All biochar is not created equal. Its physical and chemical properties and the agronomic responses reported in the literature vary by the feedstock and production process used to make it. These factors strongly influence pH, ash content, porosity, nutrient concentration, and surface chemistry. As a result, two products labeled “biochar” may behave very differently in the same soil.
Feedstock identity strongly governs biochar nutrient availability, alkalinity, cation-exchange behavior, and water-holding effects. A recent analysis reports that biochar produced from a 50:50 biosolids x eucalyptus feedstock contains more than sixty times the plant-available nitrogen of switchgrass-derived biochar, while poultry-litter biochar contains substantially more available phosphorus than pine wood biochar – outcomes that follow directly from feedstock mineral composition. Biochar made from nutrient-rich manures are consistently associated with stronger crop responses than wood-based biochars, a pattern that is mechanistically explained by their substantially higher mineral ash and nutrient content. In these cases, yield responses reflect direct nutrient inputs and liming effects rather than biochar-specific soil processes.
Pyrolysis temperature further modifies these properties, generally trading nutrient availability for increased alkalinity and carbon condensation. Lower-temperature biochars retain more exchangeable nutrients, whereas higher-temperature biochars (>600°C) exhibit increased ash content, higher pH, and reduced nitrogen retention. For instance, potassium availability from eucalyptus biochar produced at 350°C was approximately twice that of the same feedstock pyrolyzed at 550°C. While such liming effects may benefit acidic soils, they are unnecessary or potentially harmful in neutral or alkaline systems.
These findings indicate that many reported benefits attributed to biochar reflect feedstock-derived nutrient and ash effects and temperature-driven alkalinity. These varying biochar characteristics require more study to clearly match them to different soil and crop needs. In addition, biochar users must understand the specific characteristics of different biochar products to ensure it meets their particular needs and must assess whether the use of such biochar is the most cost-effective way to deliver those benefits. The former issue requires better labeling and extensive education of farmers and extension offices, while the latter issue needs more comprehensive economic analysis and, ultimately, a reduction in the price of biochar from current levels.
