# Phosphorus Dynamics in the Soil-Plant-Environment Relationship in Cropping Systems: A Review
## 3 Characteristics of Soil P Pool Processes in the Soil  
Losses and limitations of P in soil are quite high as, on the one hand, low-solubility fertilizers (Thomas slag and phosphoric rock) and high-solubility fertilizers can be used, among which there are MAP, DAP, TSP, and H\({}_{3}\)PO\({}_{4}\) used in fertigation programs. On the other hand, processes such as adsorption, precipitation, and microbial immobilization can occur due to physico-chemical and microbial conditions of the soil [45]. Phosphorus can be found in two different forms in soil, either organic or inorganic, presenting a continuous liability and availability in the soil solution [46]. Phosphate sorption is determined mainly by iron and aluminum oxyhydroxides and occurs mainly within the low-crystallinity forms and with positive charges. This adsorption occurs at Lewis acid sites, where the -OH and -OH\({}_{2}\)\({}^{+}\) groups, mono- or tricoordinatedly bound to the metal (Fe and Al), are exchanged by the phosphate [47]. Thus, the equilibrium between adsorption and desorption is mainly related to soil pH [48]. Depending on the pH, P exists in the soil solution as orthophosphates (H\({}_{2}\)PO\({}_{4}\)\({}^{-}\) and HPO\({}_{4}\)\({}^{2-}\)). According to [9], the soils of the Yellow River delta in China showed a maximum P sorption rate (43.9 mg kg\({}^{-1}\), 45.9 mg kg\({}^{-1}\), and 40.5 mg kg\({}^{-1}\) in different types of soils) with optimal pH levels of 5.5, 6.2, and 5.2, respectively. According to [48], P sorption reaches its maximum value in the pH range between 5.0 and 7.0. This was assumed to be related to the presence of soluble forms of P, such as H\({}_{2}\)PO\({}_{4}\)\({}^{-}\) or HPO\({}_{4}\)\({}^{2-}\). combined with a positive charge of pH-dependent surfaces. At high pH values, P can precipitate with Ca, forming amorphous calcium phosphates, octocalcium phosphates, and apatiites (hydroxyapatitics or fluorapatites). This is again pH dependent, increasing as the pH increases [49]. Crop productivity has been estimated to decrease by as much as 40% with marked deficiencies of P in the soil [50], in Asia [51,52], America [53,54], Europe [55], Africa [56], and Oceania [57]. The P release in agricultural soils represents an important threat for water quality [58]. P losses can be due to runoff, leaching, and erosion in soluble forms (<0.45 mm) and particles (>0.45 mm). Normally, P particles are the main nutrient transported from cropped soils to waters, representing 80% of all carriers materials [59,60]. This is because of the selective erosion of P-rich particles caused by rainfall, splash, and cutting forces of water flow over the land surface according to the topography [61,62]. On the other hand, losses resulting from surface runoff (in dissolved particles and forms), subsurface flow (leaching and flow through the soil matrix and macropores), drainage flow, or even from groundwater can boost the eutrophication of the water [63,64]. Various studies have confirmed that this high risk of surface-water pollution arising from processes such as runoff during periods of high precipitations (rainfall regime) is linked to the high solubility of these materials [65,66,67]. Authors [68] found out that the P loss due to runoff was similar for different soluble mineral fertilizers such as MAP, DAP, and KH\({}_{2}\)PO\({}_{4}\). The high fixation capacity of P in most soils in the world and the low efficiency of the use of P-based fertilizers (around 10-15%) in most crops mean that an excess of P entry tends to accumulate in soils [69]. This accumulation can become a problem due to the different types of pollution it can generate, as mentioned above, and to the high cost of the phosphatrized fertilizers. In Europe and Oceania, for instance, there is an accumulation of phosphatrized fertilizers input in agricultural soils (ranging between 560 and 1.115 kg P ha\({}^{-1}\), respectively, for the period 1965-2007).