Biochar and humic acid amendments improve the quality of composted green waste as a growth medium for the ornamental plant Calathea insignis

The effects of different percentages of biochar (BC) (at 0, 20, and 35%) and/or humic acid (HA) (at 0, 0.5, and 0.7%) on the physical and chemical characteristics of composted green waste (CGW) and on the growth and nutrition of the ornamental plant Calathea insignis were investigated. The highest quality growth medium and the best C. insignis growth were achieved when the CGW was amended with the combination of 20% BC and 0.7% HA; the lowest quality medium and the least growth were obtained with nonamended CGW. The optimum combination not only improved the particle-size distribution and adjusted the bulk density (BD), porosity, and water-holding capacity (WHC) into ideal ranges but also decreased pH and electrical conductivity (EC) and increased macro- and micro-nutrient contents and microbial biomass C and N of the growth media. In comparison with the 100% CGW, shoot fresh weight, shoot dry weight, root fresh weight, root dry weight, plant height, crown breadth, number of leaves, and total root length obtained with the optimal media were increased by 57.3, 79.7, 64.5, 82.0, 45.2, 89.1, 31.1, and 94.1%, respectively, and total nitrogen, total phosphorus, total potassium, and total chlorophyll contents of plant leaves were increased by 66.4, 55.8, 72.1, and 47.8%, respectively.