Evaluation of plants’ response to activated biochar through visual and proximal sensing monitoring

This study presents two laboratory-scale greenhouse experiments evaluating the effects of biochar addition on lettuce (Lactuca sativa) and spinach (Spinacia oleracea). Two biochar types, derived from pyrolysis at 550 °C of softwood (SW) and sewage sludge (SS), were applied at two doses (2 and 15 g/L), as both unaltered material and after physical activation with CO2 at 900 °C for 2 h. Visual monitoring included plant growth geometrical parameters, chlorophyll content (SPAD), and soil properties. Proximal sensing was based on continuous non-destructive monitoring of plant health through the normalized difference vegetation index (NDVI). Overall, the biochar application increased biomass and vegetative growth compared to control plants. Specifically, 15 g/L achieved larger fresh biomass (SS: +294% spinach, +87% lettuce; SW: +58% spinach, +19% lettuce) and SPAD (SS: +48% spinach, +14% lettuce; SW: +55% spinach, +15% lettuce) compared to 2 g/L. Activation further improved leaf number (up to +47%) and fresh biomass (+57%) in lettuce, and also in spinach (+32% leaves and +121% fresh biomass). Biochar increased soil electrical conductivity in lettuce (+6.4% SS, +1.3% SW) and moisture (SS: +5.8% spinach, +7.5% lettuce; SW: +10.2% spinach, +1.1% lettuce), while pH remained stable. Integrating NDVI trends with SPAD and geometric plant modeling supported the assessment of vegetative growth. In conclusion, biochar, particularly that derived from SS, improved plant growth and soil properties, demonstrating its potential as a sustainable growing media to enhance productivity and resilience in controlled cropping systems.