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

• Composted green waste (CGW) is inadequate as a potting medium for soilless culture.
• Biochar (BC) and humic acid (HA) were studied as amendments to improve CGW quality.
• BC and HA improved the physical, chemical, and microbial characteristics of CGW.
• BC and HA enhanced the growth of the ornamental plant Calathea insignis.
• BC and HA are useful amendments for the production of Calathea insignis in CGW.

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.