Open access peer-reviewed chapter
Abstract
Phosphorus (P) is a critical nutrient that underpins agricultural productivity and ecological balance, yet its management presents persistent environmental challenges. As a key macronutrient, P is indispensable in early plant establishment and stress responses, as well as plant growth, playing essential physiological roles, including energy transfer (ATP/ADP), nucleic acid synthesis, cellular signaling, root development, enzyme activation, photosynthesis, and other vital metabolic processes. However, its limited mobility in soil and tendency for overapplication in agricultural systems often results in inefficient use and increased risk of runoff into aquatic ecosystems. This runoff can accelerate eutrophication, leading to degradation of water quality and aquatic habitats. Unlike nitrogen (N), P lacks a gaseous phase, and its transport is governed by complex interactions with soil minerals, organic matter, and microbial processes. These interactions, along with landscape characteristics and hydrological patterns, make P losses subtle and difficult to control. This chapter examines the biogeochemical cycling of phosphorus in soil-water interfaces and highlights the multifaceted factors that influence its behavior. Emphasis is placed on emerging strategies for sustainable P management, including technological innovations, agronomic practices, and ecosystem-based approaches aimed at reducing environmental losses. In addition, the chapter addresses policy frameworks and incentive structures that can promote more effective P stewardship. By integrating scientific understanding with practical solutions, this chapter contributes to the broader effort of aligning agricultural productivity with environmental sustainability.
Keywords
- phosphorus organic fractions
- phosphorus use efficiency
- P retention
- P dynamics
- sustainable agriculture
1. Introduction
Phosphorus (P) is an essential macronutrient for plant growth and a key factor in agricultural productivity. It supports critical biological functions such as energy transfer (ATP), nucleic acid synthesis, and cellular signaling, among many others. However, its low mobility in soil often leads to inefficient use and over-application, increasing the risk of off-site transport and eutrophication of water bodies.
Unlike N, P lacks a gaseous phase and is retained primarily in soils or exported via particulate and dissolved forms [1]. Losses are typically gradual and influenced by soil characteristics, hydrology, landscape features, and management practices [2]. These challenges highlight the need to understand the complex interactions of P with soil minerals, organic matter, and microbial processes that regulate its cycling.
In soils, P exists as inorganic (Pi) and organic (Po) forms. While Pi includes mineral-bound and soluble phosphate, Po encompasses microbial biomass and organic residues. While most of the total soil phosphorus is not immediately plant-available, a significant portion can become available over time through microbial processes, weathering, and residual effects of fertilizer application. Among the factors influencing P availability, soil pH, mineralogy, and biological activity are particularly important, as they shape P speciation, solubility, and reactivity. Phosphorus sorption to iron–Fe and aluminum–Al (hydr)oxides and calcium–Ca compounds strongly governs its availability. Biologically, P cycling involves processes such as mineralization of Po, microbial uptake, and root exudation of phosphatases and organic acids that enhance P solubilization [3, 4].
Although Pi constitutes 60–80% of total P in agricultural soils, only a small fraction is present in soil solution as plant-available orthophosphate. These species are highly pH-dependent: H2PO4− dominates under acidic conditions, while HPO42− and PO43− become more prevalent in neutral to alkaline soils. Given these complex interactions, understanding the geochemical and biological processes that control P dynamics is essential for optimizing fertilization and minimizing environmental losses. This chapter reviews the biogeochemical cycling of P in soil–water systems, with emphasis on processes controlling its mobility and availability. It also highlights sustainable management strategies and policy considerations for improved P use efficiency (PUE) and environmental protection.
2. Biogeochemistry of organic phosphorus and its interactions with soil minerals
Phosphorus is a macronutrient for plant growth and a key element in many biogeochemical cycles. Although much attention is given to the inorganic chemistry of P, there are significant portions of Po in terrestrial and aquatic environments. These organic compounds are central components in soil fertility, aquatic ecosystems, and global nutrient cycles. They originate from diverse sources such as the decomposition of plant and microbial biomass, animal excreta, treated wastewater, biosolids, and other organic matter inputs across agricultural, urban, and natural ecosystems. Their environmental persistence, bioavailability, and interactions with mineral surfaces are governed more by their molecular structure and bonding (P species) than by their source. Therefore, understanding P forms and cycling is crucial for understanding ecosystem function, soil management, and developing strategies to reduce environmental impacts.
The term organic P encompasses a series of compounds containing P and some organic moiety (e.g., inositol phosphates, nucleic acids, phospholipids, phosphoproteins, phosphonic acid, and compounds associated with the humic fraction, among many others). Due to certain limitations in determining these forms, Po is often disregarded or ignored in surveys and studies. However, Po is found in relative abundance in terrestrial and aquatic environments, and its propensity to transform into bioavailable forms of P is one of the points that most emphasizes the importance of these forms.
The form or chemical species in which organic P is found determines its propensity for transformations and environmental dynamics [5, 6]. Organic P forms are commonly classified based on their dominant bond types, as well as their relative stability and abundance in soils [7]. However, while these classifications are chemically accurate, the environmental behavior of organic P compounds is strongly influenced not only by their molecular structure but also by interactions with soil minerals, humic substances, and microbial processes. Consequently, multiple subgroups emerge, each exhibiting varying degrees of lability and prevalence depending on both their chemical characteristics and environmental context. These organic forms primarily originate from the decomposition of plant and microbial biomass, animal waste, and organic matter inputs. However, their bioavailability and interactions with minerals are influenced by multiple environmental factors.
To contextualize, for inorganic P, sorption, dissolution, and precipitation processes are the major mechanisms that control P transformation in soils, which is mainly found in the form of orthophosphate. Some of the chemical behavior characteristics that make the management of orthophosphate challenging are its low solubility, relative immobility in soils, and its tendency to be fixed and become less bio-accessible.
