Spent mushroom compost (SMC) largely accumulates after the end of mushroom harvesting and must be properly managed other than by incineration or landfill. This study sought to determine the potential use of SMC from the cultivation of Volvariella volvacea as a plant substrate due to its high nutrient content. V. volvacea is cultivated for 35–40 days using empty fruit bunches (EFB) fiber as substrate. The SMC is treated with hot water before being used for plant cultivation. Capsicum annum is also cultivated using two other substrates for comparison, black soil and coco peat for 75 days without fertilizer addition. The growth of the plant, such as height, number of leaves, and pH of the substrates, is measured. The changes in chemical bonding of SMC, black soil, and coco peat were analyzed by Fourier transform infrared spectroscopy (FTIR). From the results, black soil has the highest average plant height and plant weight, 12.6 cm and 4.0 g, respectively. In addition, SMC has the highest leaf number with an average of 7.2, while coco peat has the highest root length with an average of 12.22 cm. The composition of substrates, such as primary amine, alkane, and monosubstituted alkene, decreased significantly after the cultivation process, which was analyzed by FTIR analysis. Therefore, the use of SMC as a growing medium for plant cultures is reasonable because it provides sufficient nutrients for Capsicum annuum.