = 15) were analyzed for physicochemical properties and water retention. Humic acids were extracted following the International Humic Substances Society protocol and characterized by UV–visible and FTIR spectroscopy.Amended soils showed a significantly higher field capacity than controls (38.4% vs. 27.3%), representing an increase of approximately 40% (p < 0.001, i.e., probability of error below 0.1%). Spectroscopic analyses indicated enhanced humification, reflected by significant variations (p < 0.05) in the absorbance ratios A2/4 (E2/E4, 280/465 nm) and A4/6 (E4/E6, 465/665 nm), as well as in the ΔlogK index (logA400 – logA600), which are indicators of aromatic condensation, molecular size, and structural maturity of humic substances. FTIR spectra confirmed enrichment in aromatic and oxygenated functional groups.These findings demonstrate that combining physicochemical and spectroscopic indicators provides robust evidence of improved soil water retention and long-term organic matter stabilization, supporting sustainable soil management strategies.