Combined effects of Fe-modified biochar and fly ash with fertilizer on soil fertility, yield and Cd-Pb uptake in Spinach (Spinacia oleracea L.) in semi-arid soils

In semi-arid regions, low organic carbon and coarse-textured agricultural soil limit nutrient retention and promote excessive application of chemical fertilizers, which in long run increases the risk of the heavy metal accumulation in agricultural soils and subsequent transfer into food crops. This study evaluated low-cost amendments such as rice husk biochar (BC), coal fly ash (FA) and their Fe-modified forms, with and without fertilizer, for improving soil quality, spinach ( Spinacia oleracea L.) productivity, and Pb-Cd immobilization in artificially contaminated sandy soils from Bathinda, Punjab, India. A pot experiment (with 5% w/w amendment) was conducted during winter season 2025–2026 in the field conditions, and the amendments were characterized using XRD, SEM, EDX, and FTIR. The results demonstrated that biochar combined with fertilizer (BCF) produced the greatest improvement in soil physical and chemical quality, by increasing total organic carbon by 738%, available N, P, K up to 165%, while reducing soil bulk density (up to -22.6%) compared to control. These improvements translated into substantial yield enhancements relative to control by significantly increasing plant length, root length, and fresh weight. FA-based treatments with fertilizer (FAF) produced comparatively smaller improvements in nutrient availability and crop yield because of their limited effect on soil physical properties. Compared to raw BC or FA, Fe-modified BC/FA effectively reduced DTPA-extractable fraction of Cd and Pb (Pb up to 69% and Cd up to 80%, respectively compared to CK), with greatest reduction was observed in Fe-modified FA (FeFA), followed by FeFAF and FeBCFAF treatments. This decline in phytoavailable metal fractions was associated with significant reduction of Pb and Cd concentrations in spinach roots and leaves by 57-73% and 59-87%, respectively. Overall, the combined application of Fe-modified BC or Fe-modified FA with fertilizer in pot experiments demonstrated simultaneous improvement of soil fertility, crop productivity and minimized metal transfer to crop. However, this study should be conducted in field scale and optimize the amendments dose and fertilizer application rates to validate this finding before its wider application in the semi-arid regions of Punjab.