Historically, agricultural biochar has been applied directly to soil as a solid carbon amendment to boost soil fertility, water retention, and carbon storage. However, recent advancements in mechanical ball milling, high-energy grinding, chemical extraction, and nanotechnology have transformed coarse biochar into stable water-based suspensions, aqueous extracts, and sub-micron colloidal particles suitable for spray delivery. Applying biochar directly to foliage establishes an entirely distinct biological delivery pathway that bypasses root networks. Liquid sprays target leaf cuticles and stomata directly, driving immediate physiological and metabolic responses within the plant.

Foliar sprays of biochar significantly boost plant growth and yield across diverse crop species. In Mediterranean vineyard field trials, repeated foliar applications of wood biochar suspensions at concentrations of six hundred milligrams per liter raised leaf nutrient levels—including nitrogen, potassium, sulfur, boron, and manganese—and increased grape yield per vine by up to thirty-seven percent without impairing net photosynthetic activity. Field trials on French beans using biochar sprays at five hundred milligrams per liter accelerated plant development, advancing first flower initiation and fifty percent flowering while boosting total seed yields.

The primary benefits of leaf-applied biochar manifest under severe abiotic stress conditions such as drought, heat, and high salinity. Spraying colloidal biochar suspensions on salt-stressed tomato plants elevated leaf chlorophyll and carotenoid contents by up to seventy-two percent, stabilized cell membranes, reduced electrolyte leakage, and increased fruit count and biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More. Similar foliar applications on spinach, lettuce, and wheat under combined drought and salinity stresses reduced lipid peroxidation and boosted enzymatic antioxidant defenses, including superoxide dismutase, catalase, and ascorbate peroxidase.

The mechanisms driving these physiological benefits stem from a combination of surface physical effects and biochemical interactions. Milled particles and soluble extracts adhere to the leaf cuticle, forming a protective microscopic surface film. This film alters the leaf boundary layer microclimate, diffusing intense solar radiation, lowering leaf surface temperatures, and reducing water loss through transpiration. At the biochemical level, redox-active functional groups and dissolved organic compounds within biochar extracts stimulate the production of non-enzymatic antioxidants like flavonoids and phenolics. These compounds also promote osmotic adjustment by triggering the accumulation of proline and soluble sugars, helping plants maintain cellular turgor and ionic balance under stress.

Despite these clear agronomic advantages, the effectiveness of foliar biochar remains highly context-dependent. Plant responses vary according to feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More type, pyrolysis conditionsThe conditions under which pyrolysis takes place, such as temperature, heating rate, and residence time, can significantly affect the properties of the biochar produced.   More, particle size, spray dosage, and crop species. Applying biochar sprays at excessively high concentrations can cause physical shading or block stomatal pores, leading to chlorophyll degradation and marginal leaf burns. Furthermore, engineered nano-scale biochar formulations introduce potential regulatory and safety challenges. Unstable nano-particles may cause spray drift or accumulate recalcitrant carbon in non-target aquatic environments. Current agricultural regulations in major global jurisdictions were designed primarily for soil amendments, leaving a regulatory gap for leaf-applied carbonaceous nanomaterials. Multi-season field validations, safety evaluations, and precise dosage standards are required before foliar biochar can be fully commercialized as a scalable agricultural biostimulant.

Source: Palčić, I., Javed, Q., Anđelini, D., Cvitan, D., Prelac, M., & Goreta Ban, S. (2026). Biochar Beyond Soil: State of the Art and Future Perspectives of Foliar Applications. Horticulturae, 12(8), 1042.