Knowledge about the effect of plant secondary compounds (PSCs) on belowground interactions in the more diffuse community of species living outside the rhizosphere is sparse compared to what we know about how PSCs affect aboveground interactions. We illustrate here that PSCs from foliar tissue, root exudates, and leaf litter effectively influence such belowground plant-plant, plant microorganism, and plant-soil invertebrate interactions. Climatic factors can induce PSC production and select for different plant chemical types. Climate change can therefore alter both quantitative and qualitative PSC production, and how these compounds move in the soil. This can change the soil chemical environments, with cascading effects on both the ecology and evolution of belowground species interactions, and ultimately soil functioning.