Material property control governs the performance of engineered fertilisers and plant biostimulants from biological residues

Biological residues are widely promoted as sources of fertilising materials and plant biostimulants, yet recovered nutrient mass is not equivalent to substitutable fertiliser supply. This structured narrative review examines how feedstock composition and processing determine material properties, agronomic function, contaminant behaviour, scale-up and regulatory eligibility. It covers sewage sludge, digestate, manure, ash, agri-food residues, algae and biochar, using the European Union as a detailed regulatory case and non-European evidence for comparison. Thermal, biological and chemical routes modify nutrient speciation, porosity, surface chemistry, release kinetics and contaminant mobility. Established processes can yield struvite, ammonium sulphate, ash-derived phosphates and biochar products. Their practical contribution is smaller than gross nutrient inventories imply because streams overlap, many residues are already applied to land, and water content, process losses, transport and crop availability constrain substitution. Evidence is strongest for partial, context-dependent replacement by selected phosphorus carriers. Nitrogen recovery remains constrained by economics, while many biostimulant claims lack multi-site, multi-year field validation; both areas require more standardised contaminant assessment. Regulatory comparison further shows that the same material may qualify differently across jurisdictions. Recovery becomes substitution only when controlled composition and release are combined with safety, field performance, scalable processing, viable logistics and market access. Research priorities are harmonised methods for emerging contaminants, transparent techno-economic and life-cycle boundaries, long-term field validation and integrated platforms that couple nutrient recovery with bioactive function. Nutrient recovery alone does not establish practical fertiliser substitution. Processing controls nutrient form, release kinetics and contaminant mobility. Selected recovered phosphorus carriers support partial, context-dependent substitution. Economics, logistics, field evidence and product eligibility remain binding constraints. Long-term trials and harmonised emerging-contaminant methods are priorities.