# Fertilization and Soil Microbial Community: A Review
## 1 Introduction  
Crop production is currently expanding globally due to an increased demand for food, animal feed and biofuels; the latter has been stimulated by the increase in oil prices making bioenergy crops more competitive and profitable compared to fossil fuels [1]. Currently, 47.9 million km\({}^{2}\) are devoted to agriculture, which is about 50% of habitable land [2]. Higher yields and better harvest quality can be achieved through the optimized use of fertilizers and the implementation of strategic production practices. Chemical fertilizers (also termed mineral, inorganic or synthetic fertilizers) contain a high concentration of a primary nutrient (nitrogen, N; potassium, K; phosphorous, P) as inorganic salts. Secondary elements (calcium, magnesium and sulfur) can also be added to soil by chemical fertilizers. Micronutrients (boron, manganese, iron, zinc, copper, molybdenum, cobalt and chlorine) [3] are in general absent in NPK chemical fertilizers and can be supplied by specific synthetic and expensive plant nutrients with soil or foliar applications [4]. The nutrient content in chemical fertilizers is indicated as the N:P:K rate, representing the percentages of nitrogen, total phosphorus (in the form of phosphorus pentoxide, P\({}_{2}\)O\({}_{5}\)) or total K (in the form of potassium oxide, K\({}_{2}\)O). If they also contain secondary elements, numbers in brackets specify calcium oxide (CaO), magnesium oxide (MgO), sodium oxide (Na\({}_{2}\)O) or sulfur trioxide (SO\({}_{3}\)) content. The majority of the inorganic fertilizers (with the exception of N) is extracted from rocks using physical or chemical processes. N fertilizers (mainly as ammonium--NH\({}_{4}\)and nitrate--NO\({}_{3}\)\({}^{-}\): urea, urea ammonium nitrate, ammonium nitrate and calcium ammonium nitrate) are produced by combining atmospheric N\({}_{2}\) with hydrogen (mainly from hydrocarbons such as natural gas--CH\({}_{4}\)) to obtain anhydrous ammonia (NH\({}_{3}\)), which can be used directly as a plant nutrient or converted into other different N fertilizers [5; 6]. Phosphate fertilizers (principally single superphosphate, triple superphosphate, monoammonium phosphate, di-ammonium phosphate and ammonium polyphosphate liquid) are extracted from natural phosphate rock deposits [7]. K fertilizers (potash muriate, KCl; potassium sulfate, K\({}_{2}\)SO\({}_{4}\); potassium nitrate, KNO\({}_{3}\); sulfate potash magnesa, K\({}_{2}\)SO\({}_{4}\).MgSO\({}_{4}\); kainite, KCl + NaCl + MgSO\({}_{4}\)) are produced by different chemical processes [8]. Differently, organic fertilizers (Table 1) are derived from plant- or animal-based materials or other organic constituents that are either a by- or end-product of naturally occurring processes, containing both the essential nutrients and micronutrients for plant growth. They also comprise biofertilizers (bacteria, algae, fungi or biological compounds), including plant-growth promoting bacteria [9; 10; 11]. Inorganic and organic fertilizers have an important role in increasing agricultural production, but the use of mineral fertilizers is constantly growing, with an estimated total 186.67 million tons in 2016 [12]. There is increasing concern regarding the negative environmental effects of chemical fertilizers. In fact, they can cause serious greenhouse gas (GHG) emissions and pollution of soil and water ecosystems. For example, synthetic nitrogen fertilizers have been recognized to be the most important factor contributing to direct N\({}_{2}\)O emissions into the atmosphere as a consequence of their biodegradation by soil microorganisms [13]. In addition, only 50-60% of synthetic nitrogen fertilizers added to soil is usually taken up by crops [8], and the rest runs off into water bodies (surface or groundwater [14]) due to their high dissolution properties.