Humic foliar application as sustainable technology for improving the growth, yield, and abiotic stress protection of agricultural crops. A review

The application of humic substances (HSs) promotes bioactive effects in plants, stimulating growth and development, promoting against biotic and abiotic stresses and increasing agricultural productivity. There are countless examples of fertilizers and biostimulants made from HSs that are capable of being used to form state-of-the-art intelligent agricultural technologies with increased efficiency due to their versatility and structural richness. In recent years, the phytotechnics associated with HS application to foliage have improved, and the applications have been expanded to all plant groups; however, the studies are disaggregated and still scarce, hindering the integration of data and the implementation of this technology for researchers, technicians, and specialists. The objective of this review was to gather all possible evidence related to the ability of HSs to stimulate plant metabolism when applied to foliage. This review first addressed the characteristics of foliar application and HSs. Subsequently, studies were organized by plant groups: vegetables, grasses, legumes, fruit, oilseeds, and medicinal and ornamental plants. Regardless of the plant group, HS foliar application stimulated parameters such as biomass and plant height and increased levels of photosynthetic pigments and agricultural productivity. Foliar application promoted protection against stress events, increasing the activity of peroxidase (POX), catalase (CAT), and phenyl alanine ammonium lyase (PAL) enzymes. Fruit quality also improved with HS foliar application, especially the total sugar content and the amount of oil, protein, and fiber, among others. Based on this review, we propose studies that integrate new forms and technologies of HS foliar application to plants. Experiments with various sources of origin, plant types, and environments are necessary to standardize the application forms of these compounds. Thus, we conclude that HSs are a viable technology that is environmentally friendly and highly accessible to small farmers and family farmers.