The forms of Po differ from orthophosphate primarily by having a covalent bond with a carbon (C) atom. The conversion of Po into Pi generally requires breaking this covalent bond. The distinction between inorganic and Po can be clearly identified using well-established techniques. The use of spectrophotometry after the reaction of orthophosphates with an acidified molybdate reagent has been part of a procedure used for many decades for the determination of P. Whereas organic P is usually quantified by the difference between total P, typically measured after complete sample digestion and inorganic P.
As with most methods, there are limitations and challenges [8, 9, 10, 11, 12, 13, 14]. For example, while inorganic polyphosphates do not directly react with molybdate due to their molecular structure, they can undergo hydrolysis under the acidic conditions typical of molybdate-based assays. The widely used Murphy and Riley method (1962), in particular, accounts for this hydrolysis, enabling detection of hydrolyzed polyphosphates as inorganic P. However, the strong acids often used in these reagents (e.g., >0.5 M) may also hydrolyze certain forms of organic P, potentially leading to an overestimation of inorganic P. Another common issue is the discrepancy between P values obtained from the same extract when analyzed by colorimetry or by inductively coupled plasma spectrometry (ICP). In extracts where ICP perform P determination, the values tend to be higher than those obtained by colorimetry. Another point of concern is the organic matter content in the extracts. If they are not acidified to an appropriate level, these organic compounds can influence the results obtained by colorimetry. The presence of substances in the sample, such as sulfides, or even inputs like limestone and natural phosphates, can also interfere with the chemical extraction process and lead to inaccurate results.
2.1 Forms of organic phosphorus in soils, water, and sediments
Organic P comprises a diverse group of compounds in which P atoms are covalently bonded to C. These compounds represent an important pool of P in soils, sediments, and aquatic environments, particularly in systems where inputs of organic matter from plants and microorganisms are high. A widely accepted, although not entirely accurate, classification of Po compounds includes:
Phosphomonoesters: Compounds containing a single phosphate group linked to a carbon atom via a P–O–C bond, such as glucose-6-phosphate or inositol phosphates (e.g., phytate).
Phosphodiesters: Molecules with two organic groups bonded to a single phosphate atom, such as DNA and RNA.
Phosphonates: Compounds characterized by a direct C–P bond, which are generally more resistant to microbial degradation and often found in microbial metabolites and some pesticides.
Organic polyphosphates: Although rare in environmental matrices, these include biologically important molecules like adenosine triphosphate (ATP) and adenosine diphosphate (ADP), in which phosphate groups are linked together (P–O–P) and attached to organic moieties.
It is important to clarify that condensed phosphates such as pyrophosphate (P2O74−) and polyphosphates are inorganic phosphorus forms. These compounds are composed of multiple phosphate units connected by P–O–P bonds, but they do not contain carbon and thus do not meet the chemical definition of organic phosphorus. However, as previously mentioned, these forms may be associated with organo-mineral complexes or humic substances, making their classification and bioavailability more complex than the bond-based categories suggest. While molecules like ADP contain an organic backbone, the pyrophosphate group within them remains an inorganic structure. Their presence in organic molecules does not alter the fundamental classification of pyrophosphate itself as an inorganic phosphate species.
2.1.1 Transformations of organic phosphorus
The cycling of organic P in soil involves various transformations, which can be broadly divided into mineralization, immobilization, and mineral-organic P interactions. The mineralization process is basically the conversion of Po into Pi by the activity of phosphatase enzymes. Broadly speaking, these enzymes hydrolyze Po, making Pi available to plants and organisms. There is a great diversity of enzymes; some of them hydrolyze only diester phosphates into monoesters, called phosphodiesterases, and are essential for the process of P availability. The P mineralization rate is influenced by factors such as temperature, moisture, soil pH, and microbial activity. A notable example is phytase, an enzyme highly sensitive to pH and temperature conditions, which results in low activity in most soils and contributes to the high persistence of inositol phosphates. Mineralization through enzymatic activity is not the sole mechanism responsible for the conversion of Po into Pi in environmental matrices. There is strong evidence of the abiotic role of minerals, particularly Fe and Al (hydr)oxides, in facilitating various phosphorus transformation processes—including hydrolysis, sorption, and complexation.
The immobilization process is the opposite of mineralization, meaning the conversion of Pi into Po. This conversion occurs mainly through the action of microorganisms and absorption by plants. During periods of high microbial activity or nutrient cycling, Pi may be incorporated into microbial biomass or organic matter, resulting in temporary P immobilization, as these organic forms of P can later be mineralized back into inorganic forms under appropriate conditions.
During the evolution of ecosystems, the chemistry of Po changes significantly. Under aerobic conditions, the predominant Po species are phosphate monoesters, followed by phosphate diesters and phosphonates. This order of predominance is common even in environments with different parent materials, vegetation, and climates, as evidenced by studies on chronosequences. It is linked to the build-up of labile Po by microbial biomass and to mineral alterations that occur over time, such as those caused by continuous weathering. This is evident in the study by Turner et al. [15], who investigated a 120,000-year postglacial chronosequence in New Zealand and observed that as the soil aged, the proportion of inositol hexakisphosphate (a recalcitrant form) decreased, coinciding with the crystallization of amorphous metal oxides. At the same time, more labile forms, such as DNA, increased, likely due to their incorporation into protective organic structures.
2.1.2 Role of soil minerals as catalysts in phosphorus cycling
Soil minerals play a pivotal role in P cycling, influencing both adsorption-desorption dynamics and the hydrolysis of Po. The surface properties of minerals, including charge, surface area, and reactivity, govern interactions at the solid-solution interface, where most P transformations occur.
Surface area, which depends on particle size and physical structure, ranges from <1 m2 g−1 in minimally weathered minerals to over 800 m2 g−1 in some phyllosilicates. Surface charge is either permanent (originating from isomorphic substitutions) or variable (originating from incomplete coordination at mineral edges). Variable charges can be influenced by environmental factors such as pH changes or organic matter additions, whereas permanent charges remain largely unaffected.
