# Phosphorus Dynamics in the Soil-Plant-Environment Relationship in Cropping Systems: A Review
## 8 Alternative Sources of Phosphate Fertilizers of Potential Use in Agriculture  
Of the total amount of P applied into the soils, only a small part can be recovered in the first harvest, and this leaves a remnant of potential residual P, which is not bioavailable and can be used by the plants during the following seasons [124,125]. This is an important reason for the attention devoted to developing phosphoratized controlled-release fertilizers, as these can minimize the loss outside the application area and improve the agronomic use efficiency [40,41]. A new approach to face this challenge involves coating soluble P with an insoluble layer of different materials. This creates a physical barrier that can slowdown the release rate of P, reduce its leaching, and diminish other critical processes that degrade soils, such as the surface runoff. A number of natural and synthetic polymers have been studied, ranging from starch [126,127], cellulose [128,129], chitosan [129], and P acrylic acid acrylamide/kaolin [130]. Other types of materials have been tested as well, such as strvuite (MgNH4 PO4-6H2O), which is a mineral phosphate that can precipitate in wastewaters of treatment plants. In the last decade, struvite has gained interest as a method to recover P from wastewaters so it can be used as a source of P [131]. During the manipulation and recovery of this material, different processes may be applied, which can be classified into two groups. The first group uses magnesium (Mg), most frequently as MgCl2, and the pH is adjusted by using NaOH. Magnesium in form of water-soluble salt reacts quickly with P present in the wastewaters during the crystallization process, thus enabling the formation of a highly pure product  
Figure 3: Different sources of phosphoratized fertilizers in the market and their agronomical efficiency and environmental impact on the soil. (struvite). Authors [132] used alternatively MgO and Mg(OH)\({}_{2}\) as Mg source, which is more economical than any other Mg-based salt and limits the use of NaOH. These reactions are slower than those of other Mg salts, as they have to dissolve first, so this is normally added excessively in order to boost the formation of struvite, resulting into a product with an excess of MgO or Mg(OH)\({}_{2}\)[133; 134]. Experimental tests have been also performed in different crops with products derived from metallurgy in order to decrease the use of phosphoric rocks. Metallurgic industries of Ta and Nb produce a waste of acids with an acidity of 5.8 mol L\({}^{-1}\), a high content of S [135], and a pH of around 0 [136]. This type of acid source has been used to solubilize low-reactivity apatites, involving a lower price and lower-solubility P.  
The potential to improve the formulations of fertilizers by using nanomaterials has been explored as well [44]. Carbon-based materials are often applied in agriculture [137]. Their lower solubility makes them more profitable, environmentally friendly (less leachable), and chemically and physically advantageous. One of these materials is biochar. The release of carbon dioxide as one of the main greenhouse gases is considered a key negative effect associated with the burning of agricultural waste by anthropogenic activities. Such occurrence is avoided by carbonization of agricultural waste, producing biochar [42]. Biochar is the product of the thermochemical combustion of biomass (for example, corn cob, cocoa pod, and palm kernel husk) in a limited or no-oxygen environment [138]. Biochar has received increasing interest as an approach to address the challenges of global climate change and also to ensure sustainable agricultural development [139]. This material has been recently investigated to capture P and to recover wastewaters [140; 141]. An emerging C-based material recently discovered is graphene [142], p. 20. It has attracted widespread attention for a broad range of applications including the use of energy-related materials [143], medication-administration systems [144], sensors [145; 146], and membranes [147].