Phosphorus Fractionation in Soils Fertilized with Recycled Phosphorus Products

Phosphorus (P) will be exhausted, and its recovery and cycling will play a pivotal role in agricultural systems. Different technologies have been developed to recover P from waste materials, such as mono-incineration, acid digestion, and coprecipitation of P from wastewaters. The aim of this study was to assess the plant availability of P from different recycled P products by using a sequential chemical P fractionation of soil P and to study the fate of P applied to the soil. We evaluated the plant P availability in soils fertilized with eight recycled P products in comparison with two commercial fertilizers (triple superphosphate [TSP] and rock phosphate [Rock-P]) by means of a sequential chemical extraction of soil P and combined this data with previously published information for soil isotopically exchangeable P (IEP) and P uptake by plants (PU). We sequentially extracted inorganic P with alkaline extractants and an acid solution and analyzed the increase in P fractions caused by the application of different fertilizers. P extracted by alkali solutions was positively correlated with the IEP and with PU, whereas P extracted by the acid solution was negatively correlated with these parameters. In soils fertilized with recycled P products obtained by a chemical procedure (magnesium-ammonium-phosphate), soil P was extracted mainly by alkali extractants, while in soils fertilized with P products obtained by thermal procedures (i.e. sewage sludge ashes), soil P was extracted by the acid solution and was of low plant availability. Sequential chemical fractionation of soil P is suitable for characterizing the plant availability of P from recycled P products. The increase in the alkali soluble fraction in soil characterizes the portion of P available to plants, while the acid soluble fraction characterizes the recalcitrant portion. Phosphorus from fertilizers obtained by chemical procedures, like magnesium-ammonium-phosphate, is mainly extracted by alkali solutions and is comparable to water-soluble P fertilizers, while P from products obtained by thermal processes is mainly extracted by acid solutions and remains mostly unavailable to plants.