Holding Water: A Review of Biochar and Hydrochar for Soil Amendment

Biochar (BC) and hydrochar (HC) have attracted increasing attention as sustainable soil amendments for improving soil water retention and mitigating agricultural water stress. This review synthesizes and compares the current state of knowledge on the production, physicochemical properties, and hydraulic performance of slow-pyrolysis BC, hydrothermal carbonization hydrochar (HTC HC), and hydrothermal liquefaction hydrochar (HTL HC). The mechanisms governing soil water retention are first examined, followed by a comprehensive review of the effects of amendment properties, feedstock type, thermochemical conversion conditions, particle size, application rate, and soil characteristics on field capacity, permanent wilting point, plant-available water, and water-holding capacity. The available evidence demonstrates that BC generally provides the most consistent improvement in soil hydraulic properties, particularly in coarse-textured soils, whereas the performance of HTC HC is considerably more variable and strongly dependent on hydrothermal conversion conditions and soil characteristics. HTL HC remains largely unexplored but shows promising hydraulic performance and exceptional resistance to biodegradation. Apparently contradictory findings among published studies are shown to arise largely from interactions among feedstock and conversion conditions, resulting amendment properties, soil characteristics, application conditions, and differences in hydraulic evaluation, highlighting the need for integrated mechanistic frameworks rather than interpretation based on individual factors. A comparative assessment of the three materials further considers ecotoxicity, biodegradation, life-cycle assessment, and techno-economic analysis. Overall, BC is currently the most mature soil amendment technology, HTC HC offers important advantages for wet biomass utilization, and HTL HC represents a promising but underdeveloped alternative. Future research should emphasize standardized evaluation methods, long-term field validation, and integrated mechanistic approaches linking production conditions, amendment properties, soil characteristics, and application conditions to enable predictive, application-specific design of carbonaceous soil amendments for sustainable soil water management.