# Fertilization and Soil Microbial Community: A Review
## 5 Conclusions  
Soil is of critical importance thanks to its role in several ecosystem functions, including food production. The soil microbial community has a crucial role in these functions. It also has a key role in the availability and accessibility of nutrients to plants and exerts plant bioprotection. Soil biochemical, microbiological and biological properties have been used for several years for estimating alterations in soil quality. However, the effects on soil microbial community and consequences on soil fertility have not been adequately studied at different levels. At the EU level, there is not currently a regulation regarding soil, and it is advisable that the microbial community should be considered and preserved. Chemical fertilizers are sources of NPK nutrients in their inorganic forms and do not exert beneficial effects on soil physical properties (e.g., texture, structure, porosity, etc.) and SOM content. Absence of or leftover or incorrect addition of N, P and K fertilizers can affect the absorption and use of nutrients (including micronutrients), and because it negatively affects the beneficial plant rhizobacteria, it reduces crop yield and quality. Organic and synthetic fertilizers can affect microbial community compositions, favoring species functionally adapted to the nutrient inputs and activity and, ultimately, enhancing plant productivity. It is well known that chemical fertilizers can cause different environmental problems, including biodiversity loss, as highlighted by the new EU Soil Strategy for 2030, loss of SOM, deterioration of physical and chemical soil properties and, especially with long-term application, the lowering of soil fertility. Sustainable agriculture that limits chemical fertilization but also favors crop rotation, reduced soil tillage and extensified land use is an important global issue for preserving natural microbial communities (in terms of species abundance and richness of specific beneficial microorganism groups in soil), maintaining their ecological function and sustaining soil fertility. Because it is not reasonable to use only organic amendments to support plant productivity since they do not provide high amounts of NPK, a combined addition of chemical and organic fertilizers could be the right solution, particularly for soils with low N, P and organic C contents. In particular, the so-called 4 Rs for nutrient management, i.e., \"right source, right rate, right time and right place\" could be the correct path for farmers to manage fertilization of soil. Considering that, each basic element (N, P, K) has to be applied with a balanced fertilization by organic (including bio-organic) and synthetic fertilizers considering the soil needs; the role of micronutrients is also fundamental (for example in stimulating the availability of P). In any case, because the fertilization strategy is frequently related to economic rather than agronomic evaluations, N and, to a lesser extent, P are the only nutrients used and K fertilization is underexploited, so there is still a widespread unbalanced fertilization. A combined fertilization with chemical and organic fertilizers (in particular with compost, digestate or plant residues) could be a good compromise for providing basic elements together with micronutrients and improving soil organic carbon and increasing several physico-chemical soil properties. The new era in the use of plant-growth-promoting microorganisms can lead to a decrease in use of chemical fertilizers, although this practice still needs to better investigated and adapted to specific soils and plant cultivations. Finally, it has to be taken into account that the EU Fertilizing Products Regulation (Reg. 2019/1009 EU), which governs the production and addition to agroecosystems of organic and inorganic fertilizers, organo-mineral fertilizers, soil improvers, liming materials, plant biostimulants, inhibitors, and fertilizer products, will be implemented in 2022, and the fertilizing product quality certification process will be harmonized across the EU.