Soil fertility is a multifactorial concept integrating physical, chemical, and biological aspects essential for plant nutrition and the functioning of agroecosystems. Microbial activity plays a central role in these processes by directly influencing nutrient cycling, soil structure, and plant health. Soil microbial communities, particularly plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF), interact directly or indirectly with plants, significantly contributing to the availability of essential nutrients including phosphorus, nitrogen, potassium, and micronutrients. Concurrently, edaphic factors, abiotic stresses, and agricultural practices affect the composition and functioning of the soil microbiota, potentially compromising soil fertility. Agricultural practices including crop rotation, organic fertilization, and the use of microbial biofertilizers, have proven effective in stimulating beneficial plant–microbe interactions, thereby enhancing agricultural system resilience. The growing demand for more sustainable and efficient agriculture highlights the fundamental importance of understanding plant-microbe interactions as key to the rational use of soil resources. This chapter presents a comprehensive review of the main plant-microorganism interactions, emphasizing their contribution to soil fertility and plant nutrition.