Variable charges, or pH-dependent charges, are more prevalent in highly weathered minerals such as (hydr)oxides of Fe and Al. Because of this, soils in tropical regions tend to exhibit a positive total charge, especially under conditions of high acidity (low pH), while soils in temperate regions, with a higher abundance of 2:1 minerals, have greater amounts of negative charges and a pH closer to neutrality. The net charge of a soil is given by the sum of its positive and negative charges at a given pH, and it is a fundamental attribute for understanding soil chemistry. Variable charges have several origins, including the edges and corners of crystals, as well as fractures and imperfections where charge imbalances are generated. Processes such as proton adsorption and desorption are also generators of variable charges, both on the surface of minerals and on organic matter, whose surface is rich in functional groups.
For Fe and Al (hydr)oxides, the presence of permanent charges is very low, and variable charges are abundant, especially at the edges of crystals, where Fe3+ atoms have incomplete coordination. The subsurface of these (hydr)oxides is filled with H+ or OH− ions, which play a key role in exchange reactions.
Among the oxyanions present in the soil, phosphate stands out for being highly stable and reactive, as three of its four oxygen atoms can react with colloidal surfaces. The main inorganic phosphate forms found in solution are HPO42− and H2PO4−, which predominantly bind to mineral surfaces through inner-sphere complexes, meaning they are strongly adsorbed. In some Po molecules, such as inositol phosphates, adsorption occurs in a slightly similar manner to inorganic phosphate since, despite the central region of the molecule containing organic components, its periphery is surrounded by phosphate ions, which react with the surface of Fe and Al oxides. However, due to its distinct molecular nature, the strength, form, and kinetics of adsorption can vary significantly.
Due to the strong adsorption of Po onto Fe and Al (hydr)oxides [16, 17], these minerals are far more determinant for mineralization than aluminosilicates (e.g., feldspars, micas, illite, and kaolinite). Basinski and Aristilde [18] demonstrated that the catalytic hydrolysis of phosphate esters by goethite is exceptionally high (up to 50%) compared to quartz and kaolinite. On the other hand, the persistence of phosphomonoester Po, such as phytate, is often attributed to its limited transformation potential compared to other multiphosphorylated compounds. These transformations include, but are not limited to, hydrolysis (e.g., dephosphorylation), sorption–desorption dynamics, and complexation reactions.
2.2 Phosphorus cycling in agroecosystems
2.2.1 Crop phosphorus dynamics
Phosphorus dynamics in agroecosystems are regulated by multiple factors, including the type of soil, crop plants, and management adopted. Only when we consider these factors collectively do we get a better idea of the dynamics of P in the system, especially its availability and fate.
Figure 1 shows a simplified view of the P cycle in agricultural systems, considering its inputs, outputs, and transformations. The main inputs of P in agricultural systems occur through the addition of fertilizers, plant and animal residues, and, to a lesser extent, atmospheric deposition. On the other hand, outputs occur mainly through crop removal, erosion, and, to a lesser extent, leaching, as P is highly fixed in the soil. The transformations of P in the soil are diverse, involving processes of mineralization and immobilization, adsorption and desorption on mineral and organic surfaces, as well as dissolution or precipitation with other ions.
Figure 1.
Soil phosphorus cycle in agricultural environments.
For phosphate ions, in their HPO42− and H2PO4− forms, to be absorbed, they must be available in solution. After the solubilization of phosphate fertilizers, these ions can be readily absorbed by plants or undergo adsorption onto colloids. The decomposition of organic residues, in which a significant portion of P is concentrated in organic form, as well as the desorption of labile P forms that are weakly adsorbed, are the main short-term sources of available P in solution. The process of P adsorption in the soil is complex, as discussed in this chapter, and depends on various factors involving the adsorption surface and the surrounding physicochemical environment. In this context, time is an important factor to consider, as P bonds with minerals “age,” they become stronger (occluded P), and their likelihood of reversal decreases.
The P fixation capacity is an intrinsic factor of soil nature and a key determinant in selecting the best management strategy for a given area. In general, clayey soils have a high P fixation capacity due to their higher clay content compared to sandy soils. Consequently, the “building up” of high P levels in the soil takes longer, as a larger portion is converted into non-labile forms. Conversely, in sandy soils, P retention is usually lower, resulting in higher efficiency of P utilization over time.
Figure 2 illustrates the spatial variation in available soil P levels (extracted with anion-exchange resin) on a farm of 3322 ha in Brazil. This farm cultivates soybeans in the summer, followed by either corn or cover crops in the second growing season. Figure 2 is based on 665 soil samples collected in September, after the completion of the second 2024 harvest. The data show that in areas with cover crops (Brachiariaintercropped with Sunn Hemp), P levels remained higher, likely due to enhanced nutrient cycling. In contrast, areas where corn was grown had lower P levels, probably due to nutrient removal through grain harvest. From Figure 2, it can be inferred that the management practices used, particularly the choice of plants in the system, influence P availability and can serve as an additional criterion for determining the optimal fertilizer dose and source for the subsequent crop.
Figure 2.
Spatial variation of phosphorus levels along the plots of a farm in the state of Mato Grosso, Brazil.
In addition to soil characteristics, the plants chosen for the system are fundamentally important in accurately determining the best management of phosphate fertilizers. One key aspect to consider is the average P removal by crops. In intensive systems, where two or even three harvests occur within a year, adequate P fertilization is essential to meet crop demands and prevent the system from developing a negative nutrient balance.
3. Sustainable phosphorus management in agriculture
The need for phosphorus stewardship to balance productivity and environmental health: Phosphorus stewardship in agriculture involves optimizing its use to achieve high crop yields without compromising environmental quality.
