Co-composted biochar amendments modulate soil microclimate and phenology to enhance saffron yield under saline field conditions

A two-year field study was conducted in soils with low and high electrical conductivity (EC): 0.25 dS m^–1, and 1.95 dS m^–1 respectively). The high EC soil was transported from agricultural land. Composted manures obtained from sheep/goat (SG), dairy (FYM), and poultry (PM) farms and manures co-composted with biochar made from Acacia nilotica L. wood (SG-B, FYM-B, and PM-B) were added to the soil for two consecutive years. The influence of these organic fertilizers on soil properties, flowering period, and fresh yield of saffron (Crocus sativus L.) stigma was examined. Analysis of soil collected after the harvest of the second-year crop revealed that organic fertilizers caused a significant two- to fourfold increase in the concentration of potassium and bioavailable phosphorus in both low EC soil and high EC soil. After two years of cropping, the EC of high EC soil dropped to 0.94 to 1.71 dS m^–1 under various treatments. In the low EC soil, FYM-B triggered 2–3 days of early flowering in both cropping years. However, this prolonged flowering period did not cause any increase in stigma yield. Furthermore, in this soil, for the first-year crop, all organic fertilizers reduced stigma yield by 41–44% compared with control treatment, whereas no difference among treatments was observed for the second-year crop. The stigma yield in the high EC control soil was significantly lower than in the low EC control soil by 26% and 56% for the first- and second-year crops, respectively. In high EC soil, no difference among treatments was observed in the stigma yield of the first-year crop. However, for the second-year crop in the high EC soil, all organic fertilizers prolonged the flowering period; moreover, the FYM and PM treatments also increased stigma yield by 78% and 70% respectively (p < 0.05). In conclusion, organic fertilizers influenced the flowering period of saffron and significantly increased the concentrations of potassium by 79%–218% and phosphorus by 187%–455% in the high and low EC soils. The PM in the high EC soil was found to be more suitable for stigma yield and soil properties when applied for two years; relative to control, this organic fertilizer caused the highest significant increase in potassium (218%), phosphorus (371%), and soil moisture (24%) during the critical flowering period. This treatment also prolonged the flowering period and the increased yield of stigma (70%) in the second-year crop in the high EC soil.