Adding humic acid to soil can improve soil structure and fertility, which can lead to better plant growth and higher crop yields. Extracting humic acid from compost is a sustainable and environmentally friendly way to obtain a valuable organic material. Humic acid (HA) can be extracted from compost relatively easily and at a low cost, making it an attractive option for farmers. In this study, we investigated the use of sugarcane bagasse (SB) and immature horse manure (IHM) as bulking agents for the composting of separated municipal solid waste (SMSW) and the extraction and characterization of humic acid from the mature composts produced. Fertilizing solutions containing different concentrations of humic acid were prepared and used to evaluate their effects on turnip crop growth and various biochemical parameters during cultivation. The results showed that the humic acid extracted from the composts had high yields and were rich in elemental carbon. The application of humic acid at both low and high concentrations resulted in a significant improvement in all the parameters measured except for the total protein in the roots, which did not differ significantly between the humic acid concentrations. The yield, root diameter and fresh weight increased significantly, and the leaf area was proportional to the humic acid concentration of the solution used. The highest increase in chlorophyll a content was observed in the treatment of humic acid extracted from composts C2 and C3 at a concentration of 0.1 gL-1, with an increase of 31% and 37%, respectively, compared to the control. The use of humic acid provided by co-compost can be considered a successful management strategy for degraded sandy soils and sustainable agriculture production in sandy poor soils worldwide.