Phosphorus use efficiency and environmental considerations in agricultural systems: Eutrophication is a naturally occurring, geologic process in which aquatic ecosystems gradually accumulate nutrients over long timescales, eventually supporting increased biological productivity. However, in modern times, this process has been significantly accelerated by anthropogenic inputs, particularly agricultural fertilizers, leading to harmful ecological consequences. In its accelerated form, eutrophication contributes to algal blooms, hypoxia, and loss of biodiversity in freshwater and coastal systems. Among the nutrients responsible, P plays a pivotal role due to its strong affinity for soil particles and tendency to accumulate in agricultural landscapes over time.
Phosphorus utilization in agricultural systems is often constrained by low uptake efficiency during a single growing season. Typically, less than 20% of the applied P fertilizer is absorbed by crops in the year of application [19, 20]. This apparent inefficiency, however, does not equate to loss or disappearance. Instead, the majority of applied P becomes part of the soil phosphorus pool, where it can remain available—though variably so—for uptake by plants in subsequent years. This residual P continues to contribute to crop productivity well beyond the first season.
Long-term studies have underscored the importance of accounting for cumulative P use. A 39-year field experiment in Saskatchewan, Canada, demonstrated that wheat crops eventually recovered nearly 100% of the applied P fertilizer [21]. Similarly, extensive research at Rothamsted, UK, showed that when soil P levels were managed to remain within optimal agronomic ranges, cumulative phosphorus use efficiency (PUE)—assessed through nutrient balance methods—could exceed 90% over multiple decades [22]. These findings challenge the common perception of P use as inefficient and instead emphasize the value of residual P in supporting sustainable productivity across cropping cycles.
Residual P is particularly important in systems that employ best management practices such as crop rotation, conservation tillage, and cover cropping, all of which can enhance P cycling and availability. However, as soil P accumulates—especially in systems with repeated fertilizer application or manure inputs—it can exceed the soil’s sorption capacity, a threshold known as the degree of phosphorus saturation. When this threshold is surpassed, the risk of P loss through surface runoff or leaching increases substantially [23], particularly in sandy soils, poorly drained fields, or areas with shallow water tables. These losses can contribute to nutrient loading in adjacent water bodies, thereby accelerating the eutrophication process beyond its natural geological pace.
Thus, while residual P contributes to long-term soil fertility, its mismanagement poses environmental risks. The challenge lies in balancing agronomic benefits with environmental stewardship—a task that requires transitioning from strategies focused solely on short-term crop response to those enhancing long-term PUE. This shift calls for tools and practices that include regular soil testing, site-specific nutrient management, and the integration of mineral fertilizers with organic amendments. Additionally, understanding the interactions between soil type, hydrology, and landscape features is crucial for developing effective nutrient management plans that mitigate P losses while maintaining or improving crop productivity.
Ultimately, reconciling the dual roles of phosphorus—as both an essential nutrient and a potential pollutant—demands a holistic, systems-based approach. Agricultural systems must be managed not only for yield optimization but also for their downstream impacts, ensuring that phosphorus supports food security without undermining water quality and ecosystem integrity.
3.1 Precision fertilization and soil amendments: Enhancing phosphorus use efficiency
Phosphorus availability in soil is often restricted due to strong fixation processes, especially in highly weathered or calcareous soils. As a result, PUE in the first season after fertilizer application remains a significant challenge in global agriculture, with typically less than 20% of applied P being directly utilized by crops [19, 20]. Nevertheless, P not taken up in the first year often contributes to a residual pool that remains available to crops in subsequent seasons [21, 22]. Addressing both the immediate and long-term dynamics of P use is vital for enhancing crop productivity, minimizing environmental impacts, and ensuring sustainable agricultural practices. Improved management strategies can optimize both short-term uptake and the efficient use of residual soil P over time. Finally, each situation must be assessed individually to manage the fertilization method most efficiently, reducing environmental consequences such as runoff and eutrophication.
Precision fertilization, complemented by appropriate soil amendments, presents a promising method for improving PUE by optimizing nutrient placement, timing, and formulation to align with crop requirements and soil characteristics. Advances in soil testing, remote sensing, and data-driven decision-making enable farmers to apply P more effectively, not only decreasing losses due to leaching, runoff, or fixation but also boosting farmers’ net income.
This topic examines key strategies for enhancing PUE through precision fertilization and soil amendment. It addresses site-specific P management, innovative fertilizer technologies, and soil conditioning practices that foster sustainable P usage. By combining precision agriculture with customized soil amendments, farmers can optimize P inputs, improve nutrient cycling, and encourage resilient agroecosystems across diverse soil conditions.
3.1.1 Enhancing phosphorus use efficiency through targeted fertilization
Simply put, PUE refers to the ability of plants to extract and utilize P from the soil effectively. Maximizing PUE is crucial for sustainable agriculture, as it minimizes the need for excessive P inputs, reduces costs, and mitigates negative environmental impacts. Precision agriculture technologies are key to increasing PUE, especially those involving variable rate or site-specific application.
The first step in implementing precision techniques in a field is understanding its spatial variability. To achieve this, local soil fertility surveys are essential. Initially, soil samples are collected (using grid sampling or management zones) and analyzed. The results are then processed using geospatial tools to generate nutrient distribution maps. These interpolations can be performed using inverse distance weighting or kriging equations (Figure 3). Based on fertility maps, phosphate fertilizer recommendation maps can be created, adjusting application rates according to the variability found on the farm. This approach reduces fertilizer usage and minimizes environmental pollution caused by excess application.
Figure 3.
Maps of the spatial distribution of available phosphorus in the soil as a function of interpolation methods in an area sampled on a 5-ha grid: (A) inverse weighted distance, (B) kriging.
In a study conducted by Wittry and Mallarino [24], which compared variable-rate P fertilization with fixed-rate application, the technology significantly improved PUE. It led to a 12–41% reduction in applied doses and decreased spatial variability in subsequent mappings. Usually, the greater the spatial variability, the higher the savings are achieved with variable-rate application. In this context, sampling in small grids (1 to 3 ha) allows for more precise identification of deficiency patterns and their correction through fertilization. However, simply adjusting the dose is only one aspect of improving PUE; it is also essential to establish well-defined criteria for determining these doses.
