Straw and coal gangue, primary wastes from agriculture and industry, respectively, have the potential to improve soil nutrients. The impact of plant species and microbial nutrient activation on this improvement warrants further investigation. A greenhouse experiment was conducted to examine how phosphorus-solubilizing bacteria (PSB) and mulch made from coal gangue and straw affect soil available phosphorus (AP), available silicon (ASi), and plant growth. These species include two economic crops, Gossypium hirsutum and Glycine max, and two plants used for mining remediation, Solanum nigrum and Medicago sativa. We found that the effects of mulching with coal gangue and straw on soil AP, ASi, and plant growth were influenced by plant species. The soil AP and ASi contents were significantly positively correlated in G. max and S. nigrum, and the mixed mulch significantly increased the soil AP and ASi contents for G. max and S. nigrum. The mixed mulch significantly increased the total biomass of G. max, with no significant effect on the biomass of other plants. These findings suggest that planting G. max could be an optimal strategy for improving soil with straw and coal gangue. The enhancement effects of the mixed mulch on soil AP and ASi of S. nigrum and the total biomass of G. max were negated by PSB, while PSB increased the soil AP and ASi contents of M. sativa with the mixed mulch. We demonstrated the necessity of planting suitable species and judiciously using microbial inoculants during the use of waste.