Abstract: The preparation of molded seedling substrates traditionally relies on cold- or hot-pressing technologies that require continuous external heating and specialized equipment, leading to high energy consumption and process complexity. To address these challenges, a low-energy fabrication strategy based on self-propagating high-temperature synthesis was developed for producing functional substrate blocks from agricultural biomass. Pig manure, straw, and straw char were employed as organic feedstocks, and bio-oil was introduced as a binding phase. Biomass ash, serving as the sole inorganic additive and mineral source, was incorporated at levels of 4%, 6%, 8%, 10%, and 12% (w/w). After compaction, the green bodies were ignited to initiate an exothermic reaction, during which internally generated heat drove rapid mineral transformation and organic-inorganic coupling, enabling structural consolidation without sustained external energy input. A systematic analytical framework was established to elucidate the structural evolution and functional performance of the obtained substrate blocks. Phase composition and crystallinity were determined by X-ray diffraction, while functional group variations were characterized using Fourier transform infrared spectroscopy. Microstructural morphology and pore distribution were examined through microscopic analysis. Key physicochemical parameters-including bulk density, total porosity, water-holding porosity, saturated water content, compressive strength, and leachate pH and electrical conductivity (EC)-were measured according to standardized testing protocols. Total and available fractions of nitrogen, phosphorus, and potassium were quantified through digestion and chemical extraction methods. Water-fertilizer release behavior was evaluated via sequential leaching experiments to determine nutrient release kinetics. The adaptability of the substrate blocks to seedling cultivation was assessed through controlled nursery trials. Increasing biomass ash incorporation significantly enhanced compressive strength and bulk density, accompanied by elevated leachate pH and EC values, indicating strengthened structural integrity and increased mineral alkalinity. In contrast, crystallinity, total porosity, water-holding porosity, and saturated water content decreased progressively with higher ash content, reflecting matrix densification. Mineral phase formation during self-propagating high-temperature synthesis promoted enrichment of total phosphorus, potassium, and calcium. However, the proportion of available nutrients declined with increasing ash dosage, leading to moderated nutrient release kinetics and improved release stability. At an optimal biomass ash incorporation level of 4%, the substrate blocks achieved a compressive strength of 0.82 MPa and a bulk density of 0.48 g/cm, with a total porosity of 72.80%. The equilibrium leachate pH and EC were 7.6 and 336.98 μS/cm, respectively. Saturated water content reached 106.78%, and the stable water-holding rate was 6.93%. Total nitrogen, phosphorus, and potassium contents were 7.76, 0.96, and 3.68 mg/g, respectively, while alkali-hydrolyzable nitrogen, available phosphorus, and available potassium were 0.77, 0.22, and 0.19 mg/g. Under these conditions, the substrate exhibited superior performance in seedling cultivation, reflecting a favorable balance among mechanical stability, aeration capacity, water retention, and nutrient supply. Mechanistically, biomass ash functioned as a fluxing agent through alkali metal oxides during self-propagating high-temperature synthesis, promoting melt generation, silicate network restructuring, and mineral-organic interfacial bonding. This process induced transformation of the substrate skeleton from a porous framework containing hydrophobic functional groups to a denser matrix enriched with hydrophilic functionalities. The coupled compositional and microstructural evolution enabled coordinated regulation of strength development, water-fertilizer behavior, and cultivation adaptability. Overall, this study establishes an energy-efficient pathway for high-value utilization of agricultural biomass into functional substrate blocks and provides mechanistic insight into inorganic-organic synergy in SHS-derived cultivation materials, supporting the rational design of sustainable substrates for seedling cultivation.