The effect of P application, monitored through annual grid sampling, can be observed in Figure 4. The area in question is in the northern region of Paraná sate, Brazil, and is cultivated with soybeans in the summer and cover crops in the second growing season. Figure 4A and B highlight the changes in the spatial distribution patterns of P after one and a half years. The doses applied in 2023 (Figure 4C) had a positive impact, increasing the field’s average P content from 46 mg dm−3 to 54 mg dm−3 (extracted with anion exchange resin). In this specific case, in addition to the applied doses, the presence of cover crops in the second growing season may have contributed to the increase in P availability.
Figure 4.
Maps of the spatial distribution of available phosphorus in two years (A and B), and P2O5 variable rate recommendation map (C).
Just as determining the criteria for dose and source, it is important to define the method and timing of phosphate fertilizer application. In Brazil, broadcast application of P has become popular in the Cerrado region due to time savings and operational ease. However, in areas with medium or low P levels, localized fertilization in the planting row is still more recommended, as it keeps the fertilizer granules closer to the roots and prevents losses, both from surface runoff and fixation in the clay fraction of the topsoil. In areas with high P levels, when broadcast application is chosen, it is important to consider its chronological proximity to lime application, since the topsoil may have a pH > 7.0 and a high presence of Ca, facilitating the formation of calcium phosphate complexes and making P unavailable.
3.1.2 Soil amendments and their role in phosphorus availability
Soil amendments are materials added to soils to improve their physical, chemical, and/or biological properties. Several amendments can influence P availability by altering soil pH, enhancing microbial activity, or interacting with P to prevent fixation or enhance mineralization. Some of the most used soil amendments concerning P availability include lime, phosphogypsum, and organic materials such as poultry litter, organomineral fertilizers, and biochar.
3.1.2.1 Lime
Soil acidity is one of the primary factors that influence P availability. In acidic soils, P tends to react with Fe and Al (hydr)oxides, forming insoluble complexes that are not readily available to plants. Perhaps the amendment whose relationship with P availability has been most studied is lime. This input is extremely important in regions where soil acidity is a problem, as it not only neutralizes the Al3+ toxic to plants but also raises the pH through the reaction of its carbonate ions while simultaneously supplying calcium (Ca2+) and magnesium (Mg2+). As reported in numerous studies, increasing the pHH2O to values between 6.0 and 6.5 enhances P availability, as within this pH range, the processes of adsorption and precipitation of P occur with lower intensity. Lime also enhances microbial activity, which can aid in the mineralization of Po compounds, further improving P bioavailability. There are also reports of reduced P loss through erosion due to liming. However, according to the authors, the initial P status in the soil is a more important factor than the use of limestone itself in determining P losses. While liming generally enhances P availability and modifies cycling dynamics, it is essential to consider the specific soil context and management practices to optimize these benefits.
The effects of limestone use on the biogeochemical dynamics of P have been reported in some studies. Among them, Bouray et al. [25] demonstrated that limestone application promoted microbial P immobilization and reduced phosphomonoesterase activity in the rhizospheres of Lupinus angustifolius and Lupinus polyphyllus, leading to the accumulation of organic P (Po). Bouray et al. [25] also found that carboxylate exudation and P distribution in the rhizosphere can also be influenced by liming.
3.1.2.2 Gypsum or phosphogypsum (PG)
It is a byproduct of the phosphate fertilizer industry, consisting mainly of hydrated calcium sulfate. This input has gained prominence in various regions where subsurface acidity is a problem since, due to its significantly higher solubility compared to lime, it can reach deeper soil layers, carrying sulfate ions that can react with Al3+, without changes in pH. In addition to supplying S, PG is an excellent source of Ca2+ for plants, and its use has become well established not only for these benefits but also due to its low cost and the fact that it does not require incorporation into the soil. In terms of P cycling, PG can influence P availability by promoting the formation of Ca-phosphate minerals in alkaline soils. However, despite containing approximately 1% P in its composition, the continuous use of PG can increase available P levels over time.
The use of lime combined with PG over time is a beneficial practice for Oxisols, as it not only increases crop yields, such as soybeans, but can also enhance Po forms analyzed by 31P-NMR and the activity of enzymes involved in the P cycle, as observed by Ref. [26].
While PG can improve soil fertility, improper management may lead to environmental issues, including heavy metal leaching into water bodies [27]. High rates of PG can also promote imbalances between cations in the soil, especially in soils poor in Mg2+ and potassium (K+), leading to deficiency issues due to ionic competition. Therefore, it is essential to consider sustainability aspects for the rational use of PG, its long-term effects on soil health, and potential environmental risks while enhancing P cycling in agricultural systems.
3.1.2.3 Biochar
Biochar is a carbon-rich material produced by the pyrolysis of organic matter. When applied to soils, biochar has been shown to improve soil structure, enhance water retention, and increase nutrient availability. Biochar can also influence P cycling by adsorbing P and preventing its leaching or fixation. The surface of biochar particles contains functional groups that can adsorb P, particularly under conditions where P is prone to fixation, maintaining more bioavailable P. Furthermore, biochar can modify the microbial community in soils, promoting P-mineralizing microbes that release Pi from organic sources.
One of the promising improvements that has been made to biochar is its modification with metals, which increases the P retention capacity of this amendment. A good example of this improvement was found in the study by Ref. [28], in which metal-modified biochars achieved P adsorption capacities ranging from 1.06 to 25.6 mg g−1, with Ca-modified biochar being the most effective.
The use of biochar also has marked effects on the structures of the soil microbial communities, stimulating the P cycling by increasing the populations of P-solubilizing bacteria. These changes, added to the increase in pH, increase the mobilization of P and its bioavailability [29, 30]. Although biochar has the potential to improve P cycling, issues with its recyclability and the effectiveness of P desorption following adsorption procedures still exist.
