Biochar Rate and Fertilizer Source Influence Soil Chemical and Biological Properties in Tomato Plasticulture Systems

Plasticulture tomato ( Solanum lycopersicum ) production in southern Georgia is constrained by acidic soils, low organic matter (OM), and nutrient leaching. A 2-year field study evaluated the effects of five biochar application rates (0, 11.2, 22.4, 33.6, and 44.8 Mg·ha −1 ) combined with two fertilizer sources—conventional (granular and liquid) or poultry litter—on soil chemical and biological properties. Biochar application increased soil pH by up to 1.07 units and improved cation exchange capacity (CEC) by up to 34.8% with the highest rates (33.6–44.8 Mg·ha −1 ) generally being most effective. Total inorganic nitrogen (TIN) responses varied by fertilizer source. Under conventional fertilizer, TIN peaked at 33.6 Mg·ha −1 but declined at 44.8 Mg·ha −1 ; however, under poultry litter, TIN increased steadily, reaching a 144.7% gain at the highest rate. Poultry litter also enhanced microbial activity, as indicated by CO 2 burst and active carbon, and supported greater OM accumulation across years. In contrast, conventional fertilizer treatments showed reduced biological activity and carbon stabilization over time. These findings suggest that combining biochar with poultry litter can improve soil fertility and microbial function, reduce reliance on synthetic fertilizers, and promote long-term soil health in plasticulture vegetable systems. Results can inform amendment strategies for growers seeking sustainable nutrient management practices in the southeastern United States.