Agro-environmental applications of humic substances: A critical review

Improving crop productivity in a sustainable manner represents one of the most important future challenges to provide sufficient food, feed and fiber supplies with minimum deterioration of soil quality. The term "ecological intensification" is recently introduced as an eco-friendly approach for improving crop productivity (maximizing water and nutrient supply potentials of soil) with minimum application of synthetic additives (e.g. mineral fertilizers). Humic substances (the final components of organic matter decomposition) are among the most important natural products that can help in promoting sustainable intensification of crop productivity given their high nutrients content, huge surface area, active biostimulators, stability against microbial decomposition and large number of active functional groups. Consequently, humic substances receive considerable interest for improving soil quality indices, modulating plant physiological responses and maximizing quantitative and qualitative yield parameters. On the other hand, the unique physicochemical characters of humic substances supported its application as an efficient soil amendment toward stabilizing potentially toxic elements. In this review, we will shed the light on the agro-environmental applications of humic substances taking into consideration the state-of-the-art in this regard. Additionally, we will highlight the future perspectives of the ongoing-research about novel techniques of humic substances fabrication/functionalization in several agro-environmental applications.