3.1.2.4 Poultry litter
Poultry litter is a byproduct of poultry farming rich in P, which plays a significant role in nutrient cycling within agricultural systems. Poultry litter contains both Pi and Po, indicated that approximately 80.2% of the total P in poultry litter exists as Pi [31]. The predominant inorganic forms include orthophosphate and pyrophosphate, whereas Po mainly consists of orthophosphate monoesters and diesters. The mineralization process varies significantly among different types of poultry litter. For instance, wood chip litter demonstrated a high mineralization rate of 82%, while Napier grass litter showed only 4% mineralization over a 43-day incubation period [31]. This variability highlights the importance of bedding materials used in poultry production on P cycling.
When applied based on N recommendations, excess P accumulates in soils, increasing the risk of runoff into water bodies and contributing to eutrophication. This risk is exacerbated in regions with high rainfall and topography conducive to runoff and also in systems with high rates of poultry litter application [32]. Even in Oxisols, with low natural P contents, but high fixation capacity, repeated applications of poultry litter can overcome soil P saturation capacity and lead to significant P accumulation [33], raising concerns about potential water quality issues from leaching and runoff. Also, the solubility of P is influenced by both the chemical composition of the litter and the soil environment, necessitating careful management practices to mitigate environmental risks.
3.1.2.5 Enhanced efficiency P-fertilizers (EEFs)
New formulations of P fertilizers, including slow-release and controlled-release products, can improve P availability over extended periods, reducing the risk of P fixation and losses. These fertilizers release P gradually, matching the crop uptake rates, thus enhancing PUE. Among the various technological approaches employed in EEFs, three groups stand out: (i) slow releasers; (ii) blockers; and (iii) Inducers. The first group (slow releasers) consists of fertilizers that may have coatings, scaffolds, organic matrices, or minerals of limited solubility as means to reduce the contact of P with the soil, thereby limiting the contact surface and reaction time. The second group (blockers) generally contains humic substances or maleic-itaconic polymers to disrupt P precipitation. It is envisioned that the addition of these carbon-rich and negatively charged compounds could block the fixation of the anion caused by antagonistic cations. The third group (inducers) uses a different approach, in which the stimulation of natural biochemical processes becomes the main strategy. These compounds are often nanomaterials without P in their composition, such as oxides; however, their mechanism of action has not yet been fully elucidated.
3.1.2.6 Organomineral fertilizers (OMFs)
These fertilizers can combine organic matter, biochar, and microorganisms with mineral fertilizers to sustainably enhance soil fertility and productivity. Unlike synthetic fertilizers, OMFs can improve soil health over multiple crop cycles by boosting microbial activity and organic matter accumulation. Technological advances like hydrothermal liquefaction have improved production efficiency and carbon sequestration potential. Because they are produced from organic materials, OMFs tend to have low concentrations of micronutrients in their composition, which can be beneficial in areas where plants show micronutrient deficiencies. Additionally, the organic component can help reduce P losses, thereby decreasing the risk of water eutrophication. Many OMFs also contain microorganisms that stimulate biochemical reactions in plant roots, promoting better plant development. The study by Ref. [34] highlights the potential of OMFs as valuable tools in the pursuit of more sustainable agriculture. Beyond technological aspects, the authors emphasize the environmental and social impacts of using these sources, as well as the need for standardized parameters and regulations for OMF production methods.
3.1.3 Integrating biogeochemical knowledge into phosphorus application
To optimize P application strategies, it is essential to integrate biogeochemical knowledge of P cycling into the design and implementation of fertilization and soil amendment practices. A more thorough understanding of the interactions between P and soil minerals, microbial processes, and environmental conditions can lead to more targeted and efficient P management practices. Some key considerations include:
Soil-phosphorus-mineral interactions: As discussed previously, the forms and availability of P are influenced by its interactions with soil minerals. Understanding how P is adsorbed, desorbed, and immobilized by minerals can help guide fertilizer application decisions. For example, in soils with high levels of Fe and Al oxides, applying P in a banded fashion or using enhanced efficiency fertilizers can prevent P from being fixed into insoluble forms and improve uptake by plants.
Microbial contributions to phosphorus cycling: Microbial communities in soil play a crucial role in P cycling, particularly in the mineralization of organic P. Encouraging microbial activity through practices such as the addition of organic matter (compost, cover crops), biochar or OMFs can help release P from organic forms and enhance its bioavailability. The use of microbial inoculants or P-solubilizing bacteria may further improve P availability, especially in soils with low P levels. In this scenario, monitoring phosphatase enzyme activities is of interest, as it may be possible to build a historical record and make more informed fertilization decisions.
Environmental considerations: Precision fertilization and soil amendments can also play a role in mitigating the environmental impacts of P runoff and eutrophication. By applying P more precisely and using amendments that reduce P mobility, it is possible to limit P losses to water bodies. Additionally, integrated nutrient management approaches that combine P with other nutrients, like N and K, can help optimize overall nutrient use efficiency and reduce the environmental footprint of fertilization.
Antagonism: One of the important parameters to be considered in a fertilization program is the availability of micronutrients, among which Zn is the most likely to be limited by the excessive addition of P. Given this, it is essential to assess the nutrient levels in both the soil and plant tissues to prevent antagonistic reactions that could lead to detrimental effects.
Cover crops and intercrop: There are plants with a high potential for P cycling, such as oats, lupin, buckwheat, crotalaria, and brachiarias. Therefore, in agricultural systems where crop rotation with cover crops is prioritized, or where an intercropping model is employed, it is important to estimate the amount of P that will be cycled. In some cases, P input can be reduced depending on the cover crops used, thereby improving the overall efficiency of the system.
3.2 Strategies to reduce phosphorus runoff and losses
To mitigate P losses from agricultural fields, a suite of best management practices (BMPs) has been developed. These strategies aim to enhance P retention in soil, reduce erosion, and minimize surface and subsurface transport.
3.2.1 Best management practices (BMPs) for phosphorus retention
BMPs encompass both agronomic and structural interventions. Agronomic practices include balanced fertilization, use of slow-release P fertilizers, the incorporation of fertilizers into the soil, and timing applications to coincide with crop uptake. Structural practices involve the installation of sediment control structures, constructed wetlands, and vegetated filter strips.
3.2.2 Role of conservation tillage, cover crops, and buffer strips
Conservation tillage reduces soil disturbance, thereby maintaining soil structure and organic matter content—key factors in P binding and retention [35]. No-till systems minimize erosion and surface runoff. However, they may increase P stratification at the soil surface, requiring careful management.
Cover crops provide ground cover during non-growing seasons, reducing erosion and enhancing soil structure. Certain cover crops, like legumes and brassicas, can also mobilize bound P through root exudates and microbial interactions [36].
Buffer strips, especially those vegetated with perennial grasses, act as physical barriers that intercept P-enriched runoff [37], promote sediment deposition, and enhance P uptake by vegetation.
3.2.3 Soil structure and hydrological controls on phosphorus mobility
Soil physical properties—such as texture, aggregate stability, and porosity—play a pivotal role in P retention and transport. Compacted or poorly structured soils with preferential flow paths can facilitate rapid P movement to drainage systems.
Hydrological processes, including rainfall intensity, antecedent moisture conditions, and connectivity to surface waters, dictate the timing and magnitude of P losses. Subsurface drainage systems, while beneficial for water removal, may serve as conduits for dissolved P transport if not properly managed [38].
3.3 Phosphorus recovery and recycling approaches
3.3.1 Circular economic strategies for phosphorus sustainability
The primary driver of the global P cycle is farming, thus finding sustainable ways to increase P usage across the food chain and, particularly, within agricultural systems is necessary to address this social issue. It is anticipated that between 2030 and 2040, the demand for commercially viable P reserves would surpass the supply, resulting in a scarcity of P and the emergence of the P reserve crisis [39]. Phosphorous is an essential element in our food chain with no known substitute. Phosphate rock, the primary source of P, is a finite resource found in only a few countries (China, USA, Morocco). These limitations pose various challenges on resources and food insecurity. Therefore, research interest in recovering P from different waste sources has grown. Establishing a circular economy for P can promote more sustainable agricultural practices by closing the P cycle and encouraging responsible P management at various scales. Ref. [40] suggested more sustainable 5R (1R: realign P inputs, 2R: reduce P losses, 3R: recycle P in bio-resources, 4R: recover P in wastes, and 5R: redefine P in food systems) strategy for P management. Nesme and Withers [41] proposed the creation of a framework to monitor a five-year global P facility. This framework would aim to raise awareness, set minimum standard and performance benchmarks, offer guidance on best practices for enhancing P use efficiency, and recommend appropriate economic instruments to drive change. Such a governance framework would support the development of a circular P economy grounded in green principles, innovative technologies, integrated policy, and innovative technologies.
3.3.2 Sources of phosphorus from different waste
The demand of P from phosphate rock in agriculture has been raised by 3–4% per year from 1993 [42]. According to the latest summary by the U.S Geological Survey (2021), global phosphate rock reserves are estimated at 71 × 103 Tg, while 223 Tg of phosphate rock was mined in 2020. Therefore, P recovery and recycling from waste sources such as wastewater, biosolids, and manure is crucial due to the dual challenge of depleting phosphate rock reserves and excessive P accumulation in surface waters. This imbalance not only threatens long-term P availability for agriculture but also drives environmental issues like eutrophication, making efficient recovery systems essential to restore balance in the disrupted P cycle. Wastewater and its solid fractions account for about 15% of total imported P, making them valuable targets for nutrient recovery. Phosphorus can be recovered at wastewater treatment plants from various sources, including supernatant, sludge, and ash [43]. Alongside alternative sources like manure and industrial by-products (like biosolids), these recovery processes contribute to developing a more circular P economy, though several technical and operational challenges remain.
Biosolids, the semi-solid byproducts of wastewater treatment, are produced through processes like digestion, stabilization, thickening, dewatering, and drying, resulting in pathogen-free granules suitable for land application. Around 90% of the total P present in wastewater can be captured within activated sludge. The P concentration in sewage typically ranges from 1 to 3 wt%, while its ash contains between 5 and 10 wt% P (equivalent to 11–23 wt%) [44]. These biosolids are commonly used as soil amendments in agricultural fields where they improve soil organic matter and moisture content and supply essential nutrients such as N and P.
Regarding P availability, in biosolids, P occurs in both organic and inorganic forms, with inorganic P comprising the majority, typically between 70% and 90% of the total. These inorganic forms include compounds such as aluminum phosphate, iron-bound P (e.g., adsorbed onto ferric hydroxo-phosphate surfaces), and calcium-based minerals like hydroxyapatite or tricalcium phosphate [45]. Organic P exists primarily as orthophosphate monoesters, diesters, phytates, phosphonates, and phospholipids. The availability of P to plants is largely governed by the transformation and release of labile P from these pools into the soil solution. The chemical form of P in biosolids strongly influences both its agronomic effectiveness and environmental behavior. For instance, Fe- and Al-bound P forms are relatively stable and poorly soluble, limiting plant uptake but reducing leaching risk. In contrast, Ca- and Mg-bound P forms, especially under neutral to alkaline conditions, may be more soluble and thus more readily available to plants, but also more susceptible to loss through runoff or leaching if not managed properly. Understanding these speciation dynamics is essential for optimizing biosolid application strategies to balance crop nutrition with environmental protection. Studies have shown that biosolid applications significantly increase plant-available P in soils, especially in P-deficient areas. Field experiments with wheat have demonstrated that using biosolids can achieve crop yields comparable to those produced with conventional fertilizers [46].
Animal manure is a byproduct of undigested food, rich in organic matter, N compounds, and P (primarily in the form of phytic acid). However, it also contains harmful substances like hormones, antibiotics, and pathogens, limiting its direct application as fertilizer. Poultry litter, which combines manure with feed residues, feathers, and water, provides valuable nutrients but in unbalanced N/P ratios unsuitable for efficient fertilization. Additionally, the direct use of manure can lead to nutrient runoff, water pollution, and soil aeration issues, while its slurry form is costly to transport over long distances, with its composition varying by animal type, diet, water intake, and climate [47].
3.3.3 Advances in phosphorus recovery technologies
3.3.3.1 Wastewater streams
Phosphorous recovery from wastewater can be achieved from liquids, slurries, or mixtures like secondary treated effluent, digester supernatant, sewage sludge, and its ashes. The major challenges for efficient recovery include varying P concentrations and forms, pollutant presence, and achieving high product quality. Most P recovery technologies rely on precipitation and crystallization, where Mg or Ca compounds are added to form struvite (MgNH4PO4·6H2O) or calcium phosphate like brushite and hydroxyapatite. Though crystallization offers high-quality crystalline products, it proceeds more slowly than precipitation.
Struvite crystallization efficiency is influenced by pH, temperature, Mg:N:P ratios (ideally 1:1:1), and the presence of foreign ions or organic matter. Besides fluidized and stirred reactors, newer options include ion exchange, anaerobic membrane bioreactors, adsorption, electrodialysis, forward osmosis, and constructed wetlands [47]. Direct struvite recovery after biological treatment can achieve 10–50%. The highest efficiencies (85–95%) are possible when recovering P from P-rich ashes. Though struvite technology is proven worldwide, challenges remain in improving P-removal rates, reducing costs, optimizing Mg dosing, enhancing struvite quality, and developing dynamic process models over purely thermodynamic ones.
3.3.3.2 Biosolids
Phosphorous recovery from biosolids is essential for resource conservation and environmental protection. Different approaches exist, each with unique advantages and limitations. The P recovery without chemical treatment uses materials like compost, stabilized sludge, manure, digestate from biogas, and sludge ashes from mono-incineration. These materials are directly applied to farmland, which is inexpensive although problematic due to risks from heavy metals, antibiotic residues, and pathogens. On the other hand, P recoveries with precipitation techniques use Al, Fe, Ca, and Mg from biowastes. P recovery from the biowaste using precipitation techniques reaches up to 50% from aqueous phases and up to 90% from leached sludges and ashes [48]. Adsorption with materials like biochar, agricultural waste, and aquaculture waste is also used for P recovery. Biochar absorbs P naturally due to its considerable Ca and Mg content, with enhanced sorption via chemical treatment.
Recovery of P via biological activity with organisms that accumulate P is also an alternative technique to recover P. Organisms like algae (Chlorella sp.), mussels (Mytilus edulis), and pteridophytes (Azolla spp.) can absorb P during growth. Some filamentous fungi can also store P up to 7% of their cell mass [49]. For example, polyphosphate-accumulating organism stores P, which is further usable as fertilizer. In an experiment by Vučić [50], brewery waste yeast was used to aerobically accumulate P after anaerobic release in wastewater treatment plant sludge. Consequently, the harvested yeasts provide polyphosphate-rich fertilizer comparable to mineral fertilizers.
4. Future perspectives and research needs
4.1 Bridging the gap between phosphorus biogeochemistry and sustainable management
Advances in our understanding of P biogeochemistry have yet to be fully translated into practical management tools. Bridging this gap requires the integration of molecular-scale insights with field-scale practices. This includes improving models that predict P dynamics under varying environmental and management conditions.
4.2 The need for interdisciplinary approaches in phosphorus research
Phosphorus management is inherently interdisciplinary, necessitating collaboration among soil scientists, hydrologists, ecologists, agronomists, economists, and policymakers. Integrated research approaches that consider biophysical, socioeconomic, and governance dimensions are crucial to developing effective and scalable solutions.
4.3 Policy and regulatory frameworks for improved phosphorus stewardship
Effective P management extends beyond the farm gate and into the realm of policy. Regulatory instruments, such as nutrient management plans, P index tools, and watershed-based trading programs, are essential to incentivize best practices and penalize non-compliance. International efforts, including the Sustainable Phosphorus Initiative and the European Union’s Farm to Fork Strategy, emphasize the need for a circular P economy that recycles P from waste streams and reduces reliance on finite phosphate rock reserves.
5. Conclusion
5.1 Summary of key findings
This chapter highlights the dual challenge of optimizing P use for food production while minimizing its environmental footprint. P losses from agricultural systems are driven by complex biogeochemical and hydrological processes, necessitating a multifaceted management approach.
5.2 From fixation to availability: The P continuum
P dynamics in soil involve both short-term fixation and long-term bioavailability, processes that critically influence plant nutrition and fertilizer use efficiency. Shortly after application, soluble phosphate reacts rapidly with soil constituents, especially iron and aluminum oxides in acidic soils or calcium in alkaline soils, forming poorly soluble compounds through adsorption and precipitation—this short-term fixation significantly limits immediate P availability to crops. However, over time, P bound in these forms, along with residual P from previous fertilizer applications, can slowly become plant-available through desorption or dissolution processes. Additionally, organic P stored in soil organic matter and microbial biomass undergoes mineralization via microbial activity and enzyme action, releasing inorganic phosphate that plants can absorb. The interplay between these processes underscores the importance of managing soils not only to minimize fixation losses but also to sustain long-term P availability through practices that promote organic matter turnover and efficient recycling of residual P.
5.3 The importance of integrating biogeochemical insights into phosphorus management
Integrating biogeochemical knowledge with innovative management practices can enhance PUE and reduce its mobility in the environment. Conservation tillage, cover cropping, buffer strips, and targeted fertilizer applications are among the most effective strategies.
5.4 The path forward for sustainable phosphorus use in agriculture and beyond
Looking forward, interdisciplinary research and supportive policy frameworks will be key to achieving sustainable P management. A transition to circular P systems that recycle and recover P from various sources offers a promising path to reduce environmental risks while ensuring agricultural resilience and